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The glossary pages provide definitions for over 2680 PA-related terms. 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.

L * L<number> * LAT * LAeq * Leq * Lambda * Lamp * Lampie, Lampy * Lantern * Lapel microphone * Lapped screen * Laser classes * LAT * Latency * Lav * Lavalier * Lavaliere * Law * Lay * Lay length * Lay-up * LC filter * LCD * LCR * LD * Lead * Lead sheet * Leading edge dimming * Leakage * Lectern * LED * LED display screen * Leg * Legacy * LEQ * Leslie * Level * Level meter * Leveler * Leveller * LF * LFE * LFE channel * LFH * LFO * Lift * Light * Lighting bar * Lighting fixture * Lighting instrument * Lighting plot * Lighting truss * Lightning connector * Limiter * Limiting * Lin * Lin pot * Line * Line-up * Line array * Line check * Line code * Line conductor * Line driver * Line filter * Line input * Line level * Line microphone * Line output * Line sync * Linear * Linear PCM * Linear phase response * Linear potentiometer * Linear power supply * Linear taper * Linearity * Link * Linkwitz-Riley * Lip microphone * Lip sync * Listen wedge * Littlite * Live * Lively * LM * LMID * Lo-Z * Load * Load impedance * Load-in * Load-out * Loaded * Lobar * Lobe * Log * Log pot * Log taper * Long throw * Longframe jack * Longitudinal choke * Longitudinal wave * Loom * Loom box * Loop * Loop amplifier * Loop driver * Loop system * Loss * Lossless * Lossy * Loud * Loudness * Loudness leveller * Loudspeaker * Low cut * Low end * Low frequency * Low impedance * Low pass * Low-Z * Lower mid-range * Lows * LPCM * LPDA * LPF * LR * LR-4 * LRC * LSF * LSHF * LSOF * LSOH * LSZH * LTE * LTP * Lumen * Lumens * Luminaire * Luminance * Lux * LV * lx * LX * LX tape * LXQ

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.

L
An abbreviation for 'Left channel'. See Stereo. Compare R.

L<number> (1)
In sound level measurement, the level that is exceeded for <number> percent of the time, over the period of the measurement, usually expressed in dB SPL. For example, the L10 figure is the level that is exceeded for 10% of the time during the measurement interval. See also Sound level meter. Compare Leq.

L<number> (2)
In equipment circuit diagrams (schematics), the component references for inductors usually take the form 'L followed by a number', e.g. 'L6'. Such component references are sometimes also marked on equipment circuit boards (PCBs).

LAT
See Leq.

LAeq
See the next definition.

Leq
In sound level measurement, the continuous level that is equivalent to the average of the actual levels over a stated period of measurement, expressed as a value in dB SPL. The measurement is usually A-weighted, in which case the correct term is LAeq, though 'Leq = x dB(A)' is acceptable. A measurement period of the order of 1 to 3 minutes might typically be considered appropriate for PA applications. This method of measurement is more properly called LAT, and is defined by standard IEC 61672-1.

Leq measurements are also often used to quantify sound exposure dosage for health and safety purposes, in which they specify the continuous level that is equivalent to the total sound energy exposure over the relevant time interval. For example, European regulations require the exposure for employees to be limited to an Leq SPL value of 85 dB (A) over an 8 hour period (per 24 hours), after the effects of any hearing protection measures are taken into account. For an Leq level of 88 dB (A) the maximum exposure period would be 4 hours and for 100 dB (A) it would be 15 minutes, as these exposures would give the same total sound energy dosage as 85 dB (A) for 8 hours. See also Sound level meter. Compare L<number> (1) and Time-weighted average.

Lambda
The Greek letter λ which is used as a symbol for wavelength within formulae (equations). Therefore, this term is occasionally used in place of the term 'wavelength'. For audio-related formulae involving lambda, see this entry on the FAQ page.

Lamp
The component, within a lantern, that converts electrical energy into light. Frequently called a 'bulb' by non-technical personnel (a term deprecated by lighting engineers). The term does not refer to the complete lantern. Lamps are of many different types, including incandescent filament, fluorescent, discharge and LED.

Lampie, Lampy
A slang term for a lighting engineer. Compare Soundie.

Lantern
The 'proper' name for a 'stage light' − an enclosure containing one or more light sources (and possibly other devices such as lenses, filters, shutters, gobos etc.), arranged to emit light in a controlled way. Sometimes called a luminaire.

Many different kinds of lanterns exist − see, for example, PAR can, Fresnel, Profile spot and Moving head.

With the advent of lighting and other equipment that is directly controllable using DMX, the term 'fixture' is now commonly used to refer to all such equipment; lanterns are therefore now often referred to as 'lighting fixtures'. The US term is 'lighting instrument'. See also Lamp and LED.

Lapel microphone
See Lavalier.

Lapped screen
A screen that consists of many fine strands of bare wire laid parallel with each other in a helical (or 'spiral') wrapping around the insulated inner conductor(s), rather than being woven into a braid. Some screened cables have a lapped screen consisting of two layers, one wrapped in a clockwise direction and the other anticlockwise.

Laser classes
See Classes of lasers.

LAT
See LAT.

Latency
The time delay introduced in a signal by digital signal processing or communications equipment. In the case of signal processing, this delay is generally caused by the time taken to complete the complex arithmetic operations which must be performed on the signal in order to provide the required processing function(s). In the case of digital communications, latency may result from use of a relatively low communications bit-rate. Such delays are more likely to be of significance when some but not all signals are digitally processed (or communicated) by equipment with a high latency, because the difference in timing may then be more obvious (for example, in AV systems, poor lip sync). Latency is usually most important in real time processing, for example live programme applications.

As an example from digital communications, consider a MIDI 'note-on' message sent from a keyboard to a sound generator over a single MIDI link. This message consists of three bytes, each of 11 bits (including the start and stop bits). At the standard MIDI data rate of 31.25 kbit/s, the message will therefore take just over ms to be sent. So even ignoring processing delays within the keyboard and sound generator, the latency between pressing a key and the sound beginning will be at least 1 ms. See also Time alignment.

