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Glossary of PA Terms - 0-9

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A comprehensive audio terminology reference, the PAforMusic sound engineering glossary provides alphabetically-listed definitions for over 2680 PA-related terms, many including extended explanations. Some other entertainment industry technical terms are also included, e.g. stage lighting and video terms. If you're not sure what term you are looking for, then the structured list of topics on the Training page may be a useful place to start, as there are links from there to many of the glossary entries.

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  • If you have arrived here from a search engine, or by clicking on an alphabet letter on another page of the Glossary, then click on your required term in the list below.

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.

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.

As PAforMusic is a UK-based site, this glossary uses British spelling and the definitions reflect British usage and engineering practice. However where differences exist between British and American terminology, they are generally noted in the relevant entries. The most significant differences are listed under American terminology.

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 (and then use your browser's 'Back' button to return to where you were).

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.

0 dB * 0HLS * 0 V * 1-legged * 1 microphone technique * 1 note bass * −10 dBV * 19 inch rack system * 100 volt line * 1080i * 1080p * 2-pi space * 2-TK * 2-TRK * 2-track * 2-way * 2-wire line * 24/48 * 24/96 * 24/192 * 3 dB point * 3-phase * 3-to-1 rule * 3-way * +4 dBu * 4 mm connector * 4-pi space * 4-wire line * 4K video * 5.1 * 57 * 58 * 500 series * 5th * 5th-order * 7.1 * 7.2 * 70 volt line * 720p * 75 ohm interconnection * 8K video

0 dB (1)
When applied to the level of a signal, 0 dB refers to the agreed reference level of that signal. Although this may be referred to as the signal's 'zero level', it does not mean the complete absence of any signal. When 'dB' is followed by other characters, those characters usually indicate the applicable reference level. Common examples of such reference levels are:

For more detailed information on the above, see the Decibels page. See also Metering.

0 dB (2)
When applied to the setting of a level control such as a fader, 0 dB refers to the reference setting of that control. On some level controls of some equipment, this is the setting at which the relevant section of the equipment provides unity gain. For further information see the Decibels page.

0 dB (3)
When applied to the setting of an EQ control (including a band of a graphic equaliser), 0 dB refers to the setting that has no effect on the signal being equalised (usually the mid-position of the control). See also Flat (1). For further information on decibels see the Decibels page.

0 dB (4)
When indicated on a level meter, 0 dB indicates a signal level that is equal to the relevant reference level at the point of metering. Where a meter indicates the level being supplied from a line-level output, be aware that different metering reference levels are used by different equipment. Consult your equipment handbook to check what the output metering reference level is for your particular equipment. A list of the most common reference levels used is given below.

  • +4 dBu − Most professional equipment
  • 0 dBu − Some professional equipment (e.g. some Mackie mixers)
  • −10 dBV − Most consumer/amateur equipment and some semi-pro equipment
  • 0 dB FSDigital equipment

For more detailed information on the above, see the Decibels page. See also Metering, PPM and VU.

See LSF.

0 V
A label for a terminal that is connected to a zero-voltage reference point, usually in relation to a low-voltage power connection but sometimes referring to a signal earth. The precise meaning should be determined from the relevant equipment's user manual before making connection.

See One-legged.

1 microphone technique
See One microphone technique.

1 note bass
See One note bass.

−10 dBV
See Line level.

19 inch rack system
A metal (usually steel) mounting system for equipment. Each item of equipment is secured to the rack by bolts through flanges (or 'ears') at the left and right edges of its front panel. The bolts are usually 15 mm M6 types. Plastic washers are often used under the bolt-heads, to protect the flanges from damage. The arrangements for the bolts to screw into vary between different types of rack, and include:

  • removable caged nuts which clip into holes in the vertical strips located at each side of the front of the rack. The nuts may be installed, as required, at fixed intervals in order to accommodate equipment of differing front panel heights.
  • nuts that are captive in slots incorporated in the vertical strips. The nuts are free to slide up and down, in order to align with the holes in the flanges of the equipment to be fitted.
  • threaded steel bars located behind, or incorporated into, the vertical strips. These allow the insertion of bolts at any of the standard positions.

