www.paformusic.info Glossary of PA Terms - A

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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.

A * A & E specifications * A cappella * A-B pair * A-B test * A-B-X test * A/D convertor * A-gauge jack * A-taper * A-type jack * A-weighting * A/D * A1 * A2 * A3 * A3D * AAC * AB test * ABS * Absolute time code * Absorption * ABTT * ABX test * AC * AC-3 * AC earth * AC ground * Academy curve * Accelerated-Slope * Accelerator Acceptance angle * ACOP * Acoustic * Acoustic feedback * Acoustic treatment * Acoustic wadding * Acoustics * Acquisition time * Active * Active antenna * Active crossover * Active fader * Active cross-fader * Active DI box * Active pick-up * Active monitor * Active speaker * Active wedge * ActiveX * ADA * ADAT * ADAT optical * ADC * Address * Addressing * Admittance * ADPCM * ADS * ADS-RSL * ADSR * ADT * ADU * Advanced audio coding * AES * AES/EBU * AES-X246 * AES10 * AES24 * AES3 * AES3-id * AES3-ie * AES42 * AES50 * AES59 * AES67 * AETTI * AF * AFCI * AFD * AFDD * AFILS * AFL * AFS * AGC * Air * AirPort * ALCONS * ALD * Algorithm * Aliasing * Alignment level * Alkaline battery * All-pass filter * Alt (or ALT) * Alternating current * AM * Ambience * Ambient field * Ambient microphone * Ambient noise * Ambient sound * Ambisonics * American terminology * American thread * AMF * AMI-C * Amp * Amps * Ampere * Amperes * Ampere-hour * Ampere-hours * Amphitheatre * Amplification * Amplifier * Amplifier sensitivity * Amplitude * Amplitude modulation * Analog * Analogue * Analogue mixer * Analogue to digital conversion * Analysis microphone * Anechoic * Angle of acceptance * Anode * ANSI * ANSI Lumens * ANSI S4.40 * Antenna * Antenna distribution amplifier * Antenna distribution unit * Antenna gain * Antenna splitter * Anti-aliasing filter * Anti-phase * Anti-shock mount * AoE * AoIP * AP connector * AP[n] * APA * APD * App * Apparent power * Application * Apron * Armoured cable * Array * Artefact * Articulated array * Articulation * Artist (or Artiste) * Art-Net * ASCII * ASIO * ASM * ASPEC * Aspect ratio * Assign switches * Assistive listening * Asymmetric response * Asynchronous * ATC * ATM * Atmospheric conditions * ATRAC * ATS * Att * Attack * Atten * Attenuation * Attenuator * Audacity * Audio * Audio chain * Audio group * Audio spectrum * Audio taper * Audio-frequency * Audiofool * Audiogram * Audiology * Audiophile * Audiophool * Auditorium * Auditory filter * Aureal 3D * Auto-ducking * Autoformer * Automatic double-tracking * Automatic gain control * Automatic mic mixer * Automation * Autotransformer * Aux * Aux master * Aux send * Auxiliary * AV * AVCD * AVD * Average level * Average power * AVIXA * AVL * AVO * AVoIP * AWG * Ax * Axes * Axial * Axial mode * Axis * Azimuth

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. For some notable differences between USA and UK terminology, see 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).

A

An abbreviation for 'ampere'.

A & E specifications

An abbreviation for 'architectural and engineering specifications'. The specifications relating not only to the technical performance of an item of equipment, but also to its physical characteristics such as size, weight, colour, weather resistance, mounting method(s), surface finish, etc. Usually provided for installation speakers and similar equipment.

A cappella

Singing without any musical instruments; just voices.

A-B pair

A pair of microphones positioned some distance apart (usually between 2 and 5 metres). This arrangement is used for stereo recording or for live broadcast purposes (one microphone for the left side, and one for the right), not for PA applications. The principle of this method is based on the fact that sounds will arrive at each of the microphones at slightly different times, depending on the direction of the source of each sound. (There will also be differences in the amplitude of sounds arriving from different directions, but if the distance of the microphones from the sound sources is substantially greater than the distance between the microphones then such differences will be small.) This method is sometimes called 'time difference stereo'. See also Speed of sound, Inverse square law, and Microphone technique. Compare X-Y pair, Mid-side pair, ORTF and NOS (1).

View a pictorial comparison of stereo microphone techniques.
(To view the image full-size in Explorer, hover your mouse over the image and click on the green 'expand' icon that appears in the bottom right-hand corner. Or, click when a magnifying glass containing a '+' appears.)

A-B test, AB test

A test in which two makes or models of similar equipment (referred to as items 'A' and 'B') are evaluated by comparing their performance with each other. Often the comparison is made simply by supplying the two items of equipment with an identical source signal, and listening to the resulting sound as each item of equipment in turn is switched into the signal chain. Particular care must be taken to ensure that the settings of the equipments' controls (e.g. level controls) do not unfairly favour one of the items. See also Blind test, Double blind test and the next definition.

A-B-X test, ABX test

A test in which two makes or models of similar equipment are evaluated by comparing their performance with each other in a manner that aims to exclude prejudice on the part of the person carrying out the evaluation. The set-up is similar to that of an A-B test, but includes the facility to select one of the two items without that person knowing which one it is. This method may be used for both objective and subjective tests. Typically, at each stage of the test the evaluator will record an assessment of the selected equipment's performance (which may include a judgement on whether it is believed to be item 'A' or item 'B'). At the conclusion of the test, the item actually assessed at each stage may be revealed, either by means of another person having noted which item was selected at each 'X' assessment, or by the ABX switching equipment having logged this information automatically. See also Blind test and Double blind test.

A/D convertor

See Analogue to digital conversion.

A-gauge jack

See Jack.

A-taper

See Taper.

A-type jack

See Jack.

A-weighting

See Weighting.

A/D

An abbreviation for 'analogue to digital convertor'. See Analogue to digital conversion.

A1, A2, A3

Designations sometimes used to identify the crew members with responsibility for running a PA system, or to identify specific PA-related roles. For details see Sound engineer.