Lav
A slang abbreviation for lavalier; a short-hand term for a lavalier microphone (see the next definition).

Lavalier (or, less commonly, Lavaliere)
Originally, described a small microphone that was worn by being suspended from a cord around the wearer's neck, to allow hands-free use − e.g. by presenters or actors. However, this method of attachment is now relatively uncommon, and the term is now used to refer to any chest-worn microphone − usually miniature types that are attached to clothing by means of a clip. As they are often clipped to a lapel or a tie, they are alternatively known as lapel microphones, clip microphones or tie-clip microphones. They are usually radio microphones, and are used in conjunction with a bodypack.

In some situations these microphones can be prone to problematic variations in output level, due to the changing distance from the wearer's mouth as their head is moved. To minimise this effect they should ideally be worn centrally, just below the point at which the wearer's chin would touch their chest. If they must be attached to one side, then this should be the side that the wearer predominantly faces (e.g. in an interview situation). Level variations can be further reduced by the use of omni-directional types, but these are more prone to feedback and leakage problems, especially when a high gain needs to be employed. A preferable approach is often a headset microphone.

The term 'lavalier' was taken from (by visual similarity) the name given to a type of single-stone pendant necklace. This name in turn originated from the French word 'lavalličre' for a necktie with a large knot, popularised by the Duchesse de La Valličre. See also Necklace microphone.

Law
A rule that expresses the relationship between two or more quantities. For example, the law of a potentiometer (also referred to as its taper) defines how its resistance varies as its physical setting is changed, and Ohm's law defines the relationship between voltage, current and resistance.

Lay
See the following two definitions.

Lay length
The distance taken up by each complete twist of the insulated signal conductor wires within a balanced cable. Sometimes just referred to as the 'lay'. See also Twisted pair and Star quad cable.

Lay-up
The particular physical arrangement of the conductor wires or pairs within a cable. Sometimes just referred to as the 'lay'. See also the previous definition.

LC filter
An intentional filter that is constructed using only inductors and capacitors, or an unintentional one that exists as a result of the inductance and capacitance incidentally present in a circuit. Compare RC filter.

LCD
An abbreviation for 'liquid crystal display', a device in which liquid crystals are placed under the influence of an electric field in order to control their polarisation and hence their ability to pass light (when used with appropriate polar filters). By division of the display area into segments or pixels, each under separate electrical control, information such as numbers, characters or full images can be formed. When each pixel has its own adjacent controlling transistor(s) fabricated into the LCD, the display is described as 'TFT', an abbreviation of 'thin film transistor'.

The liquid crystals themselves do not emit light; the display must either incorporate a source of light (e.g. a 'back-lit' display) or rely on the reflection of ambient light. See also IPS (2) and LED.

LCR (1)
An abbreviation for 'left, centre and right'; describes a routing arrangement that in addition to the facility for providing Left and Right mixes (for eventual delivery to the Left and Right speakers of a stereo PA system), also provides the facility for a Centre mix (for eventual delivery to centrally located speakers). Alternative abbreviation: LRC.

Some mixers that provide such routing arrangements have pan controls that are configured to pan from a Left-only feed (when fully anti-clockwise), to a Centre-only feed (when at the 12 o'clock position) to a Right-only feed (when fully clockwise). The lack of any feed to the Left and Right mix buses when the control is at 12 o'clock is in contrast to the usual stereo routing arrangement, in which an equal feed is supplied to those buses when the control is in that position. Some mixers providing LCR routing have the facility to select whether the pan controls operate in LCR mode or in stereo mode. See also Cluster.

LCR (2)
A combination of the component designations for inductors (L), capacitors (C) and resistors (R). Usually used to describe a filter which makes use of all three of these component types.

LD
An abbreviation for 'lighting designer'. See also Lampie.

Lead (1)
A common alternative name for a cable, usually one that is either fitted with connectors at both ends or (less commonly) that is permanently attached to an item of equipment. Typically preceded by a word to describe the type of cable referred to, or to describe its intended purpose, e.g. 'guitar lead', 'mic lead', 'instrument lead', 'speaker lead', 'mains lead', etc.

Lead (2)
Describes an instrument or vocal part whose sound is intended to be dominant in the overall sound of the band, as perceived by the audience, or to be dominant in at least some parts of some songs. For example, lead vocals (LV) sings the main melody line; lead guitar plays (mostly) single notes (rather than chords) and often has a solo part in at least one song. See also Instrumental break and Rhythm guitar. Compare Backing.

Lead sheet
A document that specifies the foundational elements of a song, such as the lyrics, the melody line, chords for basic accompaniment, and the song's structure (i.e. sequence of verses, choruses, instrumentals etc.). The information provided on a lead sheet can be of use to sound engineers, e.g. to indicate when a guitar solo will be played.

Leading edge dimming
See Phase-angle control.

Leakage
In general, something occurring where ideally it would not. Most often refers to pick-up by a microphone of sounds other than those intended to be picked-up by that microphone, particularly pick-up of musical instruments situated close to the target instrument. Usually undesirable in PA work (but see Creative leakage). In the sense that leakage constitutes a part of the unwanted signal present, it may be considered to be part of the noise. Alternatively known as 'spill' or 'bleed'. See also Separation, Noise gate and Earth leakage.

Lectern
A stand which supports books, documents, a laptop computer, etc., used by a person delivering a presentation. It may also support a microphone, often a gooseneck type. A lectern is often located on a dais.

LED
An abbreviation for 'light-emitting diode', a semiconductor device that emits light when a current is passed through it. High brightness LEDs are now increasingly used as light sources in lanterns, where they can provide multiple colours and much greater efficiency and longer life than incandescent lamps.