All front panel heights are a multiple of 1.75 inches (approximately 44.5 mm), referred to as a vertical unit or U. The total width of the equipment front panels (including the flanges) is 19 inches (482 mm), which is the reason for the name given to this racking system.

100 volt line
A particular type of arrangement for the connection of passive speakers to the power amplifier driving them, specifically developed to meet the requirements of public address applications. This arrangement is able to accommodate a relatively large distance between the speakers and the amplifier (sometimes up to several hundred metres), while avoiding significant power losses resulting from the resistance of the interconnecting cables. It also allows the connection of a large number of speakers to each amplifier output.

It works by supplying a substantially higher voltage to the speakers than is typically used in the low impedance speaker interconnections used in conventional PA applications. This means that the current flowing in the speaker cables (known as lines) is much reduced, resulting in much less power loss in them. In the UK the nominal voltage used is 100 volts RMS. In the USA the value is 70.7 volts RMS, and the system there is known as a '70 volt line' or as a 'constant voltage' system.

A system of this type requires a specific type of power amplifier providing one or more 100 volt outputs, appropriate cabling suitable for the high voltages present, and speakers that accept a 100 volt line input. These speakers (which usually have a relatively low power rating) are equipped with a transformer which steps the voltage down to a value suitable for the driver(s). The transformer often has several taps, which are usually marked with power ratings such as 5 W, 10 W, 15 W and 20 W. The effect of selecting a higher-power tap is that a lower impedance is presented to the line by the speaker, and so a larger current is drawn from it. The speakers are always connected in parallel to the line, which is often looped from speaker to speaker. The total connected load (usually specified in watts) must not exceed the drive capability of the amplifier, or damage to the amplifier may result.

This arrangement is generally used only in public address installations (whether fixed within buildings or for outdoor sports events and the like), where a large number of low-power speakers, located over a wide area, are to be connected to a centrally located amplifier. Historically, these types of speakers were often designed for cost-effective reproduction of voice frequencies only (rather than music), with maximum efficiency being given higher priority than sound quality. Therefore, in the past 100 V line systems have gained a reputation for poor audio quality. However, systems providing much improved audio quality are now available.

The term 'constant voltage' derives not from the output voltage of a particular amplifier of this type being constant in use, but from the fact that, in contrast to 'normal' (i.e. low impedance) amplifiers, two amplifiers of this type that differ in their power output capability do not differ in their maximum voltage output − their difference in maximum power output is accounted for solely by a difference in the amount of current that they are able to supply. This means that, when additional speakers need to be added to a system, the amplifier can be upgraded to a higher-powered model without affecting the power level supplied to the existing speakers. See also Installation speaker.



2-pi space
See Half space.

2-TK, 2-TRK
An abbreviation for 2-track.

An alternative term for stereo. A name often given to a stereo input on a mixer, intended for a playback connection from a stereo audio player (e.g. a CD player or a digital storage and playback device). Such inputs are often labeled '2-TK' or '2-TRK'. On older mixers this input may be labeled 'Tape In' or 'Tape PB', as it was intended for playback from a tape deck. Typically these are unbalanced inputs, often using phono connectors and intended to accept 'low line level' signals, e.g. nominally at −10 dBV. 2-track inputs are often equipped with a reduced set of facilities (especially as regards equalisation and routing) as compared to a channel strip.

Describes a speaker equipped with two different types of driver, each handling a different frequency range. Typically, the two types of drivers will be woofers to handle the low (bass) frequencies and horns to handle the high (treble) frequencies. Such speakers usually include a passive crossover to direct the bass and treble frequencies to the appropriate drivers, but may also be equipped with separate inputs to allow bi-amping.

Similarly, a speaker that incorporates three different types of driver, handling three different frequency ranges (low, mid-range and high) is described as a 3-way speaker. See also Concentric.

2-wire line
In a duplex communications system (e.g. an intercom) or point-to-point communications link, an arrangement in which two conductors (one pair) form a single circuit (usually balanced) that is able to convey signals (usually speech) in both directions, usually simultaneously. At each station (point of connection), a 2-wire line requires an interface unit incorporating a hybrid in order to separate the two directions of communication. See also Comms, Clearcom and Party-line. Compare 4-wire line.

