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The glossary pages provide definitions for over 2680 PA-related terms and abbreviations. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.

S-video * S/FTP * S/UTP * S100 * S200 * S400 * S4.40 * S800 * Sabin * SAC * SAC38 * SAC56 * SAC90 * SAC120 * SACD * sACN * Safe * Safety * Safety capacitor * Safety chain * Safety earth * Safety ground * Safety factor * Safety testing * Sample * Sample width * Sampling frequency * Sampling rate * SAR * SATB * SC-10 * Scaler * Scan converter * Scanner * Scarlet Book * SCART * Scene * Schematic * Schuko connector * SCMS * 'Scope * Score * Screen * Screen gain * Screened cable * Scribble strip * Scroller * SD video * SDC * SDI * SDIF, SDIF-2, SDIF-3 * SDMI * SDVoE * SE * Sealed box * SECAM * Second * Second harmonic * Second-order * Secondary winding * Sectional * Segue * SEL * Self noise * Self-powered speaker * Self-shorting jack * Semi-acoustic * Semi-balanced * Semi-parametric equaliser * Semiconductor * Semitone * Send * Sense conductor * Sense wire * Sensitivity * Separation * Serial * Serial Digital Interface * Serial effects unit * Series * Series-parallel * Servo balanced * Session * Session artist * Session musician * Set * Set list * Seventy-volt line * Sex * Sex changer * Sexless * SF/FTP * SF/UTP * SFX * Shadow * Shaped response * Shared frequency licensing * Shark fin * Sharp * Sharpie * Sheath * Shell * Shelving response * Shield * Shielded cable * Shlv * Shock mount * Shock protection * Short circuit * Short circuit protection * Short throw * Short out * Shotgun * Show * Show control * ShowNet * Showsafe * Shuko * Shunt * Shv * Sibilance * Side * Side chain * Side lobe * Side-addressed * Side-fill * Side-fills * Sideband * Signal * Signal bandwidth * Signal chain * Signal earth * Signal ground * Signal level * Signal overload * Signal path * Signal processing * Signal-to-noise ratio * Signal to quantisation noise ratio * SIL * Simplex * Sine wave * Single-coil * Single-ended * Single microphone technique * Single normalling * Single phase * Single pole * SIP * SKB * Skipping * SL * SL419 * SL61 * Slant * Slap echo * Slapback * Slave monitor * Slave speaker * Sleeve * Slew rate * Slider * SLM * Slope * Slow-blow fuse * SM * SM57 * SM58 * Smaart * Small diaphragm * Smiley face * SMPS * SMPSU * SMPTE * Snake * Snake head * Snaplock * Snare * SNHL * SNR * Soak test * Soca * Socapex * Sock * Socket * Soft clipping * Soft knee * Soft start * SOL * Solid-cored * Solid state * Solo * Solo PFL * Solo-in-place * Sone * Sonic * Sound * Sound board * Sound chain * Sound-check * Sound effects * Sound engineer * Sound engineering * Sound field * Sound hole * Sound intensity level * Sound level * Sound level meter * Sound limiter * Sound plot * Sound pressure level * Sound propagation * Sound reinforcement * Sound stage * Soundie * Source * Source impedance * SP * Spade terminal * Sparkie * Sparks * Sparky * Spatial * SPD * SPDIF or S/PDIF or S/P-DIF * Speaker * Speaker cab/cabinet * Speaker cable * Speaker cone * Speaker crossover * Speaker directivity * Speaker driver * Speaker enclosure * Speaker impedance * Speaker-level * Speaker management equipment * Speaker power * Speaker protection * Speaker Q * Speaker stack * Speakon * Special effects * Spectrum * Spectrum analyser * Speed of sound * Spider * Spike * Spill * Spiral quad cable * SPL * Split keyboard * Split output * Splitter * SPOFC * Spring line * SQNR * Square wave * Squeal * Squelch * SR * SRBP * SRCD * SRSL * SSID * Stack * Stage * Stage left * Stage microphone * Stage monitor * Stage plot * Stage right * Stage tech * Stagebox * Standard operating level * Standby * Standby switch * Standing wave * Star point earthing * Star point grounding * Star quad cable * Starquad cable * Start-stop communications * Static * STE * Steepness * Stereo * Stereo bus * Stereo buss * Stereo bus compressor * Stereo buss compressor * Stereo image * Stereo microphone technique * STI * STI-PA * STIPA * Stomp box * Stompbox * Stopband * STP * Stranded-cored * Stranding * Streaming * Strike * Studio microphone * STX * Stylus * Sub * Sub out * Sub-bass * Subcardioid * Subcarrier * Subcode * Sub-frame * Sub-group * Subjective self-deception * Subjectivism * Sub-mix * Subnet mask * Subsonic * Sub-woofer * Summing * Super-cardioid * Superposition * Supraaural or Supraural * Surface-mounted component * Surge current * Surge protection * Surge suppressor * Surge voltage * Surround sound * Suspension mount * Sustain * SVGA * SVGA connector * Sweep EQ * Sweet * Sweet spot * Switch off procedure * Switch on procedure * Switchable pattern * Switched-mode power supply * Switcher * Switching amplifier * SWL * SXGA * SXQ * Symmetrical Q * Sync * Synchronous * Sysex * System tuning

The definitions for these terms are given on the assumption of their use in the context of PA systems; many of the terms have more general meanings when used in a wider context. Where more than one definition is given for a term, the definitions are numbered (1), (2) etc.

Some of the definitions themselves use terms (such as "signal") in a specific way − most of these are links (just the first time they are used, in each definition), so just click on them to see the meanings that are intended.


An abbreviation for 'separated video components'. An analogue video connection standard in which the luminance information and the chrominance information are conveyed by separate signals, usually connected using a 4-pole mini-DIN connector. Also called 'Y/C video', 'Y' and 'C' being the respective abbreviations for luminance and chrominance. Compare Composite video, Component video and RGB.


A designation for cable containing one or more twisted pairs, indicating that the pairs are individually foil-screened (shielded) and that the cable also has an overall braided screen. The term most usually refers to cable containing four pairs, typically fitted with 8p8c RJ45 connectors. It is important to use the correct 'category' of S/FTP cable, to suit the bit-rate of the computer network or whatever other equipment it is used with, taking into account the cable lengths involved − see Category cable. See also QTP, Ethernet cable, U/UTP, F/UTP, S/UTP, SF/UTP, U/FTP, F/FTP and SF/FTP.


A designation for cable containing one or more twisted pairs, indicating that the pairs have no individual screening (shielding) and that the cable has an overall braided screen. The term most usually refers to cable containing four pairs, typically fitted with 8p8c RJ45 connectors. It is important to use the correct 'category' of S/UTP cable, to suit the bit-rate of the computer network or whatever other equipment it is used with, taking into account the cable lengths involved − see Category cable. See also QTP, Ethernet cable, U/UTP, F/UTP, SF/UTP, U/FTP, F/FTP, S/FTP and SF/FTP.

S100, S200, S400

See IEEE 1394.


See AES3.


See IEEE 1394.


A unit of acoustic absorption, named after Wallace Sabine. One Sabin is an amount of absorption equivalent to a one square foot area of 'total' absorption (e.g. an open window). A 'metric Sabin' is an amount of absorption equivalent to a one square metre area of 'total' absorption.

SAC (1)

An abbreviation for 'software audio console', a mixer that is implemented in software running on a standard PC or laptop. It is used in conjunction with appropriate audio interface hardware. See also Platform (2), Control surface, Plug-in, App and DAW.

SAC (2)

An abbreviation for 'standard audio configuration', a term used to indicate use of a specific standard for the pin-allocations of certain types of multiway connectors used in professional audio applications, most commonly EDAC 516 series connectors. In particular, refer to the following definition. See also DTRS.

SAC38, SAC56, SAC90 and SAC120

Designations referring to standardised pin-allocations used with EDAC 516 series multiway connectors, which are popular for audio applications such as for the connection of multicore cables and stageboxes. The number following 'SAC' (see the previous definition) refers to the maximum number of poles accommodated by the respective connector type in the 516 series. SAC38/8 and SAC38/12 refers to the pin-allocations of the 38-pole connector for 8 or 12 balanced circuits respectively; similarly for SAC56 (16 circuits), SAC90 (26 circuits) and SAC120 (32 circuits). These allocations may be viewed on patchbays.com's site here (external link, opens in a new window).


An abbreviation for Super Audio Compact Disc, an enhanced compact disc format. The standard for these discs is defined by the Scarlet Book (see CD standards). See also HDCD and DSD.


An abbreviation for 'streaming architecture for control networks', a DMX-over-Ethernet technology that allows up to 63,999 DMX universes to be controlled via a single network cable. sACN was developed by ESTA. Compare Art-Net and ShowNet.

Safe (1)

Describes recorded programme material that is protected against erasure or being 'recorded over' by other material. Or, describes stored equipment settings that are protected against erasure or being altered.

Safe (2), Safety

Describes a situation in which there is an absence of danger. This means that the risk of injury or death arising from any hazards present is acceptably low. N.B. This definition of the term may differ from officially recognised definitions. For further information on safety see the Safety page.

Safety capacitor

Safety capacitors are a special class of capacitors that are approved for direct connection to the mains supply. They are usually provided for interference suppression purposes, and are designated as 'X' or 'Y' types. 'X' types are for connection between Live and Neutral, and are likely to fail short-circuit. 'Y' types are for connection between Live and Earth, and are designed to fail open-circuit. 'X2' and 'Y2' types are intended for equipment that is plugged into standard domestic-style mains outlets (13 Amp BS 1363 in the UK). 'X1' and 'Y1' types have a higher surge voltage rating and are intended for commercial and industrial applications where higher-voltage surges are more likely. Compare Death capacitor.

Safety chain

A chain or wire intended to provide a safe means of support for a lantern or other suspended item in the event of its primary support method failing. Safety chains must have an adequate loading capacity to handle the weight of the item in question in the specific circumstances of use, and be of an approved type (e.g. CE marked). Other factors, such as the drop distance, may also be relevant. See also SWL and Fly.

Safety earth, Safety ground

A point that is effectively and reliably connected to the general mass of the Earth, so as to prevent anything connected to that point from persisting at a dangerous voltage with respect to Earth in the event of a fault occurring in mains-powered equipment or in mains distribution equipment. (Also called a protective earth.) Or, a conductor that provides an effective and reliable connection between Class I equipment and such a point, so as to prevent the accessible metal parts of that equipment (and anything connected to those parts) from persisting at a dangerous voltage with respect to Earth in the event of a fault occurring inside that equipment or elsewhere. (Also called a protective conductor.)

As these connections are provided for protection against electric shock (by indirect contact, i.e. for fault protection), it is essential that all such connections are maintained in proper condition, to ensure that adequate protection continues to be provided (see PAT).

The connection to the general mass of the Earth is achieved by conductors that ultimately provide a path to one or more earth rods inserted into the ground, via the main earthing terminal of the electrical installation. Part of the connection path from the main earthing terminal to such earth rod(s) (which may be located at a supply sub-station some distance away) may be provided by the electricity supplier. The means used for connection to the general mass of the Earth varies from country to country − for information on the most common means covered by BS 7671 see TN-S, TN-C-S and TT.

Safety earth connections from the main earthing terminal to the mains-powered equipment within the premises are usually provided via the fixed wiring of the building and its mains socket outlets, and are the responsibility of the owner/operator.

It is essential for every item of Class I mains-powered equipment to have its own individual safety earth connection, to ensure that its exposed conductive parts cannot reach a dangerous voltage in the event of a fault. This connection is usually provided through the 3-core mains cable to the chassis of the equipment. (It may or may not also connect to the internal signal earth of the equipment − see Earth lift.) It is never safe to rely on a signal earth connection to provide a safety earth to an item of Class I equipment.

The time for which a dangerous voltage can exist on a conductor connected to a safety earth is limited by one or more safety cut-out devices (such as a fuse, MCB or RCD) in the mains supply which, in the event of a sufficiently large fault current flowing to earth, should operate quickly enough to cut off the supply before harm might be caused. For these safety devices to operate properly, the complete path of the fault current must have an adequately low impedance − see Earth loop impedance. Aditionally, to cater for the case where the fault current is insufficient to operate the safety device(s), the safety earth connection path between the location of any possible earth fault and the general mass of the Earth (via the earth rod(s)) must have a sufficiently low total impedance to prevent anything connected to the affected safety earth arrangements from reaching a dangerous voltage while the fault persists.

Where appropriate, a safety earth may additionally serve a function of earthing for signal-related purposes (so-called 'functional earthing'). In some circumstances such as studio installations where 'clean' signal earth connections are required to be kept separate from the safety earthing, such functional earthing may be provided by means of an independent technical earth facility.

'Ground' is an alternative term for 'earth', and is the term generally employed in the USA. However, use of the term 'ground' is becoming more and more common in the UK because of the large quantity of PA equipment in use whose panel markings and documentation use that term to suit the US market. Nevertheless, the official UK term for electrical safety-related purposes remains 'earth'. An alternative term for 'safety earth' is 'protective earth'. For further information on safety matters in general, see the Safety page. See also Bonding, Circuit protective conductor, Shock protection, Earth loop and Isolating transformer. Compare Signal earth.

Safety factor

The number that the Minimum Breaking Load of an item of equipment used for flying, supporting or lifting purposes is divided by, in order to arrive at its Safe Working Load (SWL) or Working Load Limit (WLL) figure. Safety factors of at least 8 are typically used for such items.

Safety testing

In regard to electrical safety testing, for fixed installation testing see Condition report and for equipment testing see PAT.

Sample (1)

A snapshot of an analogue signal, which captures its value (usually the value of its voltage) at essentially a single instant in time − this is called its 'instantaneous value' (compare Sample (2)). The process of generating such samples is called sampling and is normally performed repeatedly, at a fixed very rapid rate (the 'sampling frequency'), in order that all the variations of interest in the instantaneous value may be registered. This is a fundamental step in the process of analogue to digital conversion. See also Acquisition time, Aliasing and Oversampling.

Sample (2)

An audio recording, usually lasting a few seconds or less. Often intended for repetitive playback − either in its original form or in a modified (processed) form. For example, samples might be taken of the sounds made by a grand piano, in order to create the stored sounds played by an electric piano. The process of generating such samples is called sampling.

Sample width

An alternative term for bit depth.

Sampling frequency (Sampling rate)

The rate at which the instantaneous voltage of an analogue signal is examined, e.g. during the process of converting it to a digital one. For further details see Analogue to digital conversion. See also Nyquist frequency and Aliasing.


An abbreviation for 'select audio return' − see Mix-minus.


An abbreviation for 'soprano, alto, tenor and bass'; the four musical parts of a traditional choral score. Also referred to as 'four-part harmony'. May be used as a description of a choir that consists of four groups of vocalists, one for each of those parts. In such a choir, each group may require separate microphones (or separate sets of microphones) in order that the parts may be appropriately mixed. See also Sectional and Contralto.


See AES24.


A video processing device that converts between two or more different video signal formats. For example, a particular scaler may have the capability to convert composite and S-video signals to SVGA format. Depending on the formats converted between, a scaler will often incorporate the function of scan conversion, which allows output signals to be provided at different vertical and/or horizontal scanning rates to the input(s).

Scalers also sometimes include a switching function, allowing one of several video sources to be selected for display, recording or transmission. Indeed, the need to switch between several sources may be one of the primary reasons for use of a scaler − e.g. when the sources are provided in different formats. Also called a scan converter. See also Raster.