Although the brightness of an LED varies with the current passed through it, such a method is generally unsuitable for accurate dimming applications (especially below around 20% power) because of variabilities between the LEDs in a group and because of undesirable changes in the colour of the emitted light. The dimming of LEDs is therefore usually accomplished using pulse width modulation or pulse density modulation techniques, which in some cases can result in problematic levels of radio-frequency interference (RFI). See also LCD. Compare Incandescent lamp and Discharge lamp.

LED display screen
Usually refers to a display screen in which the image is formed by pixels that are each made up of a red, a green and a blue LED. The light level of the individual LEDs is controlled in order to give the brightness and colour required for each pixel. Very large such screens, suitable for use in large auditoria, may be created by inter-linking smaller modules. In such screens, the pitch of the pixels is an important factor in determining the clarity of the images produced − especially for the viewers located closest to the screen. As a rough guide, the minimum viewing distance in metres can usually be considered to be slightly less than the pitch in millimetres, e.g. 6 m for a 7 mm pitch screen. It should be noted that the video signal processing associated with these screens can sometimes introduce a substantial delay in the displayed image − see Latency. The brightness of display screens is usually specified in nits.

Leg
One of the signal-carrying conductors of a balanced pair. The two legs of the pair are commonly referred to as the 'hot' and 'cold' legs, or as the '+' and '−' legs. Note that in a star quad cable two wires are used to make up each leg of the pair (making four wires in total). See also One-legged.

Legacy
Describes equipment of an earlier design or style than what is now being manufactured. This may mean that the equipment is many years old, but in the case of equipment using rapidly-changing technology (such as effects units and computer-based equipment) the term might be used of equipment even less than a year old. The term is most frequently used in a comparative sense, to refer to the earlier equipment that remains installed, or in use, after some refurbishment or extension of the system (often by new owners or new operators of a venue). Such a description does not necessarily imply that the equipment referred to is no longer fit for purpose.

LEQ
See Leq.

Leslie
Describes a speaker that incorporates an internal motor-driven rotating device which interferes with the passage of the sound emerging from the driver. A means is usually provided to switch the motor on and off, and to control its speed of rotation (e.g. start/stop and fast/slow foot switches). Effective miking of these speakers can be difficult, and is sometimes achieved by use of two microphones, e.g. at the top and side apertures of the cabinet.

Such speakers were originally used with early electric organs to produce a rapid 'vibrato-like' sound effect typical of classic 'theatre organs'. The effect is now produced internally in keyboards by digital means. Named after its inventor.

Level
The amount of something; a measurement or indication of its magnitude. In particular, the magnitude of an audible sound (usually expressed in dB SPL or in µPa) or of a signal (usually expressed in volts, dBV, dBu or dBm). By convention, such levels are nearly always expressed as RMS values. An alternative term for level is 'amplitude'.

Audio signal levels are important in a relative sense because they impact upon the perceived loudness of the sound finally produced and upon the relative contribution of each sound source in a mix. They are also important in an absolute sense: at the interconnections between items of equipment in order to ensure compatibility and within equipment to ensure that signals are neither so small that contributed noise becomes significant nor so large that significant distortion occurs as a result of overload.

Analogue audio signals at the interconnections between items of equipment in a PA system are usually at one of the following levels:

Digital audio signal levels are usually expressed as a value relative to the maximum level that can be handled by the digital signal format concerned, i.e. as a negative dB FS value.

Current levels are measured in amps (or mA, etc).

Power levels are measured in watts (or mW, etc) − but see also VA.

Sound pressure levels are measured in dB SPL (or sometimes in µPa).

Lighting levels are usually measured in lux.

See also Metering, PPM, VU, Dynamic range, Amplifier, Attenuator, Gain, Clipping, Volume, Input and Output.

Level meter
A device that indicates the level of an audio signal − see Metering. Or, an abbreviation for sound level meter.

Leveller or Leveler
A device that is intended to remove or reduce variations in average signal level, for example so as to remove or reduce variations in average programme loudness when switching between different audio signal sources. Such a device is basically a compressor with an appropriately low threshold setting and appropriate settings of compression ratio and attack and release times. Further, specific side chain filtering may be employed in order to provide a suitable assessment of loudness. 'Leveler' is the US spelling. See also Automatic gain control.

LF
An abbreviation for 'low frequency'. See Bass (1).

LFE
An abbreviation for 'low-frequency effects', typically referring to the low-frequency effects added to film sound-tracks. See Bass (1).

LFE channel
A surround sound recording channel used solely for sub-bass signals, typically for the low-frequency effects (LFE) added to film sound-tracks. Use of a separate channel for this purpose avoids a need to record the higher frequency content of the programme at a lower average level in order to accommodate high-level sub-bass signal peaks within the available dynamic range of the main recording channels. An improved signal-to-noise ratio can therefore be obtained at those higher frequencies, where noise is more prevalent and more noticeable. For proper reproduction of the LFE channel a sub-woofer is required. See also 5.1 and 7.2.

LFH
An abbreviation for 'low fire hazard', typically relating to the hazards that may be created by some types of cables when subjected to fire conditions. See also LSF etc.

LFO
An abbreviation for 'low frequency oscillator'. It is most commonly encountered in relation to modulated time-domain effects such as chorus, flanging, phasing, tremolo and vibrato, where it refers to the oscillator that runs at a very low frequency (typically between 0.1 Hz and 20 Hz) in order to produce the repetitive variations in sound that are characteristic of these effects.

Lift (1)
In the context of equalisers, an alternative term for boost.

Lift (2)
An abbreviation of earth lift.

Light
Visible electromagnetic radiation, emitted by a source such as lantern. Not the lantern itself (though non-technical personnel frequently use the term in that way, as in 'a stage light'). See also Lumen and Lux.