A3D

See Aureal 3D.

AAC

An abbreviation for Advanced Audio Coding, an elaboration of Layer 3 of the MPEG-2 and MPEG-4 specifications which define the compression algorithm for digital audio data. The additional facilities provided by AAC include an improved compression ratio, which may be used to give smaller data files (or a lower bit-rate) for the same perceived audio quality, or an improved perceived audio quality for the same size of data files (or bit-rate). It also provides more efficient encoding of multi-channel audio streams such as 5.1. AAC is available in several different 'profiles'. See also MP3, MP4 and Perceptual coding.

ABS (1)

An abbreviation for 'Absolute time code'.

ABS (2)

An abbreviation for 'acrylonitrile − butadiene − styrene', a type of plastic containing a blend of those three ingredients. This type of plastic can be very strong for its weight and is sometimes used in the construction of equipment enclosures and portable plastic racks and cases.

Absolute time code

Timing information that is carried by the subcode of a digital bit-stream. Primarily used by digital recording systems, to enable any frame of recorded data to be identified by its time from the start of the recording, which is marked as 0 h 0 m 0 s 0 f. Compare SMPTE.

Absorption

The degree to which sound is 'soaked up' by a surface that it hits, or by a medium (such as air) that it travels through, rather than being entirely reflected or transparently propagated. As a general rule, high frequencies are absorbed more readily than low ones.

If a surface is large in relation to the space in which it is situated, its absorption is likely to significantly influence the acoustics of the space, particularly as regards reverberation. For the purpose of comparison, surfaces can be designated with an 'absorption coefficient' (or 'coefficient of absorption') having a value between 0 (meaning total reflection) and 1 (meaning total absorption), but in practice the value usually changes according to the frequency of the sound and the angle at which the sound hits the surface. The total amount of absorption provided by a particular surface, room, etc. is measured in Sabins.

The absorption of sound waves as they travel through air is one of several distance-related loss mechanisms, along with dispersion, the grazing effect, refraction and diffraction. The transmission loss due to absorption alone is heavily dependent upon the combination of air temperature and relative humidity (RH), varying between about 1 dB and 5 dB per 100 metres. At temperatures around 20ºC, a loss of between 3 dB and 4 dB per 100 metres is typical. For very warm air temperatures (greater than about 25ºC), the loss is reduced with increasing RH, because the water droplets help to carry the sound. But at cool air temperatures (less than about 15ºC), the loss is increased with increasing RH. See also Speed of sound.

ABTT

An abbreviation for 'Association of British Theatre Technicians'. See also USITT, ALD and AETTI.

ABX test

See A-B-X test.

AC

An abbreviation for 'alternating current', a current (or voltage) that continually reverses its polarity. Unless otherwise indicated, the value quoted for an AC voltage or current is always an RMS value. "AC" (or "a.c."), without a value being given, is often used as a short-hand term for mains voltage (230 volts in the UK and Europe, 110 volts in the USA) but do not assume this usage as it may also refer to lower (or even higher) voltages. See also Frequency. Compare DC.

AC-3

An abbreviation for Audio Coding 3, a digital audio data compression scheme developed by Dolby and used in DVDs and CDs with 5.1 multichannel audio for 'home theatre' use; also in some high definition television (HDTV) broadcasts.

AC earth, AC ground

See Safety earth.

Academy curve

The response required of equipment to correctly process the analogue optical sound track originally used for film audio, now largely replaced by improved techniques. Also called the 'normal curve' or 'N curve'. See also X curve.

Accelerated-Slope

A type of tone-control circuit (patented and trade-marked by Rane Corporation) with a steep slope, allowing control over bass and treble frequencies whilst minimising unwanted changes to the mid-range frequencies. Compare Baxandall.

Accelerator

In the context of computer hardware, an optional card that may be installed in order to boost the processing performance of the computer, e.g. for graphics or for audio processing (DAW) applications. In the latter case, installation of a suitable hardware accelerator is required in order to run most processing-hungry plug-ins.

Acceptance angle

See Pick-up angle.

ACOP

An abbreviation for 'approved code of practice'. See Code of practice.

Acoustic

Describes something that can be heard. This term is most often used to describe things that make their 'own' sound by 'natural' mechanical means (and so can be heard without the use of amplifiers and speakers). This distinction is especially necessary in the case of musical instruments such as the guitar, piano, etc., which are available in both acoustic and electric varieties. The term is also used to distinguish feedback that involves acoustic coupling (i.e. via sound waves) from types of feedback that do not. Compare Acoustics. See also Semi-acoustic and Audio.

Acoustic feedback

See Feedback.

Acoustic treatment

The installation of one or more means to modify the acoustics of a space, with the intention of improving the experience of the audience, the performers and/or (in a control room) the sound engineer(s). For further information see the next definition.

Acoustic wadding

A soft material used for the absorption of sound. It is typically used inside speaker enclosures to reduce unwanted reflections of sound from the internal surfaces and/or to reduce resonances. It is also used in some acoustic treatments and room constructions, particularly within walls and partitions that are constructed to reduce the transmission of sound through them.

Acoustics (1)

The aspects of a space (e.g. a room, or an outdoor arena) that affect the way that sound sources in the space are heard by the audience or performers present, located at their normal listening position.

The acoustics of a space will also affect the sound that is picked-up by the microphones of PA systems or by microphones that are used for recording or for relaying sound to remote listeners. The effect of the acoustics on the picked-up sound will depend on the number and type of the microphones used, and on how each one is placed relative to the sound sources (see Distance factor). However, in the case of live sound reinforcement, the effect of the acoustics on the sound heard by the audience from the speakers is usually more significant than its effect on the sound picked-up by the microphones.

Often the most significant aspect in the acoustics of a space is the amount and nature of ambience that it provides, particularly the amount and timing of natural reverberation and echoes. For this reason, acoustics are heavily influenced by the dimensions of the space and by the extent and type of the reflection, scattering and absorption provided by the major surfaces enclosing the space and existing within it − e.g. the presence, nature and angle of walls, floors and ceilings, and the presence and nature of carpeting, curtains, soft furnishings, etc.