Scan converter

See the previous definition.


An item of equipment that is able to determine which radio frequencies are in use at a particular location. This information may assist the in the selection of suitable legal operating frequencies for equipment such as radio microphones and in-ear monitoring systems, or may be of help in determining the source(s) of radio-frequency interference. Some radio microphone receivers now incorporate a scanning function. See also Regulated frequency and De-regulated frequency.

Scarlet Book

See CD standards.


An abbreviation for 'Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs', the French organisation whose name was adopted to identify the 21-pin audio-visual connector that it specified. Also referred to as a 'Peritel' connector, it provided connections for analogue stereo audio in both directions and for various combinations of analogue video formats, including composite (in both directions), RGB (in one direction) and S-video (in one direction), as well as connections for some limited control functions such as aspect ratio switching.

In order to standardise the equipment connector pin assignments for each direction of signal flow, SCART-to-SCART cables typically incorporated a symmetrical cross-over of certain connections between its two ends, so that equipment inputs were connected to outputs and vice versa. The crossed-over pin connections were 1 to 2, 3 to 6, 19 to 20 and 18 to 17 (and also in reverse, in each case). Note that pins 10 and 12 are not crossed-over.

The SCART connector was in common use as an analogue audio-visual interface between items of consumer equipment, however with the advent of digital TV it is now considered an obsolete standard, having been largely superseded by the HDMI interface. Note that SCART cables of very poor quality were sometimes used − in particular which lacked proper screening of the conductors. (It was advised never to use types that have only an overall screen, i.e. no individual screening of the signal conductors.) A common effect of this problem was visible and audible crosstalk between the 'In' and 'Out' signals when both were present simultaneously, for example a faint unwanted picture (often drifting about) superimposed on the wanted one, and/or faint unwanted audio. Also the connectors (the cable plugs and also the equipment sockets) varied hugely in quality − a common problem was poor electrical contact on some pins due to improper mating of the connectors when either the plug or the socket was (or became) loose-fitting. The better-quality connectors had gold-plated contacts and have 'bips' in the plug shield to give a more secure fit.

View SCART image

The pin allocations of the SCART connector for the three most common analogue video formats are tabled below. (Note that the audio connections are the same for all configurations.) The asterisks refer to notes that are given after the three tables.

For composite video:

Function                 Pin  Function
Audio R out12Audio R in
Audio L/mono out 34 Audio earth
56Audio L/mono in
78 Video status I/O*
910Digital data in**
1112Digital data out**
1314Digital. data earth
Video out earth1718Video in earth
Video out19   20  Video in
Cable screen21

For S-video:

Function                 Pin  Function
Audio R out12Audio R in
Audio L/mono out34Audio earth
56Audio L/mono in
78Video status I/O*
910Digital data in**
1112Digital data out**
Chrom (C) earth 1314Digital data earth
Chrom (C) I/O1516
Lum (Y) out earth 1718Lum (Y) in earth
Lum (Y) out19   20  Lum (Y) in
Cable screen21

For RGB:

Function                 Pin  Function
Audio R out12Audio R in
Audio L/mono out34Audio earth
Blue earth56Audio L/mono in
Blue I/O78Video status I/O*
Green earth910Digital data in**
Green I/O1112Digital data out**
Red earth1314RGB status / digital data earth
Red I/O1516RGB status I/O***
Sync out earth1718Sync in earth
Sync out19   20  Sync in
Cable screen21

* Typically used as follows:

    0 to 2 V No signal, or unknown aspect ratio
    4.5 to 7 V 16:9 aspect ratio
    9.5 to 12 V    4:3 aspect ratio

** Typically used for an AV.Link interface (also known as Q.Link and by other trademarked names). Exceptionally, pin 10 may be used for field sync I/O and pin 12 for line sync I/O.

*** Typically used as follows:

    0 to 0.4 V    RGB signals not present (use composite interface mode)
    1 to 3 V RGB signals present

Scene (1)

The smallest definable section of a theatrical performance such as a play, musical or opera. Originally a section during which no major changes occurred to the stage scenery − following which a short pause in the action is required while the scenery is changed. However, it may now be a much shorter section (for example, during which specific action takes place between particular characters) which flows seamlessly with adjacent scenes. See also Set (2) and Flat (2).

Scene (2)

A specific combination of control settings, required to give the particular overall result needed at a particular point during an event. For example, the specific combination of lighting controls that is required to light a particular scene of a play (which is the origin of this usage of the term 'scene'). With the advent of digital sound and lighting equipment, scenes may now be designed and stored prior to the event, and then recalled at the touch of a button when required. See also Digital mixer and Show control.


An alternative name for a circuit diagram.

Schuko connector

Usually refers to a mains connector commonly used in some parts of mainland Europe (not used in the UK). The plug is a non-polarised connector having two round pins and (usually) two side contacts for the safety earth. A common variant of the plug also includes an earth contact hole for compatibility with French socket outlets having a projecting earth pin. Schuko plugs are most commonly rated at 10 to 16 amps. Warning: Connectors fitted to Class I equipment must only be used with outlets, extensions, adaptors etc. that are equipped with earth contact(s) providing a connection to a safety earth. See also IEC 320, CEE-form connector and BS 1363A connector.


An abbreviation for 'serial copy management system'. A system used by consumer digital audio recording equipment to limit the extent to which digital copies may be made of copyright-protected recordings. It usually allows only a single iteration of copying, i.e. digital copies may not be made of digital copies. See also DRM, SDMI, HDCP and SPDIF.


A slang abbreviation for 'oscilloscope'.


The printed musical notation for musicians or vocalists to follow when playing a particular item, or set of items. The score is divided into bars. The term is most often used in the context of classical, orchestral or choral music, in which case it details the part for each instrument and/or voice. More informally, it is frequently referred to as 'sheet music' or, confusingly, as 'music'. See also SATB.


The outer conductor of a coaxial cable or of a screened signal cable used for balanced or unbalanced interconnections. The screen is responsible for protecting the signal-carrying conductor(s) from radio-frequency electromagnetic interference (RFI) and from capacitively-coupled crosstalk and noise (notably hum and buzz). In the case of unbalanced interconnections, the screen is also responsible for providing a return path for the signal current. It also reduces the likelihood that the signals being carried by the cable will cause interference with other equipment (especially other nearby cables) by capacitive coupling.

It usually consists of a braid or lapping of fine copper strands, and/or a copper or aluminium foil or a layer of semiconductor plastic material, surrounding the signal-carrying conductor(s). Foil screens and semiconductor screens are usually provided with a drain wire. A screen is also known as a shield.

Screens are usually connected to a signal earth at one or both ends of the cable (via. the connector(s), if used), although, in theory, in some situations a degree of screening would still be obtained without such a connection (notably when multiple signal conductors of the same circuit are enclosed together by a single screen). See also Pin 1 problem and STP.

Screen gain

A numerical value that describes the amount of light reflected from a surface such as projection screen, in a given viewing direction, as compared to a standard white surface that scatters incident light equally in all directions. Unless stated otherwise, a viewing direction perpendicular to the surface is assumed. Values of up to around 1.8 are fairly readily achievable. Screen gain is more formally known as the 'reflection factor' of the screen.

The screen gain value is an important factor determining the perceived brightness of projected images − for further information see Nits.

For example, a screen that reflects twice as much light as the standard surface, in a perpendicular direction, would be said to have a gain of 2. Such a screen would of necessity reflect less light in directions away from the perpendicular than would the standard surface. Therefore, this screen's viewing angle (for optimum image brightness) would be more restricted than would be the case with a screen having a lower gain.

Screened cable

A cable that incorporates a screen. Most audio cables are screened (with the exception of speaker cables). Also called 'shielded cable'. Compare Unscreened cable.

Scribble strip

The long narrow area which runs above or below the faders on a mixer, which may be written on to identify the signal source or sub-mix which is controlled by each fader. The strip may be written on directly with a suitable erasable pen, or console tape may be used. Or, on a digital mixer display screen or computer-based equipment such as DAWs, a window, electronic display or writable area serving an equivalent function.


A lantern that is able to produce light of different colours by means of an internal colour filter consisting of consecutive colours in a continuous strip, usually held on reels. The required colour of light may be selected remotely by means of a motor rotating the reels until the required filter intercepts the beam from the lamp. Now largely replaced by LED-based lanterns operating under DMX control.

SD video

Video at standard definition, usually referring to 270 Mbit/s digital video, e.g. on an SDI interconnection. See also HDMI.


An abbreviation for 'small diaphragm condenser', a condenser microphone that has a small-diameter diaphragm, typically less than 15 mm diameter. SDCs are typically used when pick-up of fast transients is important. Compare LDC.


An abbreviation for 'Serial Digital Interface', a family of digital video interconnection standards specified by ITU-R. It can operate at choice of several standard bit-rates, most commonly 270 Mbit/s for standard definition and 1.483 Gbit/s ('1.5G') for high definition (HD). The maximum currently standardised bit-rate, for ultra high definition (UHD) applications, is approximately 12 Gbit/s ('12G'); a 24G standard will soon be available. SDI employs uncompressed encoding of component video. The data may also include audio channels and auxiliary information, and is usually conveyed over either screened unbalanced cable with a characteristic impedance of 75 ohms and terminated in BNC connectors, or over fibre-optic cable. Alternatively it may be conveyed over other types of interconnection such as Ethernet. See also Codec. Compare HDMI.


An abbreviation for 'Sony Digital Interface', an obsolete 2-channel digital audio interconnection standard. The SDIF-2 variant employs no line code and uses three cables, carrying channel 1 (typically Left) data, channel 2 (typically Right) data and a word clock (WCLK) signal respectively. The SDIF-3 variant employs a bi-phase mark line code and uses two cables, carrying channel 1 (typically Left) data and channel 2 (typically Right) data respectively. In the case of SDIF-3, the destination equipment recovers the clock signal from the two data streams.

SDIF most commonly utilised unbalanced lines at an impedance of 75 ohms, and used BNC connectors. However, a balanced version was sometimes used, usually connected via a 9-pole D-sub connector.

SDIF most commonly carried PCM-coded audio, but in some cases has been used to carry DSD-coded audio. Note that SDIF is a different interface to S/PDIF.


An abbreviation for 'secure digital music initiative', a standard for protecting the copyright of digitally stored music, by limiting the number and type of copying operations that can be performed upon it. See also and DRM, SCMS and HDCP.


An abbreviation for 'software defined video over Ethernet', a standard for distributing and managing AV signals via an Ethernet network. A specific advantage of this system is low latency. See also AVoIP.


An abbreviation for 'single-ended'. See One-legged.

Sealed box

Describes a speaker enclosure that has no ports.

The alternative term 'infinite baffle' is sometimes used to describe a sealed-box enclosure; this is because the only sound that is emitted from such an enclosure (assuming totally rigid enclosure walls) is from the front of the drivers, just as if they were mounted on a (hypothetical) rigid baffle of infinite size. However, whilst the acoustic behaviour of a sealed-box enclosure is substantially affected by its internal dimensions, such effects would be absent with an infinite-sized baffle.


A French abbreviation for 'séquentiel couleur à mémoire' (meaning 'sequential colour with memory'). Usually refers to the method used in France and some other countries for coding the chrominance information prior to creating a colour composite video signal. Or, describes a composite video signal that incorporates chrominance information coded using that method. Compare PAL and NTSC.


An alternative name for an assistant or 'junior' person.

Second harmonic

A harmonic whose frequency is twice that of the fundamental, i.e. that is an octave above the fundamental. Compare Third harmonic.


See Order.

Secondary winding

See Transformer.


A rehearsal in which only a specific section of the orchestra, band or choir are present, normally held prior to the full rehearsal. For example, just the alto section of an SATB choir might gather for an alto sectional in order to rehearse the alto part. If held in the performance venue, may be useful for sound-check purposes.


Where one performance item (e.g. song) runs straight into the next, without any gap. Pronounced 'seg-way'.


An abbreviation for 'sound exposure level', a measurement designed to determine the exposure of people to potentially harmful levels of sound. It is an A-weighted mean square measurement of sound energy, determined over a specific period of time and normalised to an 8 hour working day. See also Time-weighted average, Dosemeter, NIHL, Leq, Audiology, Tinnitus, SNR (2), SNHL and Acoustic Safety on the Safety page.

Self noise (of microphones)

An alternative term for 'equivalent noise level' (ENL) − see Microphone Noise Levels on the Microphones page. See also Thermal noise.

Self-powered speaker

Ssee Powered speaker.

Self-shorting jack

A 2-pole jack plug that is designed to provide a connection between its tip and sleeve when not plugged into anything. Typically such a plug would be used at the end of an instrument cable that connects to the instrument, and is intended to avoid pick-up of a hum or buzz when not plugged into the instrument. It is also intended to reduce the likelihood of annoying (and potentially damaging) loud clicks, thuds, etc. occurring when inserting or removing the plug from the instrument while the other end of the cable remains connected to an amplifier or PA system. However, in general it is strongly recommended that instrument cables are not inserted or removed from an instrument unless the other end has been unplugged first, or the amplifier or PA channel has been muted. See also Short circuit.


Describes a musical instrument, most usually a guitar, that produces some sound by natural acoustic means but which nevertheless is intended to be heard by means of amplification of a signal from its built-in pick-up(s). It typically has a hollow body equipped with f-holes.


Describes an interconnection similar to a 'fully balanced' interconnection, except that only one of the two signal conductors (the 'hot' conductor) is driven with a signal by the equipment at the 'sending' end. The other ('cold') conductor of the output is connected to signal earth (inside the equipment) via an impedance equal to the source impedance of the circuit driving the 'hot' conductor (each of these impedances being a half of the quoted impedance of the output). Therefore, such an output arrangement is sometimes referred to as an 'impedance-balanced' output.

As with a normal (i.e. fully) balanced interconnection, rejection ('cancellation') of common mode interference picked-up on a semi-balanced interconnection relies upon connection to a balanced input at the 'receiving' end, and upon that input having a good common mode rejection ratio. The lack of a drive signal on the 'cold' conductor does not affect the rejection of such interference, but results in a 6 dB reduction in headroom, compared with a normal balanced interconnection at the same level. (This is because, to provide the same level of output, in the semi-balanced case the sending equipment must deliver twice as much signal voltage on the hot conductor than in the case of a normal fully-balanced interconnection.) In order to maintain the same headroom, the sending level would have to be reduced by 6 dB, which would reduce the signal-to-noise ratio of the interconnection by the same amount.

The only advantages of the semi-balanced arrangement over a fully balanced interconnection are that:

The second two factors make this type of output readily compatible with both balanced and unbalanced physical interconnections (lines), which can be a useful feature in semi-professional applications.

However, in addition to the reduction in headroom mentioned above, two further disadvantages as compared to a fully balanced interconnection are:

The semi-balanced arrangement (also referred to as 'quasi-balanced'), is not often employed in professional equipment (except some condenser microphones), as most (if not all) audio interconnections are made using balanced-pair cable equipped with 3-pole connectors and so the added cost of the (generally) preferable fully-balanced arrangement is small and easily justified.

A table comparing the most common types of balanced interconnections is provided under the 'Balanced' entry. Diagrams illustrating various different types of signal interconnections are available here (opens in a new window). Compare Balanced, Ground-compensated, Quasi-floating and Pseudo-balanced.

Semi-parametric equaliser

See Parametric equaliser.