Lighting bar
A solid or hollow metal bar of rectangular cross-section, drilled at intervals to enable lanterns or other lighting fixtures to be bolted directly to it. Or, a metal tube of circular cross-section to which clamps may be attached at any point, for the mounting of lanterns or other lighting fixtures. May be referred to as just a 'bar'. See also T-bar, Truss and SWL.

Lighting fixture
See Lantern.

Lighting instrument
An American term for a lantern.

Lighting plot
A diagram (or set of diagrams), produced during the planning of an event, to indicate the stage area that is to be illuminated by each lantern during each scene, and to specify the lighting colours, textures, special effects, etc. that are to be used. Often abbreviated by lighting engineers to just 'plot'. See also Cue and Sound plot.

Lighting truss
See Truss.

Lightning connector
A flat small-format connector developed by Apple® for digital signal and charging interconnections for mobile devices. The connector has 16 poles but only the 8 on one side are used at any one time, depending on which way up the connector is inserted. Digital audio and video information may be conveyed. Adaptors to USB, and to other interfaces, are available.

View Lightning connector image

Limiter
A signal processing unit that prevents the peak level of a signal from significantly exceeding a pre-determined value (the limiting 'threshold'), either as an effect or to restrict its dynamic range in order to avoid the overload of amplifiers, speakers, recording equipment or broadcast equipment. If the limiting threshold is crossed by the input signal, the limiting action that takes place will reduce the difference between the signal's average level and its peak level, so the required headroom is reduced.

Effectively, a limiter is a compressor operating with a very high compression ratio above the threshold. This is usually referred to as an 'infinite' compression ratio (i.e. 'infinity to 1'), but in practice a ratio of 10:1 or more provides a limiting effect. See also Dynamics processor, Speaker management equipment and PPM.

Limiting
The effect on dynamic range that is intentionally produced by a limiter, or that is caused by some other means (intentional or otherwise) that has a similar effect.

Lin
An abbreviation for 'linear'.

Lin pot
An abbreviation for 'linear potentiometer' − see Potentiometer.

Line (1)
A physical signal-interconnection between two points (usually a significant distance apart), made by means of a cable. In the case of a multicore cable, the same cable incorporates several lines. Note that a line may carry any type or level of signal for which the cable is suitable (not just line-level analogue signals). The signal path over a line may be referred to as a circuit or a channel. To assist in the reduction of interference picked-up on the line, lines are often screened and/or balanced. See also Line check, Link (1), Feed, Pair, Leg, 100 volt line, Returns, Line driver, Termination (1) and Termination (2).

Line (2)
The mains power for an item of equipment, or a cable carrying such power. See also Line conductor and Line filter.

Line-up (1)
The particular set of musical instruments used by a band during a performance, or the particular set of performers involved.

Line-up (2)
The process of adjusting the relative signal levels throughout a system, so as to achieve the required gain structure. See also Tone (2), SOL and Programme level.

Line array
A speaker system consisting of a vertical column of drivers, either all contained within a single enclosure or (more usually) each driver having its own enclosure.

The array is usually flown, and may hang straight downwards or be 'articulated' into a curved shape. A common shape is the so-called 'J-curve', in which the units are angled increasingly downwards towards the lower end of the array, forming the shape of the letter 'J' when viewed sideways from stage right. (The amount of curvature necessary can be reduced by use of downfill or front-fill speakers.)

The resulting sound field has very limited dispersion in the vertical plane, but provides a broad horizontal coverage. It can be pictured as a segment of a horizontal slice of a vertical cylinder (i.e. like a wedge-shaped piece of a circular cake). The result is that, for a particular range of frequencies and up to a particular distance (the 'critical distance'), the on-axis sound level decreases by dB for each doubling in distance, rather than by the 6 dB that is applicable to a point source (see Inverse square law).

The critical distance is dependent upon the length (i.e. height) of the array and upon frequency, but opinions differ as to how best to define this distance and calculate its value. Mark Ureda (of JBL) proposed in a 2001 AES convention paper (preprint 5304 − click to open in a new window) that a suitable approximate calculation would be (array length2 x frequency) / 700, where the array length and the resulting distance are in metres, and the frequency is in Hz. However, in a 2002 paper (AES ref 5649 − click to open in a new window) he proposed a revised approach giving a significantly increased critical distance, calculated (as an approximation) using 0.006 x array length2 x frequency. These calculations show that quite long (i.e. tall) arrays are needed to obtain any useful effect at low frequencies. The high-frequency limit of the effect is determined by how close together the individual drivers of the array can be placed. See also Column speaker. Compare Cluster.

Line check
A specific part of the sound-check procedure, during which the allocation and correct operation of the lines from the stage is checked. At the conclusion of the line check, it will be proven that each microphone and instrument source is connected to the correct input of the mixer (or to the correct inputs of the front-of-house and monitor mixer, where these are separate) and that there are no problems with the connections.

Line code
A set of rules that are used by equipment to encode a digital data stream prior to making it available at an output of the equipment. (For the meaning of 'line' in this context, see Line (1).) Using a line code does not add any additional information to the data being output; rather its purpose is to facilitate transmission through the interconnection and to facilitate recovery of the original data stream by the destination equipment. Line codes are often particularly important for clock recovery and frame synchronisation. An example of a line code used for digital audio interconnections is the bi-phase mark code.

Line conductor
In mains supplies, a conductor that is intended to be at supply voltage (230 V in the UK and much of Europe) with respect to safety earth. A single-phase supply has just one line conductor, whereas a 3-phase supply has three electrically-independent line conductors. Alternatively known as a phase conductor or, less formally, as a live conductor or a hot conductor.

Line driver
A device whose function is to provide signal output characteristics suitable for connection to a line. In terms of audio lines, this usually means providing a balanced signal at line level, with sufficient drive current capability to cater for the relatively high capacitance values of lengthy cables.

In PA work, line drivers are typically integrated circuit components internal to equipment that has electronically balanced (rather than transformer balanced) outputs. There are several different types of such outputs, which behave differently to each other in some circumstances (see Balanced). For example, quasi-floating line drivers may introduce relatively high levels of noise if used to drive an unbalanced input or an unbalanced cable without linking the cold leg of the output to signal earth.