The acoustics of a space may be deliberately influenced by the introduction of absorbers to provide a deadening effect, and/or by the use of reflectors and diffusers to redirect and to spread the sound around the space, making the acoustics more lively. Alternatively, when using microphones for PA, recording or broadcasting, modified acoustics may be simulated by the use of effects such as reverberation and echo units. See also Refraction, Diffraction, Grazing effect, Haas effect and Mode (2).

Acoustics (2)

The science of sound. In particular, the study of the way in which the properties of spaces affect how sound is propagated and heard within those spaces, and the study of how such properties may be modified to improve the subjective experience of listeners (see Acoustics (1)). The properties concerned are fundamentally the size and shape of enclosed (or partially enclosed) spaces, the characteristics of the space boundaries (walls, ceiling and floor) and the characteristics and location of objects within the space. The properties of whatever gas or fluid fills the space are also of prime importance − typically air temperature, humidity and pressure are the relevant factors. Where there are variations in these quantities, the nature of such variations throughout the space must be considered.

Acquisition time

The time taken for a single sample of a signal to be captured, with a specified degree of accuracy. See Analogue to digital conversion for further information.

Active

Generally describes something containing circuitry that requires a source of electrical power in order to operate, for example an active DI box or an active crossover. In some cases, the term 'powered' is used instead of 'active', with essentially the same meaning. When describing a speaker, 'active' most usually refers to a powered speaker. However, some manufacturers use (or have in the past used) the term to refer to an unpowered full-range speaker that does not incorporate an internal (passive) crossover and which therefore requires an active crossover to be used in conjunction with the amplifiers driving it, or to refer to a switch-selected mode of speaker operation that bypasses the internal passive crossover (see Bi-amping). The opposite of active is passive. See also the following definitions and Powered.

Active antenna

An antenna that incorporates a radio-frequency (RF) amplifier. This has the advantage that the amplification is given only to the signals picked-up by the antenna, and not to any interfering signals picked-up on the cable from it. This advantage is more important when the antenna is distant from the receiver (or from the antenna distribution unit). However, such an antenna requires a supply of DC power (sometimes provided via the RF cable).

Active crossover

See Crossover.

Active fader
Active cross-fader

A fader or cross-fader in which the potentiometer does not directly control the signal level, but rather controls a DC voltage which, after filtering and sometimes other processing, is in turn used to control the signal level by means of a VCA. This approach can reduce mechanically-induced noise and give improved reliability. It also allows the possibility of a selectable taper law. Particularly popular as cross-faders on DJ consoles. See also Motorised fader.

Active DI box

See DI box.

Active pick-up

See Pick-up.

Active monitor

See Powered monitor.

Active speaker

Most usually refers to a powered speaker. However, see Active.

Active wedge

See Powered monitor.

ActiveX

A technology that allows computer software from one company to control equipment from other companies. It works by means of the equipment manufacturer publishing the details of ActiveX-format 'software objects' relating to the equipment, and the software designer then using this information to create the appropriate software.

ADA

An abbreviation for 'antenna distribution amplifier'.

ADAT

An abbreviation for 'Alesis Digital Audio Tape'. A digital audio multi-track tape recording system developed by Alesis and subsequently also manufactured by Fostex and Panasonic. It allows 8 tracks of 16-bit digital audio, at a 44.1 kHz sampling rate (i.e. CD quality) to be recorded on S-VHS video tape. See also Analogue to digital conversion.

ADAT optical

A fibre-optic digital interface originally developed by Alesis for use with their ADAT multi-track recorders. It allows up to 8 independent channels of digital audio to be multiplexed onto a single fibre-optic cable.

ADC

An abbreviation for 'analogue to digital convertor'. See Analogue to digital conversion.

Address

A code, usually numerical, that is used to identify a specific item of equipment, or one of its functions, among several (or many) possible items or functions. Depending on the particular equipment, numerical addresses may be set using decimal, binary or hexadecimal codes. On items of equipment such as DMX lighting fixtures, the addresses (in this case usually channel IDs) are commonly set up using DIP switches. See also the next definition.

Addressing

The process of allocating and setting up one or more addresses of items of equipment, in the specific manner required for a particular system.

Admittance

A rarely encountered term meaning the inverse of impedance.

ADPCM

An abbreviation for 'adaptive differential pulse code modulation'. A technique for data compression that operates by using the fact that consecutive samples are usually very similar. This means that less information is required to describe the differences between each sample and the previous one in the train, than is required to describe each and every sample individually.

ADS

An abbreviation for 'audio distribution system'. This generally refers to systems that use radio transmission to provide real-time one-way speech communication from a single person to several other people in the near vicinity, such from an official or a commentator at a sports event, or from a translator at a conference centre, to specific other personnel at the relevant event. Such transmissions are not intended for reception by the public and require special transmission and reception equipment. In the UK, VHF frequencies such as in the band 60.75 to 62.75 MHz are typically used, however note that the specific frequency/frequencies to be used must be licensed for this purpose in advance (see the next definition).

ADS-RSL

An abbreviation for 'audio distribution system − restricted service licence', a UK radio transmission licence that is specifically limited to appropriate ADS use, as defined by Ofcom.

ADSR

An abbreviation for 'attack, decay, sustain, release', the four parameters commonly used to define the envelope of musical notes produced by electronic sound generators − typically played using a piano-type keyboard. In this context the parameters operate as follows:

• Attack − Controls the time taken for the sound level to build up to its initial maximum value, after the note is struck (e.g. the key is pressed). This is often a very short amount of time.
• Decay − Controls the time taken for the sound level to reduce from its initial maximum to its steady sustain level (see the next bullet point), assuming that the key is still held down.
• Sustain − Controls the level that is continuously produced after the decay time has finished, while the key continues to be held down.
• Release − Controls the time taken for the sound level to reduce from its current level to zero (i.e. silence), from the moment that the key is released. This applies regardless of whether, at that moment, the note had reached its sustain phase or was still in its attack or decay phases.