Strictly, a material whose ability to pass an electric current falls between that of a conductor and that of an insulator. However, since certain such materials are used in the construction of electronic components such as diodes, transistors and integrated circuits (ICs), these components have themselves become known as semiconductors. See also Solid state, FET and MOSFET.


An interval of musical pitch, twelve of which make up an interval of one octave. In terms of frequency, a semitone increase in pitch represents a multiplication factor of a twelfth root of 2 (approximately 1.059, i.e. an increase of 5.9%). An interval of two semitones is a tone. See also Cent.


An output; a connection point for a signal leaving an item of equipment, or the controls associated with an output (especially fader controls). For example, a mixer output intended to supply a signal to an effects unit (see Effect send).

Or, the start of an internal path between sub-sections within a larger item of equipment, or the controls associated with supplying signals on such a path (especially fader controls). For example, an 'Aux Send' control on a mixer channel. Compare Return.

Sense conductor, Sense wire

In a ground-compensated interconnection, the conductor that conveys the signal earth voltage of the destination equipment back to the source equipment. It is usually connected through pin 3 of an XLR or the ring contact of a 3-pole (TRS) jack connector.


A measure of the level of stimulus (whether acoustic or a signal) that an item of equipment needs to enable it to produce a given level of response. When only a low level (relatively speaking) of stimulus is needed, the equipment is said to have a high sensitivity, and vice versa. For example, a microphone that is able to satisfactorily pick up very quiet sounds would be described as having a high sensitivity, as would an amplifier input that is intended to accept very low signal levels or a speaker that produces a relatively high sound level from only a moderate drive signal.

As the response (to a given level of stimulus) of equipment such as microphones and speakers varies according to the frequency of the stimulus, their sensitivity is usually quoted at a specified frequency or for a specified type of stimulus. The exact nature of this frequency-dependence is specified by the equipment's frequency response.

The sensitivity of microphones is usually specified in mV/Pa, which is the number of millivolts of output per Pascal of sound pressure level (SPL), incident on-axis. (Sometimes this is written as mV @ 94 dB SPL, because 94 dB SPL is equivalent to 1 Pa SPL.) The value of sensitivity may alternatively be expressed in dBV/Pa (see the Decibels page for more information on decibels). A microphone with a higher sensitivity will provide a higher output level for the same SPL input.

The sensitivity of speakers was traditionally specified in dB SPL @ 1 W @ 1 m, which is the SPL produced at one metre directly in front of the speaker when it is supplied with an electrical power of one watt. However in practice the amplifier supplies a controlled voltage, not a controlled power − the power taken by the speaker depends upon its impedance, which varies with frequency. Therefore, for a nominallyohm speaker, it is increasingly common for the sensitivity to be specified in dB SPL @ 2.83 V @ 1 m, 2.83 volts being the voltage required for a power of 1 W in 8 ohms. (For a 4 ohm speaker the voltage required is 2 V.) A speaker with a higher sensitivity (and the same impedance) will provide a higher SPL output for the same drive signal. See also Speaker Sensitivity on the Amplifiers and Speakers page.

The sensitivity of headphones and earphones is usually specified either in dB SPL @ 1 mW or in dB SPL @ 1 V. The difference between these two types of specification is not a fixed value; it depends on the impedance of the phones. Values vary widely between models, e.g. 80 to 125 dB SPL @ 1 mW.

The sensitivity of a power amplifier is usually expressed as the input signal level required to cause the amplifier to clip, at a given setting of the sensitivity (or gain) control and under given load conditions. Typically, power amplifiers are supplied with (or are configurable for) a professional (+4 dBu or 1.23 V), prosumer (0 dBu or 0.775 V) or consumer (−10 dBV or 0.316 V) sensitivity.


The extent to which wanted and unwanted sounds or signals are isolated from one another, at a particular location or point within a system. Often quoted as a positive value in decibels, being the amount by which the level of the wanted signal exceeds that of the unwanted one, at the same point. See also Crosstalk and Leakage.


Describes a digital interface or communications link in which a single electrical or optical path carries the data, so that, at any one point in time, only one bit of information is being conveyed. This has the big advantage of requiring a small number of conductors (or fibres) in such a cable, but the disadvantage that a very high bit-rate is required to transfer a lot of data quickly. Examples of serial interfaces are USB, AES3 and DMX. Serial links may be either asynchronous or synchronous. (As a description of effects units, see Serial effects unit.) Compare Parallel (1).

Serial Digital Interface

See SDI.

Serial effects unit

An effects unit that can be simply inserted into the signal path, or connected in a 'daisy-chain' with other serial effects units, without the need to separately mix in the original signal. To achieve this, the unit incorporates adjustable mixing of the original signal with the processed signal that is generated within the unit, so that the output of the unit contains the desired balance of original and processed sounds. Compare Parallel effects unit.


Describes the interconnection of two or more items in such a way that the total circuit current flows through each of the items in turn. Values of resistance connected in series may simply be added together to calculate their overall value, as may values of reactance (taking into account their sign), but not a mixture of resistance and reactance. Series values of impedance may simply be added together provided they all contain the same proportion of resistance and reactance (of the same sign).

Values of inductance connected in series may simply be added together to calculate their overall value. However, in the case of capacitance values in series it is necessary to sum the reciprocals of the individual capacitance values and then take the reciprocal of that sum (i.e. the same method as for parallel resistance).

When components of the same type and of identical tolerance are connected in series, the tolerance of their combined value will be the same as that of the individual components.

The series interconnection of speakers (i.e. of separate enclosures) requires special interconnection arrangements; this may be provided by a purpose-made interconnection box. Such interconnections should always have the 'to' and 'from' conductor for each speaker in the same cable, rather than using single-conductor interconnections (which have a much increased series inductance and can radiate a substantial magnetic field). Note also, that in series speaker arrangements if one of the speakers is unplugged or becomes open-circuit then any of the others that are connected in series with it will also stop working. Furthermore, problems can be experienced with poor damping factor, as the source impedance 'seen' by each speaker includes the total series impedance of the other speakers that are wired in series with it. Due to these factors, the series interconnection of speakers is very unusual and is not generally recommended.

However, series interconnection of speaker drivers within a speaker enclosure is sometimes used − in such arrangements any single-conductor internal wiring should be kept as short as possible and poor damping factor can be an issue − especially in the case of bass or lower mid-range drivers.

Series interconnection of different types of speakers or drivers is never advisable. When interconnecting speakers or drivers, always take care to observe correct polarity, and to use an appropriate gauge of cable. See also Series-parallel. Compare Parallel (2).


Describes the interconnection of several items such that a combination of series and parallel interconnection is used. This method is most frequently encountered in the interconnection of four or more identical drivers within a single speaker enclosure, the purpose being to arrange for a suitable overall input impedance at the speaker's connector. Very occasionally, this method is used for the interconnection of four or more identical speakers (i.e. separate enclosures) that are to be driven by a single power amplifier channel; in this case the purpose is usually to provide sound at several close locations, or in different directions, while maintaining an overall load impedance suitable for connection to the amplifier.

As an example, if we have four 8 ohm drivers, and connect each pair of them in series, we effectively have two 16 ohm drivers. If we now connect the two 'series pairs' in parallel with each other, we have restored the overall impedance to 8 ohms.

When interconnecting drivers or speakers, always take care to observe correct polarity, and to use an appropriate gauge of cable. Also, refer to Series above for cautions regarding the series interconnection of drivers or speakers.

Servo balanced

See Quasi-floating.

Session (1)

A single instance of continuous activity by one or more artists or technical staff, at a live event, in a recording studio, or in a broadcast studio.

Session (2)

See CD standards.

Session musician, Session artist

A musician or other artist who is employed on a session-by-session basis, rather than having a contract for employment covering an extended period.

Set (1) (Musical)

In general, a sequence of songs, arranged in the order of performance. But the term is often used more specifically to refer to the collection of songs that is planned to be performed at a particular event. It may refer to the entire collection to be performed by a particular artist or band for the whole event, but often it refers to a collection to be performed by a particular artist or band without any break in their performance. See also Set list.

Set (2) (Theatrical)

The arrangement of scenery and props on the stage, for a particular scene of the performance. See also Flat (2).

Set (3) (TV & film)

A space that is specifically constructed or arranged for filmed action to take place. This may be within a studio, within a building normally used for other purposes or outdoors.

Set list

A written, ordered list of the songs in a set.

Seventy-volt line

See 100 volt line.


See Gender.

Sex changer

See Gender changer.


See Genderless.


A designation for cable containing one or more twisted pairs, indicating that the pairs are individually foil-screened (shielded) and that the cable also has an overall braided screen and an overall foil screen. The term most usually refers to cable containing four pairs, typically fitted with 8p8c RJ45 connectors. It is important to use the correct 'category' of SF/FTP cable, to suit the bit-rate of the computer network or whatever other equipment it is used with, taking into account the cable lengths involved − see Category cable. See also QTP, Ethernet cable, U/UTP, F/UTP, S/UTP, SF/UTP, U/FTP, F/FTP and S/FTP.


A designation for cable containing one or more twisted pairs, indicating that the pairs have no individual screening (shielding) and that the cable has an overall braided screen and an overall foil screen. The term most usually refers to cable containing four pairs, typically fitted with 8p8c RJ45 connectors. It is important to use the correct 'category' of SF/UTP cable, to suit the bit-rate of the computer network or whatever other equipment it is used with, taking into account the cable lengths involved − see Category cable. See also QTP, Ethernet cable, U/UTP, F/UTP, S/UTP, U/FTP, F/FTP, S/FTP and SF/FTP.


An abbreviation for either sound effects or special effects.


An area that is less well covered by sound from the PA speakers, as compared to most of the surrounding area. This term would typically imply a larger area and a less severe reduction in sound levels than the term 'dead spot'.

Shaped response

An intentional, specifically tailored, deviation from a flat frequency response, provided in order to enhance the tonal qualities of the sound, and/or to assist in the rejection of unwanted sounds, for a particular application.

Shared frequency licensing

Licensing of radio equipment (such as a radio microphone or in-ear monitoring system) which allows use of the equipment at any site (subject to any geographical restrictions applicable). This means that other users in the vicinity may well be using the same (or conflicting) frequencies at the same time, and may therefore legally be a source of interference with your system(s). For further information see Wired or Radio on the Microphones page. See also Regulated frequency. Compare Co-ordinated frequency licensing.

Shark fin



Describes a musical note whose pitch is higher (usually, just a little higher) than it should be. See also Bum note. Compare Flat (3).


A trade-marked name for a particular type of permanent marker pen, useful for writing on smooth surfaces such as console tape, white gaffer tape and recordable discs. Sometimes used to refer to any similar type of marker pen.


The outermost protective covering of a cable, usually made from an insulating material such as plastic or rubber. Or, less frequently, the plastic sleeve that encloses each pair (with its associated drain wire) within some types of multicore. Sometimes called the 'jacket' or 'sleeve' of the cable (American terminology).


The outer casing of a line connector, especially of multi-pole connectors such as the XLR or D-sub. The main body of other types of connector, especially circular types (such as jacks), is usually termed a barrel.

A shell may be made of metal or plastic. When metal, it provides the possibility of an additional conducting path between the mated connectors, but this is usually not relied upon. A metallic shell is generally not considered to be a pole of multi-pole connectors, and is usually not connected to any of the cable conductors (though the SCART connector is an exception). In particular, most professional XLR cables have no connection to the connector shells, in order to avoid the possibility of earth loops being created by the shells of adjacent in-line connectors touching one another, or touching adjacent earthed metalwork.

Shelving response

Describes an equaliser (EQ) facility that provides control over all frequencies either below or above a specific cut-off frequency, but has essentially no effect on frequencies on the other side of the cut-off frequency. Simple 'bass' (or 'LF') controls have a significant effect on only the frequencies below a fixed cut-off frequency, while more sophisticated types allow adjustment of the cut-off frequency. Similarly, simple 'treble' (or 'HF') controls have a significant effect on only the frequencies above a fixed cut-off frequency. Again, more sophisticated types allow adjustment of the cut-off frequency. Note, however, that in the case of the simpler LF and HF controls the frequency marked adjacent to it is not its cut-off frequency. Rather, the marked frequency is that at which the amount of cut or boost indicated on the control's scale (or in the equipment manual) will be obtained. See also Baxandall. Compare Peaking response.


An alternative name for a screen.

Shielded cable

An alternative name for a screened cable.


An abbreviation for 'shelving'. In relation to shelving equalisers, see Shelving response.

Shock mount

A device intended to reduce the transmission of mechanical vibration, e.g. to avoid adverse effects on vibration-sensitive equipment such as microphones. Typically used in the mounting of a microphone to a microphone stand. Also used in the internal construction of microphones. Confusingly, also called an anti-shock mount. A specific type of external shock mount is the suspension mount. See also Handling noise.

Shock protection

In regard to protection against electric shock, this term may refer to any measures intended to guard against this occurrence, including insulation and safety earth connections. However, it most commonly refers to the provision of RCDs or RCBOs with a low enough trip current to provide protection against fatal electric shocks. In the UK, this means a trip current of 30 mA or less. Even where not mandatory by regulations, it is highly advisable for all mains circuits to be protected by such a device. See also RCD, Safety earth, Circuit protection, BS 7671 and BS 7909.

Short circuit

An electrical connection between conductors, providing a low-resistance path which enables the current to bypass its intended (or usual) route. The term is nearly always used of such connections occurring unintentionally, most often due to faults in cables or connectors (but sometimes within equipment). However, there are a few occasions when it is used of a low-resistance connection that is intentionally provided to temporarily bypass a usual current path (see, for example, Self-shorting jack).

A short circuit occurring in a signal circuit will not usually be hazardous but will result in a loss of signal at the destination end, or (in the case of a balanced circuit) may result in a reduction in signal level and/or loss of the benefits of balancing − possibly causing the introduction of hum or other interference.

A short circuit occurring in the interconnection between a power amplifier and a passive speaker can potentially cause serious damage to the amplifier − with possible serious subsequent damage to any speaker(s) connected to it. Therefore, most medium- and high-power amplifiers include a protection feature that is intended to avoid the most serious effects of a short-circuited output. Some of these mechanisms are more effective than others, so care should still be taken to avoid such short circuits.

A short circuit in any kind of power circuit will usually result in a current flow that is very much larger than that which normally flows in that circuit, because the resistance of the short circuit path is usually much lower than that of the intended route. If large enough, this excessive current may cause the operation of a circuit protection device such as fuse or circuit breaker, to disconnect the power. Indeed, in all mains power distribution arrangements, the ratings of such protective devices must be co-ordinated with the ratings of the cables and connectors so as to ensure that, in the event of a short circuit, they will operate sufficiently rapidly − before the very high current is able to raise the temperature of the conductors or surrounding insulation to a value that could cause damage or fire. See also the next definition, MCB and Speaker protection. Compare Overload (2).

Short circuit protection

An equipment feature whose purpose is to reduce or eliminate damage or danger resulting from an accidental short circuit. In PA work the term is most often used in reference to short circuits occurring in the connections between power amplifiers and passive speakers, however the term is also used in reference to circuit protection, e.g. of mains supplies to equipment, in which protection against short circuits is an essential safety feature. See also the previous definition and Speaker protection.

Short throw

Describes a speaker that has wide dispersion angles, enabling it to provide sound effectively to an audience relatively close to it (say, within 12 metres). See also Directivity. Compare Long throw.