Line filter
A filter that is intended to be used in a mains power feed, with the intention of reducing mains-borne radio-frequency interference (RFI). Such filters must be rated adequately for the current demands of the equipment supplied through them. See also Power conditioner, Distro and VA.

Line input
An abbreviated form of 'line-level input'; an input that is intended to accept only a line-level signal. The connector is usually a quarter-inch (6.35 mm) jack or a female 3-pole XLR. Line inputs may be intended to accept unbalanced signals only, or may be designed for balanced signals (at line-level). Line inputs that are intended for balanced signals can usually also accept unbalanced ones, provided that they are not supplying phantom power (rare for line inputs) and that the connecting plug is appropriately wired − check the equipment manufacturer's instructions. Compare Microphone input.

Line level
Usually refers to a signal whose nominal level is in the region of 0.3 V to 2 V (−8 to +8 dBu), or refers to an input or output connection intended to carry such a signal. Typically, line-level signals are used for interconnections between mixers, power amplifiers and outboard equipment such as graphic equalisers, effects units and active crossovers.

Nominal levels for these signals are standardised at either −10 dBV or +4 dBu.

  • −10 dBV (equivalent to approximately −8 dBu or 0.316 V) is used by most semi-professional equipment, home studios, etc. These connections are typically of the unbalanced type.
  • +4 dBu (equivalent to approximately +2 dBV or 1.23 V) is used by most professional equipment. These connections are typically of the balanced type, in which case +4 dBu refers to the level between the hot and cold conductors (not from each of these conductors to signal earth).

The name derives from the level of signals typically used on analogue lines between distant stations (such as broadcast studios and transmitter sites). See also 100 volt line. Compare Microphone-level and Instrument-level.

Line microphone
An alternative name for a rifle microphone.

Line output
An abbreviated form of 'line-level output'; an output that provides a line-level signal.

Line sync
In an analogue video signal, the synchronisation pulses that indicate the start of each horizontal line of the picture. In a 625-line picture, the pulses occur at a frequency of 15.625 kHz, an interval of 64 µs. The short black-level interval preceding the line sync pulse is called the 'front porch'. The slightly longer black-level interval following the line sync pulse is called the 'back porch' and in a composite video signal carries the chrominance reference burst; it is also used for black-level clamping. Both line and field sync pulses occupy the negative-most 0.3 volts of a standard 1 volt peak-to-peak video waveform.

Linear
Describes a system, or a component of a system, that has a truly proportional response. That is to say, the output of the system is always in direct proportion to its input (under normal operating conditions and within a practical and appropriate accuracy of measurement). So, a doubling of input produces a doubling of output, etc. Where the input is a signal, such a component or system has the important property that no new frequencies related to the original ones are introduced into the signal by it.

The term literally means 'relating to a (straight) line', which refers to the fact that if a graph is plotted of output values versus input values, under normal operating conditions, an essentially straight-line graph is obtained over the range of interest. For examples of the use of this term, see the following definitions.

A system or component that does not behave linearly in some respect may be described as 'non-linear' in that respect, and this behaviour may result in intermodulation. See also Interpolation.

Linear PCM
PCM that utilises linear quantisation, which means that equal changes in analogue sample values result in equal changes in the numeric value assigned to each sample. This scheme is used in standard CD recordings and in S/PDIF and AES3 digital audio interconnections. It is often abbreviated to LPCM. For further information see Pulse code modulation and Analogue to digital conversion.

Linear phase response
If the phase response of a system whose group delay is constant over a particular frequency range is plotted against frequency, the result will be a straight line over that frequency range; this is described as a 'linear phase response'. So this term is just another way of saying 'constant group delay', which is a good thing in high-quality audio systems. A system having a linear phase response over a particular frequency range is sometimes described as being 'phase linear' over that range. A system having a linear phase response is not necessarily a minimum phase system.

Linear potentiometer
A potentiometer (pot) whose track has a constant resistance profile along its entire length, known as a 'linear taper'. In an appropriate circuit it therefore provides an output voltage that (for a fixed input voltage) is directly proportional to the angle that the control is turned (rotary pots) or the distance it is moved (slider pots). For further information see Potentiometer.

Linear power supply
A power supply whose transformer operates at mains frequencyi.e. a power supply that is not of the switched-mode type. A linear power supply is sometimes referred to as a 'conventional' one. For further information see Power supply.

Linear taper
See Taper.

Linearity
The degree to which a system or component behaves in a linear manner. See also Intermodulation.

Link (1)
An interconnection path between equipment or between locations, a term often used when there is a considerable distance involved. Although sometimes used as short for 'radio link' (i.e. a wireless interconnection), it may also be used to refer to a physical interconnection − see Line.

Link (2)
See Jumper.

Linkwitz-Riley
Describes a crossover which uses 4th-order Butterworth filters. So-called because the engineers S. Linkwitz and R. Riley determined that this arrangement gives an improved result over 3rd-order designs − not only by virtue of a steeper slope, but also in that the low-pass and high-pass outputs are well co-ordinated in amplitude and phase through the region of the crossover frequency. This is important because in this region both drivers are operating simultaneously, and so problems in co-ordination of their drive signals can lead to undesirable variations in the overall output level and in the directivity of the speaker. Abbreviation LR, or LR-4.

Lip microphone
A microphone which is intended to be held extremely close to the mouth. It usually has a shield protruding from its top surface; this is intended to touch the user's face between the upper lip and the nose − hence the name. It is generally used only for sports commentary applications, and is usually of the noise-cancelling type.

Lip sync (1)
In audio-visual applications (such as TV or film), the degree to which the sound and vision tracks are in synchronism, characterised by the degree to which the sound of a person's voice corresponds with the movement of their lips. Lip sync may be adversely affected by the latency of signal processing equipment such as digital vision mixers, or by a large distance between the speakers and the audience (see Speed of sound).