An initial 'punch' effect to a note is achieved by a fairly short decay time and a sustain level significantly below the level achieved at the end of the attack time. As the sustain level is increased towards the level achieved at the end of the attack time, or the decay time is increased, the initial 'punch' effect is diminished. Sounds similar to those of struck or plucked acoustic string instruments, such as the piano and guitar, can be synthesised using a short attack time, a long decay time, a zero value of sustain (because such notes do not last forever when the key is held down), and a fairly short release time.

Sometimes an additional 'hold' parameter is included − the exact purpose of this varies between equipment, but commonly it controls the time between releasing the key and the commencement of the release time, in which case, during the hold time, the note continues at its current level.

Occasionally the term is used to describe the envelope of notes created by natural means, such as by an acoustic instrument.

ADT

An abbreviation for 'automatic double-tracking'. A type of delay effect which gives the impression of there being two identical sources of the signal passed through it, especially when the original and the delayed versions of the signal are placed at different positions in the stereo image. A typical setting of delay for this effect is in the region of 10 to 30 ms. Mostly used in studio recording work.

ADU

An abbreviation for 'antenna distribution unit'.

Advanced audio coding

See AAC.

AES

An abbreviation for 'Audio Engineering Society'. Their website is: http://www.aes.org. See also PLASA, ISCE and EBU.

AES/EBU

See AES3.

AES-X246

The AES standard on the application of RJ45-type connectors and quad twisted pair cable for audio interconnections. See also QTP.

AES10

An AES specification for multi-channel digital audio connections, typically between a mixer and a multi-track recorder. It carries up to 56 audio channels over two 75 ohm unbalanced screened cables, or over two optical fibres, one of which carries the data and another the bit clock. Like AES3, it can accommodate a bit depth of up to 24 bits at sampling rates of up to 48 kHz. The information bit-rate is fixed at 100 Mbit/s regardless of the sampling rate, though the 4:5 line-encoding scheme increases the line rate to 125 Mbit/s. Also known as MADI (multi-channel audio digital interface).

AES24

A standard for the control of audio equipment via computer networks, specified by the Audio Engineering Society. Previously known as SC-10.

AES3

A standard for synchronous 2-channel digital audio data interconnections using screened balanced cable with a characteristic impedance of 110 ohms and terminated in 3-pin XLR connectors, having a maximum length of about 100 metres. The nominal signal level is 5 volts peak-to-peak (terminated). The standard is specified by the Audio Engineering Society, and is also known as ANSI S4.40 or IEC60958-4. It was previously known as AES/EBU. The consumer version of this standard is S/PDIF (also known as IEC60958-3).

The AES3 standard can accommodate samples of up to 24 bits (per channel). The bit-rate is dependent upon the sampling frequency: for 32 kHz sampling (radio broadcast) it is 2.048 Mbit/s, for 44.1 kHz (compact disc) it is 2.8224 Mbit/s and for 48 kHz (DAT) it is 3.072 Mbit/s. The standard employs bi-phase mark line coding, in order to facilitate bit-clock recovery at the destination end, to remove any DC component of the signal and to make the interconnection polarity-insensitive. This means that the interface operates at twice the above rate, because two consecutive logical states are required on the line to represent each data bit.

The standard also accommodates subcode information bits, which may be used to convey information about the audio content. Together with synchonisation information and other miscellaneous bits, this makes up a total of 64 bits per 2-channel frame.

Note: Although microphone cables have the right physical connections for AES3 links, their characteristic impedance is not adequately controlled and so should not be used for digital links unless they are extremely short (preferably no more than 0.5 metres). See also AES3-id, AES3-ie and Analogue to digital conversion.

AES3-id

As AES3, but using screened unbalanced cable with an impedance of 75 ohms and terminated in BNC connectors, having a maximum length of about 100 metres without cable equalisation but up to 1000 metres with appropriate cable equalisation and signal restoration (depending upon circumstances). The nominal signal level is 1 volt peak-to-peak (terminated).

AES3-ie

As AES3, but using an optical interface. Warning − laser radiation hazard: never look into an un-mated optical fibre connector.

AES42

A digital audio interface standard for microphones. The standard specifies a balanced AES3 digital signal format and a phantom powering voltage of +10 volts ±0.5 volts able to supply at least 250 mA, referred to as 'DPP' (digital phantom power). Control codes may be sent to the microphone by means of +2 volt pulses added to the phantom power voltage. These codes may be used to set the microphone's operating parameters, e.g. sensitivity, sampling frequency, etc.. The digital output signal from the microphone is timed from the microphone's own word clock; this may be free-running ('Mode 1' operation) or brought into sync by specific control codes ('Mode 2' operation).

AES50

A standard for passing real-time multi-channel digital audio over point-to-point Ethernet connections using CAT 5 UTP cable, with a very low latency. It is typically used for the interconnection of digital stageboxes with digital mixers. See also Digital multicore. Compare AES67 and Dante.

AES59

A standard for the pin allocation of DB-25 D-sub connectors for 8 channels of balanced analogue or digital audio signals, alternatively known as the DTRS standard.

AES67

A standard for passing real-time digital media such as multi-channel digital audio over internet protocol (IP) networks. Compliance with AES67 by proprietary audio-over-IP (AoIP) systems such as RAVENNA and Dante allows audio streams to be conveyed between such systems − albeit without the availability of all the proprietary features offered by each system. For further information see the RAVENNA website at www.ravenna-network.com (this link opens in a new tab or window). Compare AES50.

AETTI

An abbreviation for 'Arts and Entertainment Technical Training Initiative', a UK organisation concerned with the development of qualifications in sound and lighting for theatre technicians. See also ABTT.

AF

An abbreviation for 'audio-frequency'.

AFCI

An abbreviation for an 'arc fault circuit interrupter', the American term for an arc fault detection device − see AFDD.