Short out

To create a short circuit, usually across a specific component or between two specific points in a circuit.


Another name for a rifle microphone.


A generic term for any type of organised live entertainment event, indoors or outdoors, including concerts, theatrical or dance performances, or any similar entertainment event in which an audience observes and (usually) listens to those taking part. In the context of PA, sporting events and exhibitions are not usually included within this term (despite its more general application to some such events, e.g. to some animal-related and horticultural events). Or, the performance taking place by a particular artist at a particular venue, date and time (as in "Who's A1 for the 3 o'clock show?"). See also Production and Gig.

Show control

A system which provides automation of some or all aspects of the changes that need to be made during an event, such as lighting changes, mixer settings (see Mixer automation), special effects (pyrotechnics, smoke machines, and the like) and even motorised scenery changes. The control system may be a dedicated item of equipment or a standard PC or laptop running appropriate show control software; in such cases a MIDI interface is often used between the control equipment and the equipment being controlled (but see also DMX). Alternatively, the control system may be built into another item of equipment such as a lighting control desk or a mixer.


A system for passing DMX commands over Ethernet networks. It is a proprietary technology of Strand Lighting. Compare Art-Net and sACN.


A brand of Socapex multiway connector, ruggedised for outdoor use and compatible with the SL419 range. It is available in 3, 5, 7 and 19 pin versions.


A mis-spelling of a type of mains connector − see Schuko connector.


As a verb, to provide a parallel path for a flow of current. Or, as a noun, a current path or an electrical component that serves such a function (whether intentionally or otherwise).


An abbreviation for 'shelving'. In relation to shelving equalisers, see Shelving response.


The sound made by speaking or singing the letter 's'. If a PA system over-emphasises sibilance the result will be a 'hissy' or 'scratchy' sound, whereas if sibilance is under-emphasised then clarity and intelligibility will be reduced. Excessive sibilance may be corrected using a de-esser.


Usually refers to one particular channel of a 2-channel item of equipment or system, such as a 2-channel power amplifier or a stereo PA system. The sides of 2-channel equipment are usually labelled '1' and '2', 'A' and 'B' or 'Left' and 'Right'.

Side chain

A part of an effects facility − especially of a facility that modifies the dynamics of a signal, such as a compressor or a limiter. This part of the facility handles the control signal for the operation of the effect; the side chain signal is never actually heard. For example, in the case of a compressor the level of the input signal is detected in the side chain and this information is used to control the gain applied to the signal passing through the facility.

By modifying this information, e.g. by applying equalisation, it is possible to change the way in which the facility modifies the signal. For example, many compressors are provided with an adjustable high pass filtering facility in their side chain, to allow control over the extent to which bass frequencies activate the compression. Some stand-alone effects units are equipped with connectors to allow the side chain to include processing external to the unit; in this case the connectors generally provide access to the signal at a point before it enters the level-detection stage.

Alternatively, the side chain input may be used to arrange for the effect to be controlled by an entirely different signal to the one to which the effect is being applied. Such an arrangement may, for example, be used to provide ducking. See also Stereo bus compressor.

Side lobe

See Lobe.


Describes a microphone whose maximum sensitivity to sound is at the side of the microphone (rather than at the end). The side-addressed design is quite common for high-quality studio microphones, but is relatively unusual for PA microphones − with the exception of some types of drum microphones. Compare End-firing. See also Polar response.

Side-fill, Side-fills

The additional sound provided by monitor speakers located in the wings of the stage in order to improve the sound heard by the performers (also known as 'cross-fill'). Such speakers are referred to as side-fills, but that term may also refer to front-of-house speakers used to provide fill at the outer edges of the audience. These are often needed in situations where the majority of the audience is served by central front-of-house speakers (usually flown). See also Front-fill.


See Modulation.


Usually refers to a representation of some kind of information (e.g. audio, video, etc.). This representation can be in the form of a varying electrical voltage carried on a wire or cable, light impulses conveyed through an optical fibre, or radio waves. The term 'signal' is often preceded by a word to indicate what kind of signal is being referred to, for example 'microphone signal'. Audio and video signals are classified as being analogue or digital. Optical signals are always digital.

The term can also be used to refer to a control indication that is passed electrically, optically or wirelessly from one point to another in a system.

In the same way that sound is said to travel through the air, or is carried by the air, it can be said that a signal travels through (or along) a cable, or is carried by the cable.

Electrical signals that are used to interconnect items of equipment are usually categorised according to important parameters such as:

See also Connector, Input, Output, Load, Termination (2), Bandwidth, Current, Characteristic impedance and the following definitions. Compare Noise.

Signal bandwidth

See Bandwidth.

Signal chain

A signal path considered across several items of equipment through which it passes consecutively, or considered across an entire system from a signal source to a final destination. The term is most often used in reference to the specific items of equipment that make up such a chain, with less emphasis on the interconnection paths between them. For example, in a PA system a signal chain would typically include a microphone, mixer, graphic equaliser, active crossover, power amplifiers and speakers. Such a chain may also be referred to as the 'audio chain' or the 'sound chain'. However, the term 'signal chain' may equally be used for the path of other kinds of signals, such as video. Compare Signal path. See also Output, Input, Drive (1), Cable, Connector, Gain and Loss.

Signal earth, Signal ground

An earthy connection point or conductor that exists solely for purposes associated with the carrying of a signal, or with the screening of a signal from interference. For example, in a screened unbalanced interconnection the cable screen is connected to signal earth in order to provide a signal return path and to reduce radio-frequency interference. Note that a conductor or connection provided for signal earthing purposes has no safety-related function and is not designed to provide one. Such a conductor or connection must always be considered unable to provide that function.

In order to provide a known point from which conductor voltages can be referenced, and to avoid the build-up of static electricity, signal earths are usually arranged to have an eventual connection path to the general mass of the Earth − hence the name. This is normally achieved by connection(s) between the signal earth and a safety earth. When such a connection occurs at more than one point, there is the possibility of an earth loop problem being created − especially when unbalanced interconnections are employed. An alternative approach, applicable to locations such as studios, is to connect signal earths to an independent technical earth system.

Or, a conductor used as the reference against which the voltages of other signal conductors in the same circuit or system are specified − even if such reference conductor has no physical connection path to the general mass of the Earth.

'Ground' is an alternative term for 'earth', and is the term generally employed in the USA. However, use of the term 'ground' is becoming more and more common in the UK, especially in relation to signal earths, because of the large quantity of PA equipment in use whose panel markings and documentation use that term to suit the US market. (Note, however, that the official UK term for electrical safety-related purposes remains 'earth'.) An alternative term for 'signal earth' is 'functional earth'. See also Screen, Chassis and Pin 1 problem. Compare Safety earth.

Signal level

See Level.

Signal overload

See Overload (1).

Signal path

The specific route that a signal is taking (or may potentially take) through a system, or within an item of equipment, at a particular time. Compare Signal chain. See also Routing, Output, Input, Drive (1), Cable, Connector, Gain and Loss.

Signal processing

The modification of a signal in any way, but especially by use of effects units or by compressors, limiters or noise gates. See also Equaliser, Dynamics processor and Digital signal processing.

Signal-to-noise ratio

A measure of the degree to which the average level of a signal exceeds the level of noise at the same point in the system (both values being RMS measurements). The higher the signal-to-noise ratio the better, because a high signal-to-noise ratio means that the noise will be at a much lower level than the signal. Signal-to-noise ratio is measured in decibels and often a weighting filter will be used in the measurement process, to approximate the measured result to that which would be perceived by the average human ear.

Signal-to-noise ratio is maximised by use of suitable equipment, adjusted to provide a suitable gain structure. See also Dynamic range, Quantisation noise, Tape noise, Ambient noise and the Microphone Noise Levels section on the Microphones page.

Signal to quantisation noise ratio

See Quantisation noise.


An abbreviation for 'sound intensity level'.


See Duplex.

Sine wave

A repetitive waveform that contains only a single frequency; a pure tone with a complete absence of harmonics. See also Square wave and Fourier analysis.

View sine-wave image


Describes a type of guitar pickup that has only one coil, and is therefore potentially susceptible to pickup of stray magnetic fields. For example, hum being magnetically induced into it from nearby combos or heads, or inductive coupling from induction loops. Compare Humbucker.


See One-legged.

Single microphone technique

See One microphone technique.

Single normalling

See Normalling.

Single phase

Describes a mains supply in which only one line conductor is provided (along with the Neutral and safety earth conductors). This type of supply is not suitable for the delivery of very large amounts of power; in such cases 3-phase supplies are employed. Single phase wiring most commonly uses the insulation colours applicable to the 'L1' phase of the 3-phase supplies, in the applicable colour-coding scheme. However, in some situations where a single phase has been derived from a 3-phase one, the line ('live') conductor may have the insulation colour of the 'L2' or 'L3' phase. In the UK (since 2004) and in Europe, this means that most commonly brown is used for the line ('live') conductor of single phase supplies (though black and grey may also be used) and blue is used for the Neutral conductor. The colour for safety earths is green and yellow striped.

WARNING: In the UK, prior to 2004, red, yellow and blue were used for the 'L1', 'L2' and 'L3' conductors, and black for Neutral. This means that black may identify a Neutral or a live line conductor, and blue may also identify a Neutral or a live line conductor, depending on which identification scheme is in use. When an installation has a mixture of the old and new schemes, extreme caution must be applied to ensure that all conductors are correctly identified.

Caution: The colours referred to above relate only to the UK and to European countries that have adopted the 'harmonised' identification scheme. Other countries are likely to employ other schemes, possibly involving different uses of the colours mentioned above. For example, in the USA the standard scheme for 208 V 3-phase is black, red and blue for L1, L2 and L3, with white for Neutral, while for 480 V 3-phase it is brown, orange and yellow for L1, L2 and L3, with grey for Neutral; in both of these cases the colour for safety earth is green or green and yellow striped. Compare Phase (3).

Single pole

Describes a device (usually a connector or a switch) having only one pole. For example, a Powerlock connector. Often abbreviated to SP.


An abbreviation for 'solo-in-place'. See Solo.


A manufacturer of hard-shell moulded cases, for protecting equipment in transit and use. However, the term is frequently used to refer to this style of case, regardless of its manufacturer. Cases of this type are often fitted with mounting strips for 19-inch rack-mounting equipment, enabling the equipment to be permanently mounted and used in-situ. (Always ensure, however, that ventilation requirements are met.) Compare Flight case.


Erratic behaviour of a playing CD, typically repeated or missing sections of the track. Usually caused by contamination or scratches of the CD's surface.

SL (1)

An abbreviation for 'stage left'. Compare SR (2).

SL (2)

An abbreviation for 'short term loudness'; usually meaning loudness measured over a 3 second window. Compare ML.

SL61, SL419

These are designations for two popular ranges of the Socapex style of connector, sometimes used by lighting engineers to refer to such connectors.


An alternative term for a wedge, because of its slanting upper surface; a slang term for a floor monitor.

Slap echo

A single echo occurring a very short time (e.g. 100 to 150 ms) after the original sound, either due to a natural early reflection or to an intentional effect. This effect is sometimes referred to as 'slapback'.


See the previous definition.

Slave monitor

A passive monitor that is driven by the power amplifier contained within a powered monitor, via a speaker cable from a slave output of the powered monitor. As with any power amplifier, care must be taken not to overload the powered monitor's amplifier. Typically, the slave monitor will be of the same make and power rating as the powered monitor that it is driven from.

Slave speaker

A passive speaker that is driven by the power amplifier contained within a powered speaker, via a speaker cable from a slave output of the powered speaker. As with any power amplifier, care must be taken not to overload the powered speaker's amplifier. See also the previous definition.

Sleeve (1) (jack plugs)

See TRS.

Sleeve (2) (cable)

See Sheath.

Slew rate

The (maximum) rate at which a signal's instantaneous voltage changes. Or, in equipment specifications, the maximum rate of change of instantaneous voltage that can be accommodated or produced by it. Usually expressed in volts per microsecond (V/µs).

This specification is a necessary supplement to the frequency response specification, because frequency response alone does not take into account the level of the signal. For a sine wave signal, the maximum slew rate occurs at the zero-crossings and (in V/µs) is 8.884 times the RMS voltage times the frequency (in Hz) divided by 1 million. For example, the maximum slew rate of a 60 V (RMS) sine wave at 20 kHz is 10.66 V/µs.


A control with an up-and-down (or, rarely, side-to-side) motion, as compared to one that turns. Often refers to a fader or to a frequency-band control of a graphic equaliser. See also Potentiometer and Travel.


An abbreviation for 'sound level meter'.


In the frequency response of a filter, a region forming the transition between the passband and the stopband, where attenuation changes relatively rapidly with frequency. This rate of change (called the 'steepness' of the slope) is often of interest, and is usually measured in decibels per octave (or per decade). It is a function of the order of the filter: first-order filters have a slope of 6 dB per octave (20 dB per decade), second-order filters have a slope of 12 dB per octave (40 dB per decade), etc. Some microphones that provide a 'low cut' facility allow adjustment of the low cut filter slope. See also Roll-off, Accelerated-Slope and Curve.

Slow-blow fuse

See Temporised fuse.


An abbreviation for 'stage manager'. See also ASM, DSM and PM.


A very popular instrument microphone manufactured by Shure, sometimes used for speech or backing vocals. It is a dynamic microphone. May be referred to as a '57'.


A very popular vocal performance microphone manufactured by Shure. It is a dynamic microphone. May be referred to as a '58'.


A trademarked name for a software-based audio analysis system from Rational Acoustics (originally designed by EAW/LOUD). Their website is www.rationalacoustics.com (opens in a new window). See also Tuning (1), MLSSA, STI, ALCONS and Measurement microphone.

Small diaphragm

Describes condenser microphones that have a diaphragm smaller than around 20 to 25 mm diameter. (There is some overlap of the small/large categories over this range.) Small diaphragm condensers are used mostly where there is more emphasis on avoiding pick-up of off-axis low frequency sources and less emphasis on low mic noise levels, such as in stage applications. Although they have reduced frequency-dependent directionality and may have slightly less colouration and/or better transient response as compared to large diaphragm types, they tend to have higher noise levels. See also Microphone Noise Levels on the Microphones page.

Smiley face

Refers to the settings of a graphic equaliser when it is adjusted to boost the Bass and treble frequencies and to cut the mid-range frequencies. The term arises from the smile-like shape made by the knobs of the equaliser's slider controls when adjusted in such a fashion. It is generally held that such a setting is indicative of problems elsewhere in the system, particularly deficient speakers.


An abbreviation for 'switched-mode power supply' (or for 'switched-mode power supply unit'). See also PSU.


An abbreviation for 'Society of Motion Picture and Television Engineers'. The term is usually used to refer to the time code standard devised by that Society, more accurately 'SMPTE 12M'. This time code consists of an eight digit number comprising two digits each for hours, minutes, seconds and video frames. When used in a digital subcode, this eight digit number is encoded as an 80-bit word. The code may also be modulated onto an audio-frequency carrier to enable it to be recorded (or transmitted) as an analogue signal. Pronounced "sim-tee". Compare Absolute time code.


A slang term for a multicore.

Snake head

A slang term for a stagebox, most often used when referring to a stagebox that is attached to the end of a multicore.