Lip sync (2)
To move one's lips in synchronism with a previously made recording of the required vocal sounds, e.g. to mime to a pre-recorded vocal sound-track. Often a microphone will be used for purely visual effect − it will generally be muted so as to avoid any accidentally-made vocalisations appearing in the mix, unless the performer wishes to add further vocals or speech at one or more points.

Listen wedge
The monitor speaker that is provided for use by the monitor engineer. It enables him/her to listen to a particular performer's monitor mix − this is especially useful when making adjustments to the mix. Typically the listen wedge is fed from the 'control room' output of the mixer, and the mix to be monitored is selected by activating the AFL of the appropriate aux send. The monitor should be of the same type as those used by the performers, and any outboard processing (most commonly graphic equalisers) that is used for the monitor mixes should be patched into the aux send inserts of the mixer, so that their effect is apparent in the listen wedge. See also Wedge.

Littlite
A trademarked name for a particular brand of miniature illumination devices for equipment and for musicians' music stands. The gooseneck variety is frequently used for illumination of mixer control surfaces, where it is often powered and supported via a BNC connector. The most popular incandescent lamp rating is 12 voltswatts. LED versions are also available.

Live (1) (electrical)
Describes an item of equipment that is currently in use, or that is connected to a source of electrical power − especially mains power. (More formally, live mains supply distribution equipment or cabling is described as 'energised'.) Or, describes a conductor that is at (or is intended to be at) a significant voltage with respect to the general mass of earth; that is, a non-earthy conductor, sometimes referred to as a 'hot' conductor, or marked '+'. Warning: In energised mains supplies, all the current-carrying conductors (including the Neutral) should be treated as being 'live', i.e. at a dangerously high voltage with respect to earth, and must never be exposed to the touch. See also Line conductor.

Live (2) (microphones)
Describes a microphone that is currently 'open' − i.e. the sound that it picks up is currently being used in some way (amplified, recorded, broadcast, etc.). See also NOM.

Live (3) (programme)
Describes a programme originating from a performance, or other event, that is happening at essentially the present time; not one previously recorded. Live programme material must be handled in real time, and latency may therefore be important.

Lively
Describes acoustics having a long reverberation time. Compare Dead (1).

LM
An abbreviation for 'lower mid-range'.

LMID
An abbreviation for 'lower mid-range'.

Lo-Z
See Low impedance.

Load (1) (electrical)
[Noun:] The item of equipment, or an input of it, that is the destination of a signal or of a source of electrical power. The impedance of the load is often significant (see the next definition), particularly in regard to speakers. Or, the amount of current drawn by, or amount of power required by, one or more items of equipment. Where the equipment has (or potentially has) a poor power factor, the load may be expressed in terms of VA rather than watts.

Or, [Verb:] The effect that connection of one or more loads has on the output of the source equipment − particularly as regards the resulting reduction in output voltage level. Specifically, to cause a current to be drawn from a source of voltage (e.g. an output of an item of equipment), or to cause power to be drawn from a source of electrical energy (such as a mains supply point).

See also Overload (1), Overload (2), Voltage-matched and Impedance-matched.

Load (2) (mechanical)
The tensile or compressive force exerted on an item of mechanical equipment such as a chain, rope, truss, hoist, staging, etc., typically specified as a weight value in kg, pounds or tons. Such loading forces must under all circumstances be less than the Safe Working Load (SWL) of the equipment sustaining them − this may require the SWL to be considerably higher than the actual weight of the item(s) to be supported. Or, a term for the equipment (or other influence, such as the wind) exerting such a force.

Load impedance
Usually, the combined input impedance value of all the loads that are connected to a particular output channel of an item of equipment, whether via a single output connector of the equipment, via multiple internally paralleled connectors, daisy-chained load equipment or other signal splitting arrangements. Or, the term may refer to the input impedance value of a particular load.

Where two or more identical loads are connected to an output channel in any arrangement that effectively connects them in parallel, their combined load impedance is the impedance of one of them divided by the number of them. In order to avoid a reduction in the performance of equipment, and/or possible damage to it, it is necessary to ensure that the combined load impedance connected is not less than the minimum specified for the output channel concerned. This is of particular importance in the interconnection of speakers and power amplifiers − see the Impedance section on the Amplifiers & Speakers page. See also Headphones, Input impedance, Output impedance, Parallel, Series, Voltage-matched and Impedance-matched.

Load-in
The process of moving a performer's equipment, or a rig, from the transport vehicle(s) into position in the venue before an event. Also referred to as the 'get-in'. Compare Load-out.

Load-out
The process of moving a performer's equipment, or a rig, from its location in the venue into the transport vehicle(s), after an event. Also referred to as the 'get-out'. See also De-rig. Compare Load-in.

Loaded (1)
Describes a signal output or an electrical supply source that has a load connected to it. See also Output impedance, Termination (2), Overload (1) and Overload (2). Compare Open-circuit (1) and Unloaded (1).

Loaded (2)
Describes a speaker enclosure or cab that has drivers fitted. Compare Unloaded (2).

Lobar
Describes a microphone whose polar response contains multiple lobes. An alternative term for a rifle microphone, because this type has such a polar response.

Lobe
An imaginary 3-dimensional region in space, emanating from a speaker or a microphone, the boundary of which area defines a given degree of sensitivity of that particular device at a particular frequency. For example, the rear lobe of a microphone is the region of pick-up on the microphone's rear axis, which occurs in the case of super-cardioid and hyper-cardioid polar responses.

Log
An abbreviation for 'logarithm'. The mathematical power to which some number (called the 'base') must be raised in order to give a particular value. Logarithms to the base 10 are generally assumed, so the log of a number is the power to which 10 must be raised to give that number. For example the log of 1000 is 3, because 103 = 1000. Similarly, the log of 0.01 is −2, and the log of 1 is 0.