AFD

An abbreviation for an 'arc fault detection', a facility provided to enable the detection of arc faults in an electrical power distribution system, and to rapidly cut off the supply to the protected circuit(s) before a fire is started. This facility is provided by an arc fault detection device (AFDD) − see the next defnition.

AFDD

An abbreviation for an 'arc fault detection device', a device whose purpose is to detect the typical current characteristics of an arc fault in an electrical power distribution system, and to rapidly cut off the supply to the protected circuit(s) before a fire is started. The American term for this class of devices is 'arc fault circuit interrupter', abbreviated to AFCI. A fuse or MCB is unable to provide this function in cases where the arc current is too small to operate the relevant device. An RCD is unable to provide this function in cases where the arc occurs between the Live and Neutral conductors. However, some AFDDs incorporate the function of an RCD and/or an MCB within the same device.

AFILS

An abbreviation for 'audio-frequency induction loop system'. See Induction loop.

AFL

An abbreviation for 'after-fade listening'. A facility of a mixer, allowing the operator to listen, via headphones, to the sound being handled by an individual channel, selection of channels, or group, taking into account the setting of the channel or group fader(s) (and sometimes also the pan control). This means that when listening to several channels, they are heard at the same relative levels as they will have in the main mix.

The aux send AFLs of a mixer that is used as a monitor mixer should be post-insert, so that the effect of any outboard processing (typically graphic equalisers) patched into the aux send inserts is apparent in the monitor engineer's listen wedge. See also PFL and Solo.

AFS

An abbreviation for 'automatic feedback suppressor'. A device that analyses the signal(s) passing through it, attempts to detect feedback frequencies and sets the parameters of its notch filters so as to cut the offending frequencies. The effectiveness of these devices is much debated, as they are sometimes very helpful but sometimes less so.

AGC

An abbreviation for 'automatic gain control'.

Air (1)

A descriptive term used to refer to an upper frequency range that controls the 'airyness' (or 'lightness') of a sound. Some types of audio equipment provide an 'Air' control to enable adjustment of this aspect of the sound signal being handled. The simpler versions will usually provide a gentle high frequency shelving boost, starting at somewhere between 8 kHz and 12 kHz, but more sophisticated 'air effects' also exist. This term is generally used more in recording work than in PA work, and most often in the context of vocals processing. However, it can also be applied to instruments.

Air (2)

Air Music Technology, a manufacturer of audio equipment, specialising in virtual instruments, music software and effects. Their website is www.airmusictech.com (opens in a new tab or window).

AirPort

See IEEE 802.11.

ALCONS

An abbreviation for 'articulation loss of consonants'. This is a subjective measure of speech clarity, assessed as the proportion of consonants in speech that are likely to be misheard, for given locations of the sound source(s) (direct and/or amplified) and of the listener, in a particular space. It is usually expressed as a percentage value. This measure is a useful way of specifying the required acoustic properties of a space, from the perspective of speech intelligibility. The lower the ALCONS value, the better the intelligibility. Maximum acceptable values of ALCONS range from around 5% to 15%, depending on the application.

To be a meaningful measure of acoustics, it is assumed that the characteristics of the talker's speech (sound level, accent, speed, etc.) and of the listener's hearing and comprehension are not significantly influencing the result − i.e. are such that a figure very close to 0% would be obtained for that talker and listener under ideal acoustic conditions for speech intelligibility. See also Reverberation time. Compare STI.

ALD

An abbreviation for 'Association of Lighting Designers'. See also ABTT and USITT.

Algorithm

A set of rules, precisely specified (often in a published standard), that define a repeatable means to perform some operation towards completing a desired process. In many cases, such as in digital signal processing, the operation to be performed is a sequence of mathematical computations.

Aliasing

A phenomenon that can occur during sampling, whereby spurious frequencies, that did not exist in an original analogue signal, are introduced into the sampled signal. It happens when the original analogue signal contains frequencies higher than the Nyquist frequency, that is, frequencies higher than half the sampling rate used. This can occur either when an analogue signal is converted to a digital one (see Analogue to digital conversion) or when a digital signal or file is processed to reduce its bit-rate or file size by a reduction in sampling rate.

Any frequencies present that are higher than the Nyquist frequency 'fold down' around the Nyquist frequency. This means that, after sampling, any such frequency appears as an alias frequency that is below the Nyquist frequency by the same amount as the original was above it (see the example below). So, the higher the original 'out of bounds' frequency, the lower its alias. As aliasing produces frequencies which are below the original ones, they are (in the case of audio) often more noticeable − in fact the original frequencies may have been inaudible. The effect of aliasing on audio signals is frequently the introduction of an annoying squeaky background 'twittering' artefact ('birdies') which varies in correspondence with the wanted audio but which is not itself intelligible. This is because the folding process effectively removes any intelligibility from the aliased parts of the signal, making the phenomenon highly undesirable (except for use in special effects). Therefore, an anti-aliasing filter is normally employed, to essentially remove any frequencies above the Nyquist frequency before the signal is subjected to sampling.

As an example, if a signal is being sampled at 30 kHz, and contains a frequency at 18 kHz (3 kHz above the Nyquist frequency of 15 kHz), the result will be an alias frequency of 12 kHz (3 kHz below the Nyquist frequency).

Alignment level

See SOL.

Alkaline battery

A type of non-rechargeable battery that has a longer operating life and storage life than the cheaper zinc-carbon types, and so is often used in battery-powered equipment such as bodypacks.

All-pass filter

A filter that has no significant attenuating effect on the signal passing through it, but affects only its phase. The amount of phase-shift introduced will usually be different at different frequencies.

Alt (or ALT)

An abbreviation for 'alternate', or 'alternative'. Usually refers to a facility (often a signal path) that may, if required, be used in place of a 'main' or 'primary' facility of a similar kind. For example, some small mixers provide a rudimentary audio grouping facility by enabling each channel to be routed either to the main mix or to an 'Alt' mix.

Alternating current

See AC.

AM

An abbreviation for 'amplitude modulation'. See Modulation.