A range of single-pole locking mains power connectors intended for applications where a higher current rating is required than that provided by CEE-form connectors. They are rated at 250 amps and manufactured by ITT Cannon (Veam brand). In common with other types of single-pole power connectors, they must not be mated or de-mated whilst the circuit is live, and all poles of the circuit must be mated before the circuit is energised. The poles should be mated in the sequence Earth first, then Neutral, finally the Phases − and be de-mated in the reverse sequence. The labelling and colour-coding of the connectors must conform to the up-to-date standards of the country concerned. Note: not to be confused with several completely different products of the same name, such as some low-current wiring connectors and some pipework couplers. See also Power distribution and MDU. Compare Powerlock and Camlock.

Snaplock image


A specific type of drum, fitted with a wire mesh (also called a snare) on the underside. Usually requires its own microphone; sometimes separate microphones are used above the drum (for the skin sound) and below it (for the snare sound). See also Phase (1).


An abbreviation for 'sensorineural hearing loss', a specific type of hearing deficiency originating in the inner ear or in the nerves leading from it to the brain. This type of hearing loss usually affects the ability to discriminate between different frequencies of sound and is usually irreversible. When caused by exposure to very high sound pressure levels (or to prolonged exposure to moderately high sound pressure levels), it may be referred to as NIHL. See also Audiology, Presbycusis, Tinnitus, Auditory filter and Acoustic Safety on the Safety page.

SNR (1)

An abbreviation for 'signal-to-noise ratio'.

SNR (2)

An abbreviation for 'single number rating', a single figure attributed to hearing protection devices such as earplugs, in order to indicate their effectiveness. Higher numbers indicate greater attenuation of sound. See also NIHL, Audiology, Tinnitus, SNHL and Acoustic Safety on the Safety page.

Soak test

A general term for any testing procedure that is carried out on an item of equipment for a relatively long period of time − typically for at least an hour but often for much longer. Soak tests often attempt to simulate typical conditions of real usage of the relevant item, e.g. a typical operating temperature. For example, in the case of a power amplifier the test may be performed under typical loading conditions.

A soak test is generally performed either in an attempt to discover a problem that only occurs during long periods of use, or to confirm the absence of such a problem after a repair. It may also be performed when it is required to establish the reliability of an item of equipment, especially a critically important one, under typical conditions of long-term use.


A slang abbreviation for 'Socapex' − see the next definition. Compare Sock.


A make of multi-way power connector, popular for mains distribution to stage lighting, powered speakers, etc. The most common types are the 7, 19 and 37 pole versions of the SL61 or SL419 range. The 19 pole version, nominally rated at 25 amps per contact, is generally used for mains power and provides for 6 independent circuits (or 8 circuits when three poles are used for a common safety earth connection) − it is commonly fitted to pre-wired 6-way lighting bars.

Warning: In some systems, multi-way Socapex connectors are used for balanced audio interconnections, or for interconnections between power amplifiers and speakers (especially when multi-amped). To avoid possible dangerous and/or expensive errors, it is highly advisable to ensure that types used for different purposes in the same system cannot be mated with each other.

Socapex, a member of the Amphenol group of companies, manufacture many other types of connectors (e.g. XLRs) besides these particular multi-way types, but the company name has become synonymous with them. Ranges of compatible connectors are also available in other brands, such as Showsafe®. See also IEC and CEE-form.

View Socapex (37 pole) image


A slang term for a windshield. Compare Soca.


A fixed connector, i.e. one that is integral with an item of equipment or is attached to a fixed surface such as a wall, into which the plug of a cable is inserted. Typically (but not always) a socket has female contacts. (For example, 'output' audio XLR sockets have male contacts.) Mains power sockets are also known as 'outlets'. The approximately equivalent US term is 'receptacle'. See also Jack. Compare Plug.

Soft clipping

Clipping in which the 'flattening' of the waveform occurs gradually as the top and bottom of each cycle is approached, giving somewhat 'rounded' corners to the flattened part(s) of the waveform. For further information see Overload (1). Compare Hard clipping.

Soft knee

See Knee.

Soft start

A design feature of an item of equipment, provided in order to reduce the equipment's inrush current. Such a feature is useful in equipment such as high-power power amplifiers, as without it their inrush current would typically be high enough to cause frequent or occasional inappropriate operation of fuses or MCBs at switch-on of the equipment.


An abbreviation for 'standard operating level'. A term used mostly in broadcast audio engineering, especially in the USA, to refer to the level of a continuous test tone which gives a known reading on a specified type of level meter for the purpose of aligning (or 'lining-up') the levels of audio signals between interconnected equipment − especially when the equipment is in different locations. In the UK it is more commonly known as 'alignment level'. The actual level used varies between different countries and broadcast companies, and also depends on what type of standardised level meter is being used. The level chosen is used only as a reference for correctly setting up the equipment interconnections, so that a particular level indication on one item is also given on the other. It does not imply the specific standard level of the actual programme signals that are to be passed over those interconnections.

In early US analogue systems using VU meters, a level of +4 dBu (or +8 dBu in some cases) was commonly arranged to indicate 0 VU, while UK systems mostly used an alignment level of 0 dBu to produce the same indication. The UK test level would therefore indicate -4VU on a typical US-calibrated meter. In UK analogue systems using mechanical BBC PPM meters, a level of 0 dBu is arranged to give a reading of '4' on the scale of 1-7, sometimes written as 'PPM4'. For digital audio, most organisations use a SOL of −18 dB FS in Europe or −20 dB FS in the US. [The US figure of −20 arose from a standard of +24 dBu as analogue clipping level (equivalent to 0 dB FS), putting the US SOL of +4 dBu at 20 dB below that figure.] A sine wave tone at a frequency of 1 kHz or 997 Hz is nearly always used. (997 Hz is popular because it isn't an integer factor of any of the common digital audio sampling rates and because there is neither any cut nor boost provided by K-weighting at this frequency.) See also Reference level, Programme level and Gain structure.


Describes a cable having non-stranded core conductors. These cables are typically used only when regular flexing is not required, for example for fixed-installation purposes. For further information see Core. Compare Stranded-cored.

Solid state

Describes equipment whose active components are all semiconductors, i.e. equipment which contains no valves. So-called because in semiconductor components the electric current flows entirely through solid material, rather than through a vacuum as in the case of a valve.

Solo (or Solo PFL, or Solo-in-place)

A facility of a mixer, enabling an individual channel (or a combination of channels, if several of these buttons are in the 'on' state) to be listened to. Sometimes it is also provided on groups. The details of the arrangements for providing this facility, and the names given to the various flavours provided, vary substantially between different models of mixer. This makes precise definition of the terms impossible, but in general the following flavours, or a combination of them, may be encountered:


A quantified measure of loudness − sound level as perceived by an average human ear, based on the subjective judgement that perceived loudness doubles for each 10 decibels increase in sound pressure level.

A loudness of 40 phons is assigned a value of 1 sone, so 0.5 sones is equivalent to 30 phons, 2 sones is equivalent to 50 phons, 4 sones to 60 phons, 8 sones to 70 phons, etc.. Not often used in PA work.


Relating to sound. For example, the 'sonic properties' of a space refers to its acoustics. See also Ultrasonic and Infrasonic.


A flow of vibrational energy, carried from its source in the form of a longitudinal pressure wave. The term usually refers to such a wave in air, consisting of travelling regions of increased pressure (compression) and decreased pressure (rarefaction). It is generally used only in respect of waves that are detectable by the (human) ear − that is, those of audio frequency. The level of sound is usually specified by its sound pressure level (SPL), and its loudness is usually specified in phons. See also Speed of sound, Sound pressure level, Ultrasonic, Dispersion, Absorption, Reverberation, Reflection, Refraction, Diffraction, Grazing effect and Haas effect.

Sound board (1)

A rather outdated name for a mixer. (A term usually now used only by people not involved in sound engineering.)

Sound board (2)

In a stringed musical instrument, that part whose purpose is to vibrate in sympathy with the vibrations of the strings and so enhance the quality and volume of the sound produced. In traditionally designed instruments, it is usually made of wood. In most stringed instruments it forms part of the body of the instrument, and the air contained within the body also vibrates and amplifies the sound produced − the amplified sounds escape through one or more 'sound holes' provided in the sound board.

When placing microphones to pick up the sounds from such instruments, the position of the microphone in relation to different points on the sound board − especially in the vicinity of the sound holes − makes a great difference to the timbre of the sound picked up. See also Bridge (1) and F-hole.

Sound chain

See Audio chain.


A procedure by which a sound engineer checks the channel allocations for a particular set of performers (sometimes called a line check) and makes initial adjustments to suit that set of performers. The overall purpose is to enable the desired amplified sound to be achieved from each source and to create the required monitor and front-of-house mixes.

The sound-check adjustments will involve setting up the gain, EQ, monitor levels, effect levels and fader settings on each channel, making the channel-group assignments and setting up the effects units as required.

Preferably, adjustments are made on a one-channel-at-a-time basis in the first instance, starting with percussion and moving through strings to solo instruments and finally vocals. As this method requires all other sound sources to be silent, it is not possible to achieve the best results if the band insist on using the sound-check time as a rehearsal. Rehearsal time should be scheduled after the sound-check.

When a single set of performers are the only users of a permanent PA installation, the full procedure is necessary only before their first performance, because the system can subsequently remain set to meet their particular requirements. In all other situations, however, the procedure is vital prior to each performance. When multiple bands are to play in the same show, using a digital mixer allows the mixer settings to be stored after their respective sound-checks, and then rapidly restored at the appropriate times during the show. Compare Rig check.

Sound effects

Pre-recorded sounds that are played back at the appropriate times during a performance. Often abbreviated to 'SFX'. See also Grams, SXQ and Special effects. Compare Effects.

Sound engineer

A person responsible for mixing sound, whether for recording or for a live audience. In the case of live sound, the same engineer may also be responsible for the design and/or assembly of the PA system, or others may have these responsibilities. The principal sound engineer may be referred to as the 'A1', while those with supporting roles (or who are understudying the A1) are known as 'A2s'. In set-ups where a monitor engineer is used, he/she may be designated the 'A2' and any additional supporting person(s) may then be designated 'A3'. See also FOH engineer, Soundie, Noise boy, Techie, Mic wrangler and the next definition. Also see the introductory Mixing Engineers and System Assemblers pages.

Sound engineering

The practical business of working with sound to achieve specific desired results, whether for recording or for a live audience. In the case of live sound, good results can only be obtained with good-quality equipment and with skilful personnel involved in the design, assembly, and operation of a PA system. See also Sound engineer and the Training page.

Sound field

The resultant sound, due to all sources present, at a particular point in space, taking account of all effects caused by the space concerned (reflections, absorption etc.) in the given circumstances. The same sources, similarly placed in a different space or under different circumstances (e.g. a smaller audience), would produce a different sound field at a corresponding point in the space.

Sound hole

See Sound board (2).

Sound intensity level

A measure of the flow of sound power per unit area in a particular direction at a particular point in space within a sound field, regardless of how loud the sound might appear to an average listener (which is dependent upon many factors, especially upon what frequencies are present). It is normally expressed as an average power density value in watts per square metre or in decibels (dB) relative to an intensity of 10−12 W/m2 (1 pW/m2). It is very rarely used in PA work − sound pressure level (SPL) is used instead because it is much easier to measure, and because it correlates directly with the output level of microphones and with the mechanisms responsible for noise-induced hearing loss. In a free field, 1 pW/m2 corresponds to an SPL of 20 µPa (200 pbar). Sometimes abbreviated to 'SIL'.

Sound intensity level measurements are most commonly used, in combination with analysis equipment, to determine the total sound power emitted by a particular source, such as an industrial machine. Its measurement requires a specialist dual-microphone probe and a compatible meter; a conventional sound level meter (SLM) cannot be used.

In contrast with sound pressure, sound intensity reduces with distance from a point source, in a free field, according to the inverse square law. For example, a value of 1 µW/m2 at 1 m would reduce to 0.25 µW/m2 at 2 m. Nevertheless, in a free field its measurement in dB correlates over such varying distances with measurements of SPL in dB, because all measurements in dB relate to the (effective) power level of the quantity concerned − i.e. SPL measurements in dB relate, in a sense, to the 'effective' sound intensity of the measured sound pressure. However, note that this does not mean that sound intensity values in dB can in general be inferred from the dB readings of an SPL meter.

Sound level

An abbreviation for 'sound pressure level'.

Sound level meter

An instrument which measures the amplitude of sound, that is, the sound pressure level. The value is usually displayed in decibels SPL. Most modern types have a digital (i.e. numeric) display, but models with an analogue (moving needle) display are still available. Some types allow a number of different plug-in measurement microphones to be used. The instrument is usually hand-held and battery-powered.

These devices are available with a wide range of sophistication and precision (with a corresponding wide variation in cost). Professional types are now designated as 'Class 1' and 'Class 2' (Class 1 being more accurate) and must comply with standard IEC 61672. Basic types will usually provide a means of selecting the full-scale reading, a choice of A or C weighting and fast or slow response, and may have a maximum-hold facility. Advanced versions are likely to offer various time-averaged measurements (often automatically timed), unweighted (sometimes called 'linear') readings, maximum and minimum levels, sound exposure measurements, RMS or peak readings, etc. May be abbreviated to 'SLM'. See also L<number> (1) and Leq.

Sound limiter

A term most commonly used in a non-technical sense to refer to any item of equipment whose function is to provide protection against excessive sound levels in a venue. This may be either for the purpose of avoiding harmful sound exposure to the venue employees or to avoid unacceptable environmental noise pollution to residents or employees outside the venue boundaries (or for both of these purposes). Sound limiters may either be of a type that restricts the input level to permanently installed power amplifiers feeding permanently installed speakers or, more commonly, may be of a type that monitors sound levels using a microphone and cuts off the power to the PA system (or to its power amplifiers) if a pre-set sound level is exceeded for a pre-set length of time. In the latter case, a visual indication (usually green, yellow and red lights) is normally provided to system operators, to assist them in keeping sound levels below the cut-off threshold. The installation of sound limiting equipment is mandatory for some licensed premises in certain districts of the UK. For audio signal level limiters (of the more general kind), see Limiter.

Sound plot

Information produced during the planning of an event to specify all the details relating to sound. This may typically include:

However, some users of the term (especially in the theatre) use it to refer primarily, or solely, to the last item in the list above.

Information relating solely to the layout of performers and equipment on stage may be provided in a separate stage plot.

Often abbreviated by sound engineers to just 'plot'. See also Technical rider, Digital mixer, Mixer automation, Show control and Lighting plot.

Sound pressure level

A measure of the actual magnitude of a sound field at a particular point in space, regardless of direction or frequency content. This is technically not a measure of how loud the sound at that point might appear to a particular listener, which is dependent upon many factors, especially upon what frequencies are present. In regard to an average listener, an approximate indication of loudness can be obtained by use of a sound level meter (SLM) that applies a suitable frequency weighting.

Sound pressure level is usually abbreviated to 'SPL'. It is normally expressed as an RMS value in Pascals or in dB SPL.

In contrast with sound intensity, the reduction in sound pressure with distance from a point source (in a free field) is not in accordance with the inverse square law. For example, a value of 1 Pa at 1 m would reduce to 0.5 Pa at 2 m, not to 0.25 Pa. Nevertheless, in a free field its measurement in dB changes, over varying distances, in the same way as measurements of sound intensity in dB, because all measurements in dB relate to the (effective) power level of the quantity concerned. That is, SPL measurements in dB relate, in a sense, to the 'effective' sound intensity of the measured sound pressure, which requires the change factor in Pa to be squared. Therefore, when looking at SPL figures in dB it appears as though the inverse square law were being followed. (Note that this does not mean that sound intensity values in dB can in general be inferred from the dB readings of an SPL meter.)