24/48, 24/96, 24/192
Identifies a digital audio bit-stream, or stored data, with a bit depth of 24 bits and a sampling frequency of 48, 96 or 192 Kbit/s. Such identifiers may for example be used to indicate the bit depth and sampling frequency used during an analogue to digital conversion process, or those used during digital signal processing.

3 dB point
See Cut-off frequency.

See Phase (3).

3-to-1 rule
The rule-of-thumb that recommends that when two or more microphones are used to pick up different sound sources, the distance between the microphones should be at least three times the distance between each microphone and its respective sound source. Or, to put it another way, each microphone's distance from its respective sound source should be no more than one third of the distance that the mics are from each other.

This is to ensure that each sound source is picked up predominantly by just one microphone, and so minimise the extent of undesirable comb filtering effects which can occur when a sound source is picked up at similar levels by two or more microphones whose outputs are subsequently mixed. See also Microphone technique.

See 2-way.

+4 dBu
See Line level.

4 mm connector
A single-pole connector sometimes used (in pairs) for speaker connections, so-named because the nominal diameter of the male prong is 4 mm. Usually the female connector can be unscrewed by hand for alternative use as a binding post. The male connector is commonly referred to as a banana plug.

Some 2-pole speaker connectors employ a pair of 4 mm prongs mechanically linked; in this case the prong centres are normally 0.75 inches (approximately 19 mm) apart.

4 mm connectors are now deemed unsafe for use in a domestic environment because they will fit into unshuttered European-type mains outlets (e.g. Schuko), which would result in a serious shock hazard. The replacement type of connector for domestic speakers is the BFA connector, though binding posts (used with cable conductor ends that are either fitted with spade terminals or simply stripped bare) are far more common. See also Speakon.

4-pi space
See Full space.

4-wire line
In a duplex communications system (e.g. an intercom) or point-to-point communications link, an arrangement in which four conductors (two pairs) form two circuits (usually balanced) − one to convey the signals (usually speech) in one direction and the other to convery them in the other direction, usually simultaneously. Note: Do not confuse with the use of star quad cabling, which has an entirely different purpose and application. See also Comms, Clearcom and Line. Compare 2-wire line.

4K video
This term may refer to a number of different ultra high definition (UHD) video resolutions having approximately 4000 pixels of horizontal definition. See also 8K video. For a list of common resolutions and their aspect ratios, see VGA.

The multichannel digital audio encoding scheme developed by MPEG to provide a surround sound capability for films in many formats, including DVDs, tape and celluloid. Also used in some high definition television (HDTV) broadcasts. Alternatively, it may refer to proprietary encoding schemes serving the same function, most commonly those developed by Dolby® or by DTS®. It incorporates five full-range (20 Hz to 20 kHz) channels, for the left front, right front, centre front, left rear and right rear speakers, and one sub-bass (20-120 Hz) channel, hence the designation '5.1'. A 4-pole 3.5 mm jack connector is sometimes used for 5.1 digital connections. See also 7.1, 7.2, LFE channel and AC-3.

A slang term for a Shure SM57 microphone.

A slang term for a Shure SM58 microphone.

500 series
A range of plug-in audio processing modules, available from many manufacturers, that are compatible for insertion into the slots of rack systems designed to accept modules complying with the 500 series standard. The standard was originally developed by API®.

See Fifth.

See Order.

A multichannel digital audio scheme similar to the 5.1 scheme but with the addition of 'Left Side' and 'Right Side' channels.

A multichannel digital audio scheme similar to the 7.1 scheme but with support for two sub-bass (LFE) channels. Note that the presence of two LFE channels does not provide stereo sub-bass, as there is no positional information available at these frequencies. See also Stereo image.

70 volt line
See 100 volt line.


75 ohm interconnection
See Impedance-matched.

8K video
This term may refer to a number of different ultra high definition (UHD) video resolutions having approximately 8000 pixels of horizontal definition. See also 4K video. For a list of common resolutions and their aspect ratios, see VGA.

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This page last updated 02-Oct-2017.