Ambience

In the context of acoustics, the additional sound that is heard, or picked-up by a microphone, as a result of reflections and scattering from the surfaces surrounding and within the space, i.e. sound that does not travel direct from the source to the listener. The nature of this additional sound will be characteristic of the space, particularly its dimensions and the nature of the surfaces present, as regards the degree to which they reflect, absorb, or scatter various audio frequencies. The dominant component of ambience is usually reverberation.

The amount of ambience, relative to direct sound, that is heard by a listener, or picked-up by a microphone, is heavily influenced by the distance between the sound source and the listener or microphone, though the effect of changes in this distance depends upon the directionality of the source and of the microphone − see Distance factor for the effect of using microphones having various polar responses. But note that the type and extent of ambience experienced by a listener will also be influenced, to some degree, by the location of the sound source and of the listener (or microphone) within the space.

In more general contexts, ambience can be understood to include non-audio related aspects of a space, such as visual aspects and even smell and touch.

Ambient field

See Diffuse field.

Ambient microphone

A microphone that has been located and directed with the specific purpose of picking-up the acoustic ambience of a space, in preference to the direct sound from any particular sound source(s). Most usually the intention of employing such a microphone is to add its signal to a mix of closely miked instruments and vocals, in order to create a more live-sounding mix for recording or broadcast. Rarely used in live PA work, with the exception that it is sometimes provided for inclusion in in-ear monitoring mixes in order to help reduce disorientation effects, and/or for inclusion in mixes for 'overspill' rooms that are separated from the main auditorium.

Ambient noise

The level of unwanted sound that exists in an area used by an audience and/or by performers; the 'background' sound level of the space. Likely sources of ambient noise include HVAC equipment, equipment cooling fans, audience conversations (and movements, etc.) and sounds from outside the auditorium (such as road traffic noise). Noise levels from PA system speakers are not usually considered to be contributors to ambient noise, and in low-level applications such as speech reinforcement, ambient noise levels may significantly exceed such system noise and may therefore be the dominant factor in the overall signal-to-noise ratio perceived by the audience. See also Auditorium.

Ambient sound

The normal 'background' sound present at a specific location. Or, the sound that exists at that location, prior to the addition of sound from a particular sound source. Ambient sound consists of ambient noise plus the pre-existing sound (if any) originating from intentional sound sources.

Ambisonics

A proprietary surround-sound system, developed in the UK.

American terminology

Historically, there have been differences in some PA and related terminology between the USA and the UK. As a result of the extensive use of some American equipment in the UK, use of some of the American terminology is now fairly common in the UK, whilst other terms remain specific to the USA. Furthermore, as the original UK terms are also still used, some American terms are used alongside their UK equivalents.

The table below lists some American terms and their UK equivalents. The American terms that are generally well understood in the UK are indicated with an asterisk in this table. Please email me with other PA-related American / UK terminology differences, to help to expand this table for the benefit of other users.

Note that confusion may also arise as a result of different spellings and/or pronunciation of the same term. For example, in the USA 'routing' is pronounced "r-owt-ing" whilst in the UK it is pronounced "r-oot-ing". Some spelling differences are given in the second table.

American term	UK equivalent
3⁄32″ jack	2.5 mm jack
1⁄8″ jack	3.5 mm jack
110 volts (as a general reference to mains voltage)	230 volts
60 Hz (as a general reference to mains frequency)	50 Hz
70 volt line (as a general reference to a constant voltage distributed speaker system)	100 volt line
AFCI (arc fault circuit interrupter)	AFDD (arc fault detection device)
autoformer	autotransformer
AWG	CSA (mm²) − see AWG for conversion table)
B supply	HT supply
barrier strip	terminal strip
C clamp	hook clamp
Cinch connector	phono connector
constant voltage system	100 volt line system
cord	lead (or cable)
dBv (now superseded by dBu)	dBu
EFI (earth fault interrupter)	RCD (residual current device) or RCCB
electrical .... (cable / plug / outlet / supply / etc.)	mains ....
foldback*	monitor / monitoring / monitor mix
gauge (AWG value)	CSA (mm²) − see AWG for conversion table)
GFCI (ground fault circuit interrupter)	RCD (residual current device) or RCCB
GFI (ground fault interrupter)	RCD (residual current device) or RCCB
ground*	earth
grounding	earthing
ground isolator*	earth isolator
ground lift switch*	earth lift switch
ground loop*	earth loop
ground spike	earth rod
hot (electrical power conductor)	live; line conductor
howlround / howlback	(acoustic) feedback
intermission	interval
jacket (of cable)	sheath
light tree	lighting boom
lighting instrument	lantern (or luminaire)
lighting pipe	lighting bar (or barrel)
NTSC (as a general reference to composite video signals)	PAL (phase alternating line)
outlet*	socket
patch cord	patch cable
pin jack	phono connector
plate	anode
power* .... (cable / plug / outlet / supply / etc.)	mains ....
power cord	mains lead
RCA connector	phono connector
receptacle	socket
safety ground	safety earth
shield* (of cable)	screen
sleeve (of cable)	sheath
SR	PA
Tech	Techie
TRS jack*	3-pole jack (or 'stereo' jack)
TS jack	2-pole jack (or 'mono' jack)
TT jack	Bantam jack
tube*	valve
turnaround	gender changer
wireless microphone*	radio microphone

* These terms are generally well understood in the UK.

American spelling	UK equivalent
aluminum	aluminium (and different pronunciation)
analog	analogue
center	centre
color	colour
equalize, equalizer, equalization	equalise, equaliser, equalisation
leveler	leveller
optical fiber	optical fibre
polarized, polarization	polarised, polarisation
program (audio or video material)	programme
quantize, quantization noise	quantise, quantisation noise
theater	theatre

American thread

A term for a screw thread that is used in the USA; in PA work it is most commonly used to refer to the 5⁄8 inch diameter (27 threads per inch) thread used for attaching microphone clips (and other accessories such as boom arms) to microphone stands. See also Thread adaptor. Compare Euro thread.