High values of SPL, or moderate values for long or frequent periods, can cause permanent damage to hearing. Note that when using headphones or earpieces (e.g. in-ear monitoring systems), high values of SPL can be produced at very low power levels. For further information see About Sound Levels on the Decibels page. See also Sensitivity, Measurement microphone and Weighting. Compare Loudness (1).

Sound propagation

See Propagation of sound.

Sound reinforcement

The 'proper' name for a PA system that is primarily used to amplify live sound sources (such as bands, orchestras, theatrical performances, etc.). The term 'sound reinforcement' is sometimes used to emphasise the suitability of a system for music applications (in contrast to low-bandwidth speech-only public address systems), but this usage is technically incorrect because the term could apply to reinforcement of any kind of sound. The term 'PA' is generally preferred by users simply because it is short and much easier to say, so we use 'PA' throughout this website. (The abbreviation 'SR' for 'sound reinforcement' is very rarely used.)

Sound stage (1)

Another name for the stereo image.

Sound stage (2)

A performance area (for film recording or for live performances) that is equipped with suitable microphones positioned such that performance sound may be adequately picked-up without the need for visible microphones in front of, held by or worn by (the majority of) the performers. Typically such microphones will be fixed rifle types located above the stage, suspended types, boom-mounted types and/or low-profile floor types, though hidden sub-miniature body-worn types are sometimes also used. Hand-held or stand-mounted microphones are sometimes additionally used on a sound stage, either to provide some close-miked pickup of specific performers or purely as non-functional performance props. See also Boundary microphone.


A slang term for a sound engineer. See also Noise boy and Techie. Compare Lampie.


In mains power distribution, describes a connector that is wired (ultimately) to a supply of power. It may be a fixed connector or may be attached to a cable. The mating connector is described as a drain type. See also Distro.

Source impedance

The output impedance of a signal output.

SP (1)

An abbreviation for 'single pole'.

SP (2)

An abbreviation for 'signal present', e.g. as used on some mixers to identify LEDs provided to indicate the presence of signal at a usable level.

Spade terminal (1)

A push-on connector sometimes used for the connection of individual wires to large components inside equipment, for example to drivers and crossovers inside speaker enclosures. They are available in various sizes, and the female part may be insulated or uninsulated. They are readily mated (hence are also sometimes referred to as 'quick-connect' connectors), but most types are not readily de-mated as they are intended for semi-permanent connections within equipment (while allowing disconnection for maintenance purposes) and so frequently incorporate a locking feature. Alternatively, the trademarked name 'Faston' is used. The name 'spade' derives from the flat and thin shape of the male prong, which is usually referred to as a tab. The most frequently used size has a tab width of 6.35 (or 6.3) mm, but other widths such as 2.79 (or 2.8), 4.75 (or 4.8) and 11 mm are also fairly common. See also Terminal.

Spade terminal (1) image (example of usage)

Spade terminal (2)

A flat, forked termination for single cable conductor ends. They are typically used to terminate speaker cables, in order to facilitate their connection to binding posts. Some types are angled to provide additional clearance from the equipment surface. Their use is particularly common for low and medium-power speaker cables for applications such as domestic Hi-Fi systems. See also 4 mm connector and BFA. See also Terminal.

Spade terminal (2) image

Sparky, Sparkie or Sparks

A slang term for an electrician. See also Distro and LX.


Describes something that relates to physical space. This may be in reference to just one dimension (e.g. width only, as in stereo), to two dimensions (e.g. width and depth only, as in most surround sound systems), or to all three dimensions (width, depth and height). Compare Temporal.


An abbreviation for 'surge protection device', a device or item of equipment that is intended to protect an electrical supply installation, or part of it, or specific equipment supplied by it, from damage due to the effects of voltage surges. Such surges usually originate from the electrical distribution network as a result of faults, switching operations or electrical storms (lightning), and manifest as very large increases in the supply voltage occurring very suddenly and briefly. Depending on the specific purpose of an SPD, it may consist of a discharge device or a semiconductor component such as an MOV. Further information is provided by BS 7671 (2011).


An abbreviation for 'Sony / Philips Digital Interface', a standard for the interconnection of consumer digital audio equipment, based on the professional AES3 standard. The interface is defined by specification IEC60958-3, and uses screened unbalanced cable with a characteristic impedance of 75 ohms and terminated in phono connectors, having a maximum length of about 10 metres. The nominal signal level is 0.5 volts peak-to-peak (terminated). The bit-rate is 2.8224 Mbit/s for a 2-channel signal at the compact disc sampling frequency of 44.1 kHz (3.072 Mbit/s at 48 kHz). The line code used is bi-phase mark.

Note that S/PDIF is a different interface to SDIF. See also Coaxial, TOSLink, Analogue to digital conversion and SCMS.


An item of equipment for converting an electrical signal into audible sound. On this website, we use this term to mean an enclosure containing one or more drivers and often incorporating a crossover, connectors, etc. (i.e. we are not referring to just the drivers themselves). However, be aware that the term may elsewhere be used to mean just a driver. When communicating with non-technical personnel, e.g. about the stage management of events such as conferences where one or more people speak to the audience from the stage, take care to avoid confusion between the PA speakers and the people who are to speak; in such cases it may be helpful to use the original full-length term 'loudspeaker'. See also Front-of-house, Monitor, Passive speaker, Powered speaker, Full range, Bass bin, Sub-bass, Mid-range, Tops, Horn, Stack, Cluster, Array, Line array, Trapezoidal, Woofer, Concentric, Short throw, Long throw, Fly, Top hat, Rating, RMS, PMPO, Sensitivity, Frequency response, Directivity, Q (4), Power amplifier, Bi-amping, Bridging, Baffle, Port, Direct radiator, Inverse square law, Speakon, Gauge, Speaker protection, Processor-controlled, Speaker management equipment, A & E specifications, Leslie, Transducer, 100 volt line and the definitions following this one. See the Amps and Speakers page for further information.

Speaker cab, Speaker cabinet

A term most commonly used (especially in the abbreviated form) to refer to a complete speaker, i.e. including its driver(s) etc., rather than to just the enclosure itself. Musicians sometimes use this term to refer to a backline speaker that is used in conjunction with a head, in contrast with a combo, in which case the usual term is simply 'cab'. See also Loaded (2) and Unloaded (2).

Speaker cable

An unscreened cable that is intended for use in the interconnection between a power amplifier and a passive speaker. Most importantly, such cables are rated for the higher currents and voltages employed by these interconnections, as compared to other types of interconnections − other types of cables must not be used. Speaker cables are most frequently terminated in Speakon connectors, but binding posts, 2-pole 6.35 mm jacks and XLRs are sometimes used. The Rean/Neutrik NYS225 'jumbo' series are popular as speaker jack connectors, as they provide ample room for terminating larger cables such as speaker cables.

The two conductors are most usually laid in a twisted fashion within the cable, in order to reduce its series inductance and the level of radiated magnetic fields. 4-core, 6-core and 8-core versions are available, to accommodate multi-amping and other multiple-circuit arrangements. High-power applications require a heavy gauge of cable − for information on the size (thickness) of cable required for various applications, see Gauge. Increased flexibility of heavy gauge cables is often provided by the use of a very large number of very fine strands to make up each conductor. Audiophiles place great store by the quality of speaker cables. To obtain proper bass response when more than a single speaker is used, it is essential that all speaker cables are wired with correct polarity; this is facilitated by their cores being colour coded or numbered. See also Oxygen-free copper and 100 volt line. Compare Microphone cable.

Speaker cone

See Cone.

Speaker crossover

See Crossover.

Speaker directivity

See Directivity.

Speaker driver

See Driver.

Speaker enclosure

The outer casing of a speaker − see Enclosure.

Speaker impedance

See the Impedance section on the Amplifiers & Speakers page. For a general definition of impedance, see Impedance.


Describes an analogue audio signal at a level suitable for direct connection to a passive speaker. Such a signal will typically be at a level of 20 to 100 volts RMS, and may therefore pose an electric shock risk. The term also implies that the signal is supplied from a sufficiently low source impedance to be suitable for driving a passive speaker.

Signals at this level are used only between power amplifiers and passive speakers, and the cables and connectors used to carry them must be suitable for the voltages and currents involved (for cables, see Gauge). To avoid possible equipment damage and/or danger, power amplifier speaker outputs should be connected only to passive speakers. See also Speakon and 100 volt line. Compare Line-level, Instrument-level and Microphone-level.

Speaker management equipment

Signal processing equipment whose function is to modify the drive signal fed to one or more power amplifiers, so as to obtain the best performance from the associated speakers − and often also to provide some degree of speaker protection. Such equipment may be referred to as speaker processors, and the associated speakers may be described as processor-controlled. The facilities of speaker management equipment will typically include crossovers, delay, compression, limiting and sometimes feedback suppression.

Some types monitor the output of the amplifiers they feed, so as to assess the drive level(s) supplied to the speaker(s). Nearly all types now operate digitally. Many speaker manufacturers strongly advise the use of specific models of speaker processor with their speakers, and may provide details of optimum processor settings for use with each model of speaker. See also Outboard.

Speaker power

See the Power Ratings section on the Amplifiers & Speakers page. For a general definition of power, see Power.

Speaker processor

See Speaker management equipment.

Speaker protection

A facility that automatically safeguards a speaker, to some degree, against the effects of certain undesirable circumstances. Some types of protection (such as horn overload protection by fuses, self-resetting solid-state devices or protection lamps) may be incorporated within the speaker itself − often as part of a passive crossover unit. Other types of protection may be provided by the amplifier, such as automatic disconnection of the speakers during power amplifier switch-on and switch-off or in the event of a power amplifier fault causing a large DC voltage to appear at its output. When speakers are processor controlled, some types of speaker protection may be provided by the speaker processor. See also Compression (1) and Limiting.

Speaker Q

This can refer either to a speaker's (or a driver's) directivity index or to its resonance − for the latter see Q (3). Note that these are two entirely unrelated parameters.

Speaker stack

See Stack.


A type of locking heavy-duty connector designed specifically for the connection of speaker cables between amplifiers and speakers. The name is a trade-mark of Neutrik.

The most common types have either two or four poles, but 8-pole types are also available. The Neutrik part numbers NL2, NL4 and NL8 are frequently used to refer to these types, whether or not the particular connector is manufactured by Neutrik. Most designs have a different shape of central spigot for the different number of poles, to prevent mating of incompatible types. For example, a 2-pole plug can be mated with a 2-pole or 4-pole socket, but a 4-pole plug cannot be mated with a 2-pole socket. This is to prevent a 4-core speaker cable (assumed to be feeding a bi-amped speaker system) from being plugged into a full-range amplifier output. Therefore, under normal circumstances, 4-pole plugs should be fitted only to 4-core cables.

The standard for identifying the connections is as follows: 1+ is the 'positive' (i.e. hot) connection for circuit No. 1 and 1− is the 'negative' (i.e. cold) connection for that circuit; likewise, 2+ and 2− are the corresponding pair for circuit No. 2, etc. Failure to observe correct polarity will result in impaired bass response in multi-speaker systems. Important: To avoid possible damage to speakers, in a multi-amped system be sure to check for correct correspondence between the Speakon circuit allocations for each frequency band on all speakers, amplifiers and cables used, before making the connections between the equipment.

A common colour code for the allocation of multiway speaker cable cores to Speakon terminals is given below, but as this is not a published standard always be sure to check that the same allocations are used at both ends of the cable.

Red 1+
Blue 1−
Green 2+
Yellow 2−
White 3+
Black 3−
Brown 4+
Violet 4−

Originally all equipment was fitted with female connectors and all cables were fitted with male connectors at both ends, requiring the use of a coupler if cables were to be joined. However male chassis-mounted connectors and male-to-female cables are now available (e.g. Neutrik STX series).

The current rating of the Neutrik NL series is 30 amps RMS, corresponding to a continuous average power of 7.2 kW into a single 8 ohm load, or 3.6 kW into 4 ohms. The STX series is rated at 40 amps RMS, allowing even higher powers. These figures of course assume that the cables used with the connectors are of adequate gauge to carry the relevant current. Speakon connectors must not be used for mains connections or other power connections.

View Speakon (2-pole) image

Special effects

A generic name for visual effects such as smoke, fog, mist, dry ice, strobes, pyrotechnics, etc. See also Effects.


A particular range, usually of frequency, between indicated (or implied) upper and lower limits. For example, the 'audio spectrum' is the range of frequencies that can be heard − i.e. those described as audio-frequency. A smaller range within a spectrum is usually referred to as a 'band'. The plural of spectrum is spectra.

Spectrum analyser

An item of equipment or software that is designed to indicate the level of a signal within a number of specific frequency ranges, or bands, rather than its overall level. The equipment may be equipped with a microphone, to allow the direct analysis of sounds.

The equipment usually operates (in principle) by passing the signal through a number of bandpass filters, and indicating the level at the output of each filter. An analyser having a large number of narrow bandwidth filters is able to distinguish between closely-spaced frequencies better than an analyser having a smaller number of large bandwidth filters. The centre frequencies of the filters are often arranged logarithmically, to coincide with the centre frequencies of a graphic equaliser. Likewise, the bandwidth of the filters is arranged in proportion (maintaining a constant Q), giving continuous coverage of the entire spectrum required. As there is inevitably some overlap between the ranges of frequencies covered by adjacent filters, it would not be unusual for a high-level single-frequency (i.e. sine wave) signal to produce an indication at the output of two (or even three) filters. (This is exactly equivalent to the fact that two (or even three) controls of a graphic equaliser may well have a noticeable effect on the level of such a signal, if each in turn is moved substantially from its flat setting.)

In practice, some analysers operate by sweeping a single filter through the spectrum, and arranging for the display of the output signal level to co-ordinate with the filter's centre frequency at each point in time. However if the filter is narrow then for accurate results the sweep-rate must be relatively slow, and this precludes the continuous rapid-response analysis of a rapidly changing programme signal. Because of this deficiency, the term 'real time analyser' (RTA) was devised by manufacturers to describe products that are claimed to give a display across the whole spectrum that correctly responds to programme changes in 'real time', i.e. as soon as they occur.

Although, as described, such analysers can be used to analyse real programme material, a more practical use during system set-up involves feeding pink noise into the system (via. a channel with flat EQ!) and then adjusting the graphic equaliser to give a flat response (within the frequency limits of the system) on the analyser. See also MLSSA, STI, ALCONS, Fourier analysis and Auditory filter.

Speed of sound

In air at room temperature (20ºC), this is approximately 343 metres per second (767 miles per hour). Put another way, this means that sound takes approximately 30 milliseconds (0.03 seconds) for every 10 metres that it has to travel. It also gives the relationship between frequency and wavelength: dividing 343 by the frequency (in Hz) gives the wavelength (in metres), and dividing 343 by the wavelength (in metres) gives the frequency (in Hz).

For temperatures other than 20ºC, the speed of sound in air can be calculated using the formula 331 + 0.6T, where T is the air temperature in degrees Celsius. The scientific name for speed is velocity (though, strictly speaking, velocity also takes into account the direction of travel). See also Propagation time, Time alignment, Delay and Haas effect.

Spider (1)

See Suspension mount.