Or, an abbreviation for 'logarithmic', which describes a component, scale, or response in which the 'output' varies in accordance with the logarithm of the 'input' (or approximately so, or within defined limits). An example of this usage is 'log pot' (see Potentiometer).

The main relevance to sound engineering is in relation to decibels, which is a logarithmic scale − see the Decibels page. See also Taper.

Log pot
See Potentiometer.

Log taper
See Taper.

Long throw
Describes a speaker that has narrow dispersion angles, enabling it to provide sound effectively to an audience at some distance from it (say, greater than 12 metres). See also Directivity. Compare Short throw.

Longframe jack
See Jack.

Longitudinal choke
An inductor whose purpose is to introduce an opposition to signals carried by all the conductors of a cable, rather than to the signals appearing between the conductors. Most often used to reduce common-mode radio-frequency (RF) electromagnetic interference (EMI) picked up by, or radiated by, computer equipment or other digital signal processing equipment. In such cases the choke is usually implemented as a ferrite cylindrical or toroidal core through which the whole cable is passed either once or twice; this is seen as a cylindrical 'bulge' close to one (or both) ends of the cable. Sometimes called a 'common mode choke'. ('Choke' is just another term for an inductor.)

Longitudinal wave
A physical wave in which the displacement of the medium occurs in the same direction as the travel of the wave. Or, more formally, occurs along the same axis as a line drawn between successive crests. For example, a sound wave in air is a longitudinal wave consisting of alternate regions of compression and rarefaction. See also Propagation of sound and Speed of sound.

Loom
Short for wiring loom or cable loom. A group of wires or cables secured together, either permanently or temporarily. For example, the fixed cabling that inconnects items of equipment permanently installed in an equipment rack or behind interconnected patch bays. Or, a set of cables, temporarily secured together using cable ties or PVC tape, that run as a group between two points. Or, the grouped-together wiring internal to an item of equipment, which interconnects its circuit boards, panel connectors, controls, etc. Alternatively known as a harness (wiring harness, cable harness). See also the next definition. Ribbon cable.

Loom box
A compartment of an equipment flight case, intended to contain the cables connecting to the equipment so that they may remain connected while the equipment is in transit. It is usually located at the rear of mixer flight cases, and usually has a hinged or removable lid. The slang term 'dog box' is sometimes used.

Loop (1)
Short for Induction loop.

Loop (2)
Short for Effects loop.

Loop (3)
A recording that, when played, is arranged to restart each time that it reaches the end of the recorded material.

Loop amplifier / driver
See Induction loop.

Loop system
See Induction loop.

Loss
The amount of attenuation given to a signal between two specific points in a system or between two points within an item of equipment. Or, the amount of reduction in response (or sensitivity) of an item of equipment at a particular frequency, as compared to its response at another frequency (often a standard reference frequency such as kHz) − see Frequency response. Loss is usually measured in decibels. Compare Gain.

Lossless (1)
See Compression (2).

Lossless (2)
Describes equipment or an interconnection having no attenuation.

Lossy (1)
See Compression (2).

Lossy (2)
Describes equipment or an interconnection having a significant attenuation.

Loud
A subjective term, describing sound judged to be of high loudness relative to an implied, specific or arbitary reference. See also Sound pressure level. Compare Quiet (2), but take note of Quiet (1).

Loudness (1)
The apparent magnitude or 'volume' of an audible sound, as perceived by an average human listener (therefore also called 'perceived level', or 'perceived volume'). On average, loudness is subjectively judged to double for each 10 decibels increase in sound pressure level. Usually measured in phons (occasionally in sones). Contrast with sound pressure level (SPL), which (in unweighted form) is the actual magnitude of the sound waves, regardless of how an average listener might perceive their level. (Note, however, that SPLs may also be quoted as weighted values, which do provide an approximation to the perceived sound level.) See also VU and Weighting.

Loudness (2)
A control, usually found on a domestic Hi-Fi amplifier, which may be operated to activate equalisation that partially compensates for the apparent reduction in sound quality which occurs when listening at low sound pressure levels. It works by boosting the bass frequencies to compensate for the ear's reduced sensitivity to those frequencies at low levels, and also boosting the treble frequencies for the same reason and to compensate for the masking effect of low-level ambient noise. See also Fletcher-Munson curves.

Loudness leveller
See Leveller.

Loudspeaker
See Speaker.

Low cut
The operation of reducing the bass content of an audio signal. Or, the name of a switch or control(s) that provides this function (e.g. on a mixer, where it may be considered to be a part of the channel EQ section). Some types of microphone are equipped with switches providing a low cut (or bass roll-off) function, sometimes giving control over the cut-off frequency and/or slope; this facility is generally provided only on condenser types. May be abbreviated to 'LC', or may alternatively be referred to as 'high pass' (HP). On a mixer, a variety of forms are found, depending on the mixer sophistication − for details see Equalisation on the Mixing Facilities page. For cross-references to more filtering-related terms see Filter (1). Compare High cut.

Low end
The bass end of the audio-frequency spectrum, also called the 'bottom end'. Compare High end.

Low frequency
See Bass (1).

Low impedance
Usually describes equipment or interconnections in which the source impedance is, or is intended to be, no greater than 600 ohms. Sometimes written 'Low-Z' or 'Lo-Z', because Z is the symbol for impedance. Note that inputs intended for the connection of low-impedance microphones usually have an input impedance significantly greater than 600 ohms (typically kilohms), but are nevertheless referred to as 'low-impedance inputs'. For further information on the impedance of microphones, see the Microphones page. A speaker that is not a 100 V line type is sometimes referred to as a 'low-impedance speaker'. See also Matching. Compare High impedance.