AMF

An abbreviation for 'automatic mains failure', usually referring to power distribution equipment that monitors the normal supply (whether from the mains or a local generator) and implements an automatic switch-over to a standby source (usually a standby generator) in the event of a supply failure. It is sometimes known as ATS (automatic transfer switch), though this may also refer to the actual switching device. See also UPS and Power conditioner.

AMI-C

An abbreviation for 'Automotive Multimedia Interface Collaboration' (for those interested in mobile entertainment systems).

Amp, Amps

An abbreviation for either amplifier(s) or ampere(s).

Ampere, Amperes

The unit of current, frequently abbreviated to 'amp' (but beware confusion with the more prevalent abbreviation of 'amplifier' to 'amp'). For those with a scientific interest, the number of amperes represents the number of Coulombs of electrical charge that are passing per second.

Ampere-hour, Ampere-hours

A unit for specifying the charge-storage capacity of a battery. Note that the amount of electrical energy stored by a battery depends not only on the amount of charge stored but also on the voltage of the battery.

Amphitheatre

Originally (long before the advent of electronic amplification), an outdoor auditorium that is specifically designed and constructed so as to enable a reasonably large audience to adequately hear performers or presenters, or else which makes use of a natural geological feature (sometimes artificially enhanced) in order to provide the same effect. However, in most cases a fairly loud voice (or other sound source) is a requirement. In modern times there is no reason why amplification should not be additionally employed in such venues, however special considerations may apply. Amphitheatres are most commonly provided with a circular or semi-circular banked audience seating arrangement. The American spelling is 'amphitheater'.

Amplification

The process of increasing the level of a signal or a sound. The equipment or circuitry that performs this function is an amplifier. The amount by which the level is increased is termed the gain. An item of equipment that includes amplification circuitry (as a secondary function) is sometimes described as 'active'. Compare Attenuation.

Amplifier (Amp)

In general, any equipment, or internal part of equipment, whose main purpose is to increase the level or the load-driving capability of a signal. Therefore the term may be used to refer to any of the following:

• A power amplifier − this meaning is the most common in live sound engineering.
• A combo − this meaning is the most common among musicians.
• A head.
• A distribution amplifier.
• A pre-amplifier.
• A repeater.
• An induction loop amplifier.
• Internal circuitry that provides an increase in signal level (called gain) within a more complex item of equipment (such as a mixer). Such circuitry is also known as a 'gain stage', and operates using transistors, valves or integrated circuits.

N.B. 'Amp' is also an abbreviation of 'ampere', the measurement unit for electrical current, though this is not the usual meaning of 'amp' in the context of PA systems.

Amplifier sensitivity

See Sensitivity.

Amplitude

See Level.

Amplitude modulation

See Modulation.

Analogue, Analog

In PA work, usually describes a signal whose instantaneous voltage varies in direct proportion to the instantaneous air pressure variations constituting the sound wave represented by the signal. Or, describes audio equipment that processes or carries such a signal, or a control or indicator that operates in a continuous fashion − i.e. smoothly rather than with defined steps.

Similarly, for other types of signal such as video or lighting control, an analogue signal is one whose instantaneous voltage directly corresponds to image content or to light levels respectively.

In general, the term 'analogue' describes any arrangement in which the amount of a quantity is directly and continuously represented by, controlled by, or indicated by the amount of some other quantity. Note that the American spelling is 'analog'. Compare Digital.

Analogue mixer

A mixer whose basic functions of mixing, equalisation etc. are performed by analogue circuitry. For example, the faders are potentiometers directly carrying the relevant analogue signal, and mixing is achieved by simple signal voltage addition. However, some analogue mixers incorporate one or more effects units that function by means of Digital signal processing. Compare Digital mixer.

Analogue to digital conversion

The process by which an analogue signal is converted into a digital one conveying essentially the same information. There two stages to this process, sampling and coding. Samples of the analogue signal are taken at a regular rate (the sampling frequency), and each sample is assigned a numeric value based on its magnitude, according to some agreed rules (the coding scheme). The number is expressed in binary form, and the digits of it are placed into the bit-stream of the digital signal being generated, together with any extra bits necessary to carry other information.

There is an inevitable degradation introduced as a result of the assignment of discrete numeric values to a continuously variable quantity − this is called quantisation noise. However, by using an adequate number of bits to represent the value of each sample, the size of the steps between adjacent discrete levels (and therefore also the quantisation noise) is reduced, enabling the magnitude of each sample to be digitally expressed with sufficient accuracy and resulting in a good quality of the final sound. The number of bits used for each sample is often referred to as the 'bit depth' or the 'sample width'. (Compact disc digital audio uses a bit depth of 16, i.e. 16 bits are used to represent the value of each sample of each channel of the stereo signal.)

The sampling frequency must be at least twice the highest frequency of interest in the original analogue signal, but usually the actual sampling frequency used is somewhat higher than this. The highest frequency that can be properly sampled (i.e. half the sampling frequency) is called the Nyquist frequency, after the person that discovered this. It is important that the signal contains no frequencies above the Nyquist frequency (at any significant level), in order to avoid aliasing, therefore an anti-aliasing filter is used to drastically attenuate the level of any such frequencies.

As an example, consider the conversion of an analogue signal into an S/PDIF digital signal. Given that the highest frequency of interest is 20 kHz, the analogue signal must be sampled at least 40,000 times per second. In practice, considering the compact disc stereo audio format, a rate of 44.1 kilo samples per second is used, referred to as 44.1 kHz sampling. (This strange figure derives from the early days of digital audio, when VCRs were used for recording purposes.) The anti-aliasing filter must allow through signals up to 20 kHz without any significant attenuation, but must effectively block signals above 22.05 kHz. Each sample is converted into a 16-bit binary number, and a further 16 bits are required for synchronisation and control purposes, giving a total of 32 data bits per sample. However, there are a total of 64 bits to fit into the time allocated for each sample, because both the Left and Right channels must be multiplexed into the bit-stream. This works out at a final bit-rate of 2.8224 Mbit/s for a 2-channel S/PDIF signal at the compact disc sampling rate.