Spider (2)

A multicore cable (usually short in length, for local links between equipment) that is equipped with tails at one or both ends.


A very short duration impulse in a signal voltage or mains power voltage waveform. Usually refers to any such undesirable phenomena resulting from impulsive electrical interference, such as is produced by equipment being switched on or off, vehicle ignition systems, lightning strikes, etc. It takes its name from its shape, as seen when the affected signal waveform is viewed on an oscilloscope. Somewhat longer duration spurious increases in mains voltage are referred to as voltage surges. Take care to avoid confusion with 'earth spike'. Compare Transients.


Another name for leakage, but sometimes more specifically refers to sound from speakers (monitors or front-of-house) that can be heard in locations where ideally it would not be heard.

Spiral quad cable

An alternative name for star quad cable.


An abbreviation for 'sound pressure level'. For further information see About Sound Levels on the Decibels page.

Split keyboard

An electronic keyboard instrument that is configured such that the right-hand ('upper') and left-hand ('lower') ranges of the keyboard can produce an entirely different type of sound, at the same time. Note that, in such a configuration, notes played in the upper range of the keyboard may sometimes have a lower pitch than those played in the lower range. (This arrangement can be a potential source of confusion to sound engineers when more than one keyboard is in use.)

Split output

An output of a mixer, found on each channel, which allows the balanced signal that is supplied to the channel to be also fed to another destination − usually another mixer such as a monitor mixer. In effect, a mixer with such outputs incorporates an onboard two-output microphone splitter, with one of the two outputs feeding the mixer's own channels. The split output is normally a male XLR. An earth lift switch may be provided to disconnect pin 1 of the output from the mixer's internal earth connection.

Usually this facility is implemented by means of a hard-wired interconnection between the balanced input connector and the split output connector. Therefore, it is important to remember that if phantom power is activated on a channel, the power is likely to appear on the channel's split output as well as on its balanced input. Compare Direct output.


See Microphone splitter. See also Y-connector.


An abbreviation for 'silver-plated oxygen-free copper', a type of cable conductor claimed by some to give superior performance to standard oxygen-free copper (OFC) cable. See also Audiophile.

Spring line

A type of reverberation effect, so called because it may be produced by passing sound vibrations through a metal spring (though the effect is now often produced digitally). A spring line unit, especially one internal to a backline amplifier, is sometimes referred to as a 'tank'. See also Plate.


An abbreviation for 'signal to quantisation noise ratio'. See Quantisation noise.

Square wave

A repetitive waveform having only two instantaneous values of voltage within each cycle, with the transitions between those values occurring very rapidly (relative to the period of the waveform). A true square wave has equal times at the positive (or 'high') voltage and the negative (or 'low') voltage (in which case it contains only odd-numbered harmonics), but in practice the term is also used when this is only approximately the case. See also Sine wave.

View square-wave image


A term sometimes used to describe some types of undesirable feedback.


In a radio link (such as is used within radio microphone and in-ear monitoring systems), a facility which automatically mutes the audio output of a receiver if the level of radio-frequency (RF) carrier signal being received is inadequate for satisfactory operation.

Typically this situation would occur when the transmitter is switched off (or its battery expires), or when the transmitted signal is being unduly attenuated by excessive distance from the receiver, by intervening obstacles, or due to the transmitter being incorrectly held or worn (or a combination of these factors). Automatic audio muting is essential under these circumstances, in order to avoid high levels of noise and/or unwanted signals appearing at the receiver output. It is important that the squelch threshold of the receiver is correctly adjusted, so that muting always occurs when necessary but not when unnecessary.

For the purpose of assessing the adequacy of the received carrier level, this level may be monitored directly or the noise level at the output of the demodulator may alternatively be monitored. The latter method is referred to as 'noise squelch'.

In more advanced systems, an alternative (or additional) method used to establish the presence of a compatible switched-on transmitter is the reception of an appropriate pilot tone.

Or, a control that sets the threshold of RF signal level, or of audio noise level, at which such muting occurs. See also Diversity.

SR (1)

A little-used abbreviation for 'sound reinforcement'.

SR (2)

An abbreviation for 'stage right'. Compare SL.


An abbreviation for 'synthetic resin-bonded paper', an insulating material commonly used in connectors.


See RCD.


An abbreviation for 'short-term restricted service licence', a UK radio transmission licence that is specifically limited in purpose, transmission range and duration of validity. Each licence is issued by Ofcom for use under specific conditions relating to the specific licence application concerned. An example is the broadcast of programme material relating to a specific sporting or music event, for reception in the FM broadcast radio band (88-108 MHz) over a very limited area around the location of the event.


An alpha-numeric name associated with a particular Wi-Fi router or access point. It is used by devices (clients) to select the particular router or point that is to be used for the provision of a needed Wi-Fi service, and may be up to 32 characters long. SSID stands for 'service set identifier'.


Short for 'speaker stack', a slang term for a vertical arrangement of floor-standing speakers, or speakers and amplifiers, set up as for use. The term is usually used to refer either to such an arrangement of front-of-house (FOH) speakers located at each side of the stage, or to such an arrangement of backline equipment, generally for either electric guitar or for bass.

In the case of FOH speakers, historically the usual arrangement was for a stack to consist of bass bins at the bottom, mid-range speakers on top of the bass bins, and high-frequency horns at the very top. However, since the advent of capable full range speakers, an FOH stack will now typically consist of several full-range units − often with the addition of sub-bass speakers (subs) either below them or located elsewhere (e.g. at floor level centrally). In large systems, flown line arrays are now frequently used in place of stacks (again, with the addition of suitably placed subs).

In the case of backline equipment, the term generally refers to an arrangement of one or more speaker cabinets (cabs) with a head placed on top, or sometimes to a combo with an extension cab placed either below or above it. The arrangement of a head placed on top of a single cab (as opposed to a vertically stacked pair) is often referred to as a "half stack". Compare Cluster.

Stage (1)

A platform or raised area on which performances take place. See also Stagebox, Stage left, Stage right, Up-stage, Down-stage, Rake, Apron, House tabs, Dais, Front-of-house and Auditorium.

Stage (2)

A section of circuitry, internal to an item of equipment, through which a signal must pass before reaching the next internal section (or reaching an output connector). See also Input stage, Output stage, Block diagram and Amplifier.

Stage left

The left-hand side of the stage as viewed by performers facing the audience. That is, the right-hand side as viewed by the audience, where the Right-channel speaker stacks are located. Commonly abbreviated to SL. In theatre, alternatively referred to as 'prompt side' (PS). Compare Stage right.

Stage microphone

A microphone that is intended for, or is suitable for, use on-stage (usually in contrast to one that is more suited to use in other situations). For example, one that is suitably rugged in construction or that is suitable for close pickup use (so as to reduce the leakage and feedback problems that are often encountered in live sound reinforcement applications). Compare Studio microphone.

Stage monitor

A monitor speaker that is located on a performance stage, or that is designed for that application. Stage monitors are usually floor monitors, commonly termed wedges.

Stage plot

Information indicating the layout of performers, equipment, etc. on stage, usually in the form of a diagram. Typically, a stage plot may be supplied in advance by a performance act as part of their technical rider. For PA purposes, more complete information is provided by a sound plot. Regarding stage lighting, see Lighting plot.

Stage right

The right-hand side of the stage as viewed by performers facing the audience. That is, the left-hand side as viewed by the audience, where the Left-channel speaker stacks are located. Commonly abbreviated to SR. In theatre, alternatively referred to as 'opposite prompt' (OP) or 'opposite prompt side' (OPS). Compare Stage left.

Stage tech

A technically competent person who assists with technical matters on stage, such as positioning and connecting microphones, instruments and monitors, reacting to technical problems occurring on stage during a performance, etc.. See also Techie, Crew and Monitor engineer.


A box at the stage end of a multicore, provided with connectors (usually female XLRs) suitable for the direct connection of balanced low-impedance microphones and other balanced low-impedance signal sources such as DI boxes and backline amplifiers with balanced DI outputs. The number of such connectors is usually a multiple of 4. In small systems, the same stagebox is often equipped with a smaller number of male XLRs for use as returns to the power amplifiers and/or to powered speakers. Stageboxes are most commonly hard-wired to the multicore, but sometimes a multiway connector such as an EDAC is used. The slang term 'snake head' is sometimes used to refer to a stagebox.

Or, the term 'stagebox' may be used to refer to a digital stagebox.

In the case of a powered multicore, the power amplifiers are located at the non-stage end of the multicore, and passive speakers are connected directly to the stagebox.

Standard operating level

See SOL.

Standby (1), Standby switch

'On standby' is a term describing equipment that is potentially available for use but is currently not being used, or that is in a preliminary state of preparedness for use but not (yet) in a fully operational state. Note that this is a broad term describing many different scenarios, see for example Cold standby and Hot standby. Some equipment has the facility to automatically enter a standby state under certain circumstances − see APD.

'Standby' is the usual label for a switch, on an item of equipment, whose purpose is to switch between a preliminary state of preparedness and a fully operational state ready for use. Such switches are generally provided in addition to the main power switch, which is usually labeled 'Mains' or 'Power'.

Terminology caution: Usually, when the terms 'on' or 'off' are used in respect of a standby mode or switch, 'on' refers to the equipment being switched fully on (i.e. not in standby mode), and 'off' refers to the equipment being switched to standby mode. However, the opposite meanings for standby 'on' and 'off' may also be encountered.

A Standby switch is most commonly encountered on valve combos and heads, where a common arrangement is for it to control the internal HT supply to the valves (sometimes to just the output stage valves). In such cases, this switch allows the valve heaters to be switched on (by the Mains switch), to bring the valves to a stable operating temperature, before their HT supply is applied. Warning: On some types of equipment, however, the Standby switch does not control the HT supply, but operates by other means. A dangerous voltage may therefore be present at the valves regardless of the position of the Standby switch.

On valve equipment that has a Standby switch, before the Mains switch is moved to 'ON' it is important to check that that the Standby switch is in the position that puts the equipment in standby mode. Sufficient time (at least 30 seconds, but preferably at least 2 minutes) should then be allowed before moving the Standby switch to the position that switches the equipment out of standby mode, i.e. to the fully on state. See also APD.

Standby (2)

An instruction to performers or equipment operators to be prepared for an imminent action that they will be required to take. Or, an advance warning of an imminent cue. Such instructions are typically given via an earpiece or headphones. See also Comms. Compare Cue (1).

Standing wave

A stationary sound wave that exists between two parallel surfaces. It is produced by the reflective reinforcement that is given to an acoustic stimulus when the distance between the two surfaces is an exact whole number of wavelengths of the stimulating frequency.

Star point earthing, Star point grounding

An earthing arrangement in which signal earth interconnections between items of equipment are strenuously avoided, with the intention of preventing earth loops. Instead, an earth connection is provided separately from each item of equipment (and/or each rack of equipment) to a single common point, known as the 'star point', which is connected to the main installation earth. This arrangement is most often found in studios, and is most likely to be useful when many items need to be connected using unbalanced interconnections.

Or, a technique used in the internal wiring of equipment, in which internal signal earths, DC power returns, chassis connection(s) and the incoming safety earth are all wired to a single common point rather than being arbitrarily interconnected.

These arrangements and techniques operate by avoiding common impedance coupling.

Star point earthing is rarely suited to the arrangements for interconnecting items of PA equipment, because the large distances between the items make it impractical to run a separate earth connection from each item to a common point, because safety considerations preclude such long (and perhaps vulnerable) safety earth conductors, and because the high levels of radio-frequency interference often encountered in performance venues requires signal cable screens to have a good connection with the chassis of the equipment at both ends of the cable. As many of these items of equipment require their own independent safety earth connection (usually provided via their power cables), the existence of earth loops is often inevitable. However, the problems that could potentially be caused by these loops are avoided by the use of balanced interconnections (but see Pin 1 problem). 'Grounding' is an alternative term for 'earthing', and is the more common term in the USA. See also Technical earth and Class I.

Star quad cable, Starquad cable

A type of cable with especially good immunity to the pick-up of hum and other interference, when used in a balanced interconnection. This is achieved by the use of four signal-carrying conductor wires (rather than the usual two), laid in a 'star' configuration inside the screen of the cable (hence the name). The conductors on opposite sides of the star are connected together at each end of the cable, to give the usual two signal connections of a balanced interconnection. So, each leg of the interconnection is made up of two opposite wires of the cable. This arrangement (often combined with a short lay length) reduces the effective internal loop area between the legs, so reducing pick-up of induced interference, and also means that interference pick-up on each leg is more equal, so improving cancellation by the balanced input of the destination equipment.

This type of cable is used mostly as a microphone cable in situations requiring very high quality, such as recording and broadcast studios, or when local interference levels are very high. The term 'star quad' is sometimes written as a single word, or hypenated. Alternative terms for this type of cable are 'spiral quad', 'quad', and 'double balanced'. See also Pair.

Start-stop communications

See Asynchronous.

Static (1)

A slang term for an electrostatic discharge.

Static (2)

The radio-frequency interference caused by electrostatic discharges (especially large ones, such as occur during electrical storms) which, after demodulation by electronic equipment, produces a characteristic 'crackling' effect.


An abbreviation for 'stereo'.


See Slope.


Short for 'stereophonic'. Describes a sound which contains spatial information in one dimension, i.e. the sound does not appear to come from a single point in space, but has width (though no depth or height) − this is known as a 'stereo image'.

Or, describes equipment which handles, or provides, signals capable of representing such sounds. Usually this operates by handling or providing two independent signals (usually different) − one of which is intended to be ultimately supplied to the Left front-of-house speaker and one which is intended to be ultimately supplied to the Right one. These are often referred to as the Left and Right channels. Note that the 'Left' channel is the one destined for the speakers at the left as viewed by the audience (i.e. those located at stage right).

A PA system configured in stereo generates two different mixes (usually through the use of pan controls), whose signals are separately amplified and separately supplied to the Left and Right front-of-house speakers. The sound sources in the mix then appear to take up locations in the stereo image according to their relative level within the signals of the Left and Right channels. So, a source which is panned slightly left of centre has a slightly higher level in the Left channel signal than in the Right, and its resultant slightly higher sound level from the Left speaker than from the Right causes that source to appear to be located at a point slightly left of a point centrally between the two speakers − provided that the listener is located equidistant from the two speakers.

The two signals may each have their own connectors (with separate cables or a single cable with separate lines), may be connected through a single stereo connector such as a 3-pole (i.e. TRS) jack, or, in the case of digital signals, may be multiplexed onto a single line. In the case of separate connections, the Left channel is often identified by the colour white (occasionally black) and the Right by the colour red, while in the case of a 3-pole jack the Left channel is usually carried on the tip and the Right channel on the ring.

See also Side, Balance control and LCR (1). Compare Mono and Surround sound.

Stereo bus, Stereo buss

A pair of buses carrying, or intended to usually carry, a stereo mix. The two buses of a main mix pair are usually designated Left and Right (see Stereo). Where buses are numbered (as in the case of audio groups) the usual arrangement is that each odd numbered bus is Left and the next highest (even) numbered bus is the corresponding Right of the pair.

Stereo bus compressor, Stereo buss compressor

A compressor unit that is designed for use with a stereo signal, typically by connecting it at the main mix Left and Right Insert points of a mixer. Such a unit provides two channels of compression, whose side chains are linked to ensure that equal amounts of compression are always applied to both the Left and Right signals of the mix. Such units generally have a single set of controls, applying to both channels. See also the previous definition.