Low pass
Describes a filter that attenuates ('cuts') frequencies higher than a particular value, but allows frequencies lower than that value to pass through relatively unaffected. The boundary between the range of frequencies allowed to pass and those cut is called the cut-off frequency, however in practice the transition between 'cutting' and 'passing' is not abrupt, but takes place over a range of frequencies. The cut-off frequency of a low pass filter is usually considered to be the frequency at which the attenuation is dB greater than the average attenuation at frequencies that are low enough to be clear of the transition region. A low pass filter is sometimes referred to as a 'high cut' filter. For cross-references to more filtering-related terms see Filter. Compare High pass.

Low-Z
See Low impedance.

Lower mid-range
Describes an audio frequency at the lower end of the mid-range frequencies, typically between 250 and 800 Hz. Often abbreviated to 'LMID' or 'LM'. Compare Higher mid-range.

Lows
Another name for bass frequencies. Compare Highs.

LPCM
An abbreviation for 'linear PCM'.

LPDA
An abbreviation for 'log-periodic dipole antenna'. A type of antenna that is sometimes used with receivers for radio microphones or with transmitters for in-ear monitoring (IEM) systems. Its directional behaviour provides some gain in the forward direction and (when receiving) provides some rejection of interference in the opposite direction. Transmit antenna gain can be particularly useful for improved signal integrity in IEM systems, because IEM receivers are usually non-diversity types.

This type of antenna is sometimes constructed in the shape of a paddle-shaped or fin-shaped plate, and these types are therefore sometimes referred to as 'paddles' or 'shark fins'. Note, however, that some paddle-shaped antennae are omni-directional types, not LPDA ones. See also Antenna distribution unit. Compare Helical antenna.

LPF
An abbreviation for 'low pass filter'.

LR, LR-4
See Linkwitz-Riley.

LRC
See LCR (1).

LSF, LSOF, LSHF, LSOH, LSZH
An abbreviation for 'low smoke and fume'. Describes a cable that emits little smoke or hazardous fumes when exposed to extreme heat or to fire. Such cables are sometimes specified for permanent PA installations in certain types of building. Some types of cable may be referred to as LSOH, LSZH or OHLS (low smoke, zero halogen) or LSHF (low smoke, halogen-free), or as 'plenum cable'. However, as considerable differences exist between the smoke and fume emissions from different types, always be sure to install an appropriate type of cable for the situation concerned. See also LFH.

LTE
An abbreviation for 'long term evolution', a standard for public wireless data communications that uses fixed base-stations connected to the public telecommunications network. It has versions known as '3G', '4G' etc. Care must be taken to avoid possible interference between LTE equipment and equipment used in PA systems, particularly wireless equipment such as radio microphones and in-ear monitoring (IEM) systems.

LTP
In stage lighting, an abbreviation for 'latest takes precedence'. This refers to the method used to determine the DMX control values sent by lighting desks to fixtures such as moving-head lanterns, as the more usual 'highest takes precedence' (HTP) method would not be applicable to control parameters such as pan, tilt and colour-wheel. In the LTP method, the most recent value determined by the desk is always output, regardless of whether it is higher or lower than any other values that might be concurrently relevant for the same DMX channel. Compare HTP.

Lumen, Lumens
The unit of measurement of total perceived light output from a source, or present within a defined beam. More strictly, it is called the 'luminous flux', and is abbreviated to 'lm'. Scientifically speaking, a value in lumens is determined by weighting the overall spectrum of the electromagnetic radiation emitted by the source in accordance with the spectral response of an average human eye.

This unit is commonly used to specify the light output of video projectors. As there are several possible ways to make the measurement, the method laid down by ANSI is often used, and values measured this way are usually quoted in 'ANSI lumens'.

The amount of illumination (strictly, 'illuminance') provided onto a surface from a given number of incident lumens, uniformly distributed, will depend upon the area of the surface and is measured in lux. How bright the surface appears to be as a result, when viewed from a particular direction, depends on the light returned from the screen in that direction per solid angle, and is measured in nits. This value will depend on the screen's optical properties, particularly the amount of reflection and the extent of scattering that it provides. The effect that a front-projection screen has on the contrast (i.e. white to black ratio) of a projected image will depend on the amount of light returned from the screen's surface in a given direction as a result of the projected light, as compared with the amount it returns in that direction from other sources of incident light.

Luminaire
See Lantern.

Luminance (1)
The part of a composite video signal that carries the monochrome (i.e. black and white) information, that is, the brightness information. Abbreviation 'Y', thought to be because of its appearance being indicative of converging beams of the three additive primary colours (red, green and blue), whose total illuminating strength equals the value of luminance. See also S-video, Grey scale and IRE. Compare Chrominance.

Luminance (2)
The apparent brightness of an illuminated or light-emitting surface, when viewed from a given direction. In other words, the amount of light returned or emitted from one square metre of the surface in that direction, per solid angle. In the case of an illuminated surface, its luminance depends on the amount of light falling on the surface per unit area (measured in lux) and upon the optical characteristics of the surface. For more information see Nits, the (unofficial) unit of luminance.

Lux
The unit of measurement of illumination (strictly, 'illuminance') provided onto a surface, or returned from a surface. The illuminance, in lux, provided by a given total amount of incident light on a particular surface (measured in lumens), uniformly distributed, is given by the number of lumens divided by the illuminated area of the surface (in square metres). For example, 2000 lumens uniformly illuminating an area of 4 m2 will provide an illuminance of 500 lux. Abbreviated to 'lx'.

LV
An abbreviation for lead vocal(s). Compare BGV and BV.

lx
An abbreviation for lux.

LX
Originally an abbreviation for 'electrics' ('LectriX). However it now refers to all aspects of stage and auditorium lighting, due to the extensive electrical supply cabling and equipment that are involved with lighting. May be pronounced 'lex'.

LX tape
Electrical insulating tape, usually made from PVC. The name 'LX tape' derives from its use by lighting engineers and electricians (see the previous definition). For further details see PVC tape.

LXQ
An abbreviation for 'lighting cue'. See also LX and Q (5).

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