Stereo image

When listening to a stereo sound from a good listening position (see Sweet spot), you can imagine (especially if you close your eyes) the various sound sources spread out from left to right in front of you, according to the position that each sound seems to be coming from. Sources that are panned towards the left in the mix are louder from the left speaker than from the right, and therefore appear to be positioned left of centre, and vice versa.

This mental 'picture' of the sound sources is called the 'stereo image', or sometimes the 'sound stage'. The extent of the panning within a mix is referred to as the 'width' of the stereo image; i.e. a mix that contains sources that are panned substantially to the left or right is referred to as a having 'wide' image.

It is important to note that the position of the various sources in the stereo image need have no relationship to the physical positioning of the corresponding performers on stage, but in live situations the audience's listening experience will often be improved if there is such a relationship, because the sound that they experience then correlates with their visual experience. (However, the physical layout of many live venues renders them unsuitable for stereo sound.) See also Pan rule and Psychoacoustics.

Stereo microphone technique

See Microphone technique.


An abbreviation for 'speech transmission index', an objective rating of speech intelligibility specified by standard IEC 60268-16. An STI measurement is made by playing a suitable test signal through a speaker complying with specific requirements, and analysing the sound picked up by a measurement microphone at the listening location(s). The STI has a value between 0 (zero intelligibility) and 1 (perfect intelligibility).

As measurement of STI is complex and time-consuming, two simplified versions have been developed:

STIPA is generally reckoned to give more accurate and meaningful results than RASTI in public address applications. Compare ALCONS and MLSSA.


See the previous definition.

Stomp box (1), Stompbox

A slang term for an effect pedal.

Stomp box (2)

A simple percussion instrument, typically consisting of a shallow hollow wooden box intended to be tapped with the foot. May be fitted with an internal microphone or pick-up.


The range of frequencies that is effectively prevented from passing through a filter, due to the large attenuation of the filter at those frequencies. Compare Passband.


An abbreviation for 'shielded twisted pair', a type of cable containing one or more twisted pairs and one or more integral screens (also called shields). Although that definition would include typical balanced audio cable, the term STP is most usually used in reference to data cable, such as is used to interconnect computers (and some other computer-related equipment) in computer networks. However, STP is only a very general term for such cables; it should not be used to specify a particular type of cable because it fails to indicate the way in which the cable is screened. The terms that should be used are: F/UTP, S/UTP, SF/UTP, U/FTP, F/FTP, S/FTP and SF/FTP. See also Ethernet cable, Category cable, QTP and FST. Compare UTP.


Describes a cable having core conductors that each consist of several bare metallic strands, usually twisted together. These cables are typically used when regular flexing is required. For more detailed information see the following definition. See also Core. Compare Solid-cored.


The particular arrangement of bare metallic strands making up each core of a cable conductor − in particular the number of strands used per core, which has a large impact on the flexibility of the cable (for a given gauge of its conductors). It should be noted, however, that the overall flexibility of a cable also depends on other factors, such as the type of insulation used and the ambient temperature. The manufacturer's guidance for its use must always be followed.

IEC/EN 60228, BS 6360 and VDE 0295 specify four classes of stranding, as follows:


The continuous transfer of information in digital format. Usually refers to a transfer at the (net) bit-rate required for listening (or viewing) of the programme as it is received. (This process involves the use of a suitable codec.) See also Bit-stream.


To dismantle and/or remove something. For example, to 'strike the drum mics' would mean to disconnect the drum microphones and remove them from the drum kit. 'Strike the piano' would mean to remove the piano from the stage.

Studio microphone

A microphone that is intended for, or is suitable for, use in a recording studio (usually in contrast to one that is more suited to use in other situations). For example, one that is of extremely high quality or that is unsuitable for use on-stage due to it being insufficiently rugged in construction. Compare Stage microphone.


A high-performance range of Speakon connector.


See Vinyl.


Short for sub-bass or sub-group. The term is often used to refer to a speaker designed specifically to handle sub-bass frequencies, otherwise known as a bass bin. See also Cardioid sub.

Sub out

An output connection on an item of equipment, either for an audio group mix or for a sub-bass signal.


The very lowest audible (or feelable!) bass frequencies − typically those between about 20 Hz and around 80-120 Hz, but may extend to as low as 15 Hz. Correct reproduction of these frequencies is essential for sound reinforcement of such instruments as electric bass (lowest note B0 is approximately 31 Hz), piano (lowest note A0 is 27.5 Hz) and kick drum. Good quality sub-bass reproduction from speakers is often very difficult (i.e. expensive) to achieve. See also LFE channel and One note bass.


Describes the polar response of a uni-directional microphone whose sensitivity to sound decreases from the front to the back more gradually than in the case of a cardioid type, such that it still has some significant pick-up even at an angle of 180º measured from the front axis (i.e. on the rear axis). The name arises from the fact that its pattern of sensitivity, when plotted as a polar response graph, is somewhere between that of the heart-shaped cardioid pattern and an omni-directional pattern. See the Microphones page for further information. See also Super-cardioid and Hyper-cardioid.

View uni-directional polar responses illustration


A carrier which, after modulation by the information to be carried, itself becomes the modulating signal for another carrier (the 'main' carrier). Usually the frequency of the 'main' carrier is much higher than that of the subcarrier. Often, the subcarrier is not the only signal modulated onto the 'main' carrier − there may also be Baseband signals or other subcarriers.

An example is the chrominance subcarrier in an analogue television channel. In the UK PAL system, this subcarrier is at 4.433 MHz. Another example is the 38 kHz subcarrier used in analogue FM stereo radio transmissions in the UK, to carry the L-R information. See also Composite video.


Additional information bits, that are not part of the audio or video programme, embedded in a digital bit-stream. Examples of such additional information are track titles, copyright information and time codes. Each frame of the bit-stream contains a specific number of bits that are reserved for subcode information. See also AES3, SMPTE and Absolute time code.


See Frame (1).


Another name for an audio group.

Subjective self-deception

A term devised by PAforMusic (the only such term in this Glossary!) to refer to the strange phenomenon in which someone makes a small adjustment to a control and is convinced that this has had the desired effect − only later to discover that the wrong control had been adjusted, or that the adjusted control was bypassed or was otherwise ineffective. Rarely, if ever, does anyone admit to having experienced this phenomenon.


A position held by many audiophiles, which (broadly) maintains that objective scientific analysis of equipment performance is of little value in comparison with a listener's opinion of its performance, and that such analysis is unable to account for subtle differences that are claimed to be subjectively discernible.


Another name for an audio group.

Subnet mask

See IP address.


Describes something whose speed is less than that of sound (in a given medium, usually air, under given conditions). This is generally of more interest in the realm of air travel than of PA systems! However, the term is included here as it is sometimes incorrectly used in place of the term 'infrasonic'. See also Speed of sound.


A woofer designed specifically to handle sub-bass frequencies.


The combining together (i.e. 'addition') of two or more quantities − usually signals. For example, mixer channels may be summed onto audio group buses. In the group-mix section the group buses may be summed to produce a main-mix, and the Left and Right main-mix buses may be summed to produce a mono mix. Summing is usually performed by passing the signal from each source through its own summing resistor to a common point, and following this with a summing amplifier in order to make up for the losses in the resistors and to provide a low impedance summed signal. Note that, in general, once signals are summed they cannot later be separated (unless there is no overlap of their frequency ranges, in which case separation may be possible by the use of filters). Compare Multiplex.


Describes the polar response of a uni-directional microphone whose sensitivity to sound decreases more rapidly from the front to the sides than is the case with a cardioid type, and then increases again towards the back − though only to a fraction of its sensitivity at the front. Its useful angle of acceptance (measured from side to side) is around 115º. Its minimum sensitivity to sounds is at an angle of around 125º, measured from the front axis (i.e. 55º from the rear axis). Greater directionality is provided by the hyper-cardioid type. See the Microphones page for more detail.

View uni-directional polar responses illustration


See Principle of superposition.

Supraaural or Supraural

Describes headphones that are worn with the pads resting on the outer part of the ears. These headphones are usually of the open back type. Compare Intraaural and Circumaural.

Surface-mounted component

A component that is designed to be attached to a PCB by its contact pads being soldered to copper pads on the same side of the PCB as the component. Surface-mounted components must be of exactly the correct physical size to match the spacing of the PCB pads. They may be fitted to either or both sides of the PCB. Compare Through-hole component.

Surge current

See Current surge.

Surge protection

A generic term for a facility whose purpose is to absorb mains voltage surges, or at least partly to do so, in order to protect equipment against the damage that such surges may cause. Surge protection may be provided by dedicated equipment (see SPD), or by facilities within other equipment − most commonly within power distribution equipment such as MDUs and power conditioners. It operates by use of surge suppression components such as MOVs.

Surge suppressor

See SPD and Power conditioner.

Surge voltage

The maximum voltage reached by a voltage surge. Some components, such as safety capacitors, have a surge voltage rating, meaning that they are designed to withstand surges up to a specified voltage.

Surround sound

Any scheme which provides the acoustic illusion of being 'within' a region of action − typically used with films. The usual arrangement for 'home theatre' set-ups is left front, right front, centre front, left rear and right rear speakers, plus a sub-woofer. See also 5.1 and 7.1. Compare Mono, Stereo and 3D sound.

Suspension mount

A type of external shock mount which employs elastic strips or loose springs to suspend a microphone away from direct contact with its stand or support arm, thereby providing good isolation from troublesome vibrations that might otherwise be mechanically coupled through that route. This type of shock mount is sometimes used for drum overheads, but otherwise is generally used only in recording and broadcast studios (often for vocals and speech). Commonly referred to as a 'spider' mount.


The continued sounding of a musical note. The term is most often used in reference to the extension of the time for which a note sounds, beyond that which would "normally" be expected for the musical instrument in question (achieved, for example, by a guitar effects pedal). Also, one of the parameters commonly used to define the envelope of a musical note − for details see ADSR.


An abbreviation for 'super video graphics adaptor'. A standard interface for the connection of display equipment (such as monitors and projectors) to computers. The standard gives a resolution of 800 pixels horizontally and 600 pixels vertically. The aspect ratio is 4:3. See also VGA, XGA, SXGA, WXGA, UXGA and DVI. The most commonly encountered standards, their resolutions and aspect ratios are tabled under the entry for VGA.

SVGA connector

Usually refers to a DE-15 15-pin high-density D-sub connector, as this type is commonly used for SVGA analogue video interconnections. As SVGA was a common standard, the term continues to be used even when the connector is employed for signals of different resolution. These connectors are also commonly referred to as 'HD15' or 'VGA' connectors; there is typically no physical difference between a connector or cable described as 'VGA' and one described as 'SVGA'. For pin allocations see VGA connector. For further information see VGA.

Sweep EQ

A type of equalisation facility, often provided on mixers for control of the mid-range frequencies, but sometimes also for treble (HF) and/or bass (LF). It provides the facility to cut (that is, reduce) or boost a band of frequencies whose centre frequency may be adjusted (or 'swept') through a particular range.

The sweep equaliser has at least two controls: one to set the amount of cut or boost and another to set the centre frequency. When a Q control is also provided then it is more properly referred to as a parametric equaliser. When there is no Q control, the Q is usually fixed at a value of around 2, and the equaliser may then be described as 'semi-parametric' (or 'quasi-parametric'). See also Peaking response.


Describes a sound that is generally pleasing to the ear; not harsh. An absence of distortion or imbalance.

Sweet spot

In a stereo speaker arrangement, a listening position which gives a much improved stereo image as compared to other listening positions. If the system is set up to give the same sound level from the Left and Right speakers, then assuming left-right symmetry in the room acoustics the sweet spot will always be a point that is the same distance from the two speakers. What this distance should be will depend on multiple factors, including the design of the speakers and the manner in which they are angled.

Switch on/off procedure

In order to avoid possible damage to equipment (especially speakers) by switch-on and switch-off transients, the component parts of a PA system should be switched on and off in a specific sequence, as follows:

Switching on:

  1. Check that all power amplifiers and powered speakers are switched off.
  2. Switch on all sources, mixers, outboard equipment (effects, dynamics processors etc.) and graphic equalisers.
  3. Switch on active crossovers and speaker management equipment.
  4. Check that all power amplifier level controls are set to minimum.
  5. Switch on all power amplifiers and powered speakers.
  6. Slowly turn up each power amplifier level control to its required setting.
Switching off:
  1. Set all power amplifier level controls to minimum.
  2. Switch off all power amplifiers and powered speakers.
  3. Switch off active crossovers and speaker management processors.
  4. Switch off sources, mixers, outboard equipment and graphics.

Some types of mains distribution unit (MDU) provide a sequential switching facility, enabling automatic sequencing of the power supplied from each of their outlets.

Switchable pattern

Describes a microphone whose polar response may be selected from two or more options by means of a switch.

Switched-mode power supply

See Power supply.


A slang term describing either a switched-mode power supply or a switching amplifier.

Switching amplifier

An alternative name for a Class D power amplifier. For more information on that type of amplifier see Pulse width modulation. For a list of common amplifier classes, see Amplifier Classes on the Amplifiers and Speakers page.


An abbreviation for 'Safe Working Load', the maximum loading (expressed as a weight) that can safely be supported by a chain, rope, truss, hoist, cherrypicker, etc.. An SWL value may be a maximum value specified by a manufacturer, requiring possible reduction depending on the circumstances of use (i.e. similar to a WLL value), or may be a value determined by a competent person taking into account such conditions as the working environment and method of use (but in any case never greater than the manufacturer's value).

The SWL value incorporates a suitable safety factor to allow a substantial margin between between the permissible loading and the load at which the equipment might be expected to fail − generally at least a factor of 8 for such kinds of equipment. As a rule of thumb, it is recommended that flown equipment be supported by equipment having an SWL of at least 5 times the actual weight to be supported, though in some circumstances such a high further factor may not be necessary or practicable. Compare WLL. For further information on safety see the Safety page.


An abbreviation for 'super extended graphics adaptor'. A standard interface for the connection of display equipment (such as monitors and projectors) to computers. The standard gives a resolution of 1280 pixels horizontally and 1024 pixels vertically. Note that this format gives a 5:4 aspect ratio, not the more common 4:3. See also VGA, SVGA, XGA, WXGA, UXGA and DVI. The most commonly encountered standards, their resolutions and aspect ratios are tabled under the entry for VGA.


An abbreviation for 'sound cue' or for 'sound effects cue'. See also LXQ. Compare FXQ.

Symmetrical Q

See Constant Q.


An abbreviation for 'synchronisation', a means of ensuring that two or more events, or ongoing processes, have the appropriate timing relationship to each other. For example, a signal which provides timing information to achieve that function. The plural term 'syncs' may be used when there is more than one type of synchronisation information present, as in the case of a composite video signal. Sometimes written 'synch' (but still pronounced 'sink'). See also Lip sync, Word clock, Line sync, Field sync and DARS.


Describes something in which the timing of events is controlled by a fixed timing reference (usually, a clock). In synchronous communications, it is the start of transmission of each word of information which occurs synchronously; in practice this usually means that the bit-stream is continuous, as, for example, in SPDIF digital audio. Compare Asynchronous.


An abbreviation for 'system exclusive', a type of MIDI message whose purpose is specific to a particular make or model of equipment. See also Proprietary.

System tuning

See Tuning (1).

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This page last updated 21-Feb-2021.