Note that the extreme steepness of the anti-aliasing filter required for 44.1 kHz sampling can have undesirable side-effects on the wanted high frequency components of the original signal. It is therefore preferable for a higher sampling rate to be used (generally 48 or 96 kHz) − this is called over-sampling. Such a digital signal may be subsequently filtered digitally to remove components above 20 kHz, and then down-converted to a 44.1 kHz rate if required (e.g. to produce a CD). See also dB FS, Over, Digital mixer, Digital signal processing, DAW, Acquisition time and Oversampling. Compare Digital to analogue convertor.

Analysis microphone

See Measurement microphone.

Anechoic

Describes a space in which there are essentially no reflections of sound from any surfaces, including the walls, ceiling and floor. Such rooms (often called anechoic chambers) are specially created for the testing of speakers and microphones by manufacturers, and so may be mentioned in equipment specifications. That is the only relevance for PA personnel, because such an environment does not exist in the 'real world' of PA system use.

Angle of acceptance

See Pick-up angle.

Anode

The final positive electrode of a valve. In a conventionally constructed valve, this is the outermost part of the valve's internal structure. In the USA, the term 'plate' is used instead. In order to function correctly, the anode of a valve needs to be supplied from a high positive DC voltage called the 'HT supply' (or, in the USA, the 'B supply'). See also Red plating.

ANSI

An abbreviation for the American National Standards Institute, a body responsible for the setting of standards in many areas of technology. See also IEC.

ANSI Lumens

See Lumen.

ANSI S4.40

See AES3.

Antenna

The technical term for an aerial. The plural is 'antennae'.

Antenna distribution unit
Antenna distribution amplifier

A device that accepts multiple RF radio-microphone signals received by a single antenna (or pair of antennae in a diversity system) and provides multiple RF outputs for connection to the individual receivers. This avoids the necessity for a large number of antennae when many radio-microphone systems are in use simultaneously − especially useful when the antennae need to be located in an elevated position for reliable pick-up. The device may also provide some amplification of the RF signal, to counter the losses incurred in the connecting cable. BNC connectors are usually employed. Sometimes called an 'antenna splitter'. May be abbreviated to 'ADU' or 'ADA'.

Antenna gain

The improvement in transmission or reception performance obtained by use of a particular directional antenna, in the direction of best performance, as compared with use of a non-directional (omni-directional) antenna. Usually expressed as a value in decibels.

Antenna splitter

An alternative name for an antenna distribution unit.

Anti-aliasing filter

A filter that is provided to prevent (as far as practically possible) the occurrence of aliasing. See also Analogue to digital conversion.

Anti-phase

This term is most often used to describe the situation in which the changes in a signal's instantaneous voltage occur at essentially the same time but in the opposite direction to those of some reference signal carrying the same information. For example, the signal's instantaneous voltage changes in a positive direction as the reference signal's instantaneous voltage changes in a negative direction. (The signal's changes may, additionally, be of different level to those of the reference signal.)

Or, the situation in which the changes in a sound wave's instantaneous pressure occur at essentially the same time but in the opposite direction to those of a reference sound wave carrying the same information, at some specified location(s). For example, the wave's instantaneous pressure increases as the reference wave's instantaneous pressure decreases. (If two such sound waves combine they will tend to cancel one another, giving a decrease in sound pressure level; this is termed 'destructive interference'.)

However, the above situations are more accurately referred to as a difference in polarity between the two signals (or sound waves) because, strictly speaking, a change in phase requires a time shift (i.e. a delay). Such a difference in polarity may occur intentionally or accidentally (see Polarity reversal). In order for a change in phase to provide the same effect, there would need to be a time-shift equivalent to half a cycle (180º) of the signal or sound wave; this is only possible in the case of a single-frequency content, i.e. a sine wave.

Note that an anti-phase situation is often referred to using the term 'out of phase' although, properly speaking, 'out of phase' has a much more general meaning. See also Phase reversal, Balanced, Dead spot and Centre tap. Compare In-phase and Out of phase.

Anti-shock mount

See Shock mount.

AoE (1)

An abbreviation for 'audio over Ethernet', referring to any scheme enabling single or multiple channels of audio to be carried over an Ethernet-based connection. Not to be confused with systems such as Format A or any other arrangement using QTP cable, which do not use Ethernet protocol but may use cables and connectors (usually RJ45) that are also suitable for Ethernet connections. Also, take care to avoid confusion with AoE (2), although AoE (1) is the more common meaning in PA work. See also AoIP, AVoIP and SDVoE.

AoE (2)

An abbreviation for 'ATA over Ethernet', referring to a network protocol enabling the interconnection of storage devices and servers in an Ethernet-based storage area network (SAN). ATA stands for 'advanced technology attachment', a standardised interface for the connection of storage devices, also known as IDE (integrated drive electronics). Take care to avoid confusion with AoE (1), which is the more common meaning in PA work.

AoIP

An abbreviation for 'audio over internet protocol', referring to any scheme enabling single or multiple channels of audio to be carried over any connection using the internet protocol (IP). Such connections are usually set up in advance over a local or global network, but may alternatively be point-to-point connections. Most commonly, 'AoIP' refers to a scheme complying with the AES67 standard, and/or in many cases complying with the RAVENNA standard. See also Dante.

AP connector (AP3, AP4, AP5, AP6, AP8)

A series of high-power latching connectors manufactured by Amphenol^®. This series has a durable plastic shell. A specific feature is their ability to accept large diameter cables. Once popular for connections to passive speakers, but now largely replaced by the Speakon. They are available in 3, 4, 5, 6 and 8-pole versions, referred to as AP3, etc. A further digit may be added to indicate male or female gender, cable or panel mounting, etc., e.g. AP5-12. The 3, 4, 5 and 6-pole versions are rated at 20 amps and at least 200 volts RMS; the 8-pole version is rated at 15 amps and 100 volts RMS. The 5-pole version was useful for amplifier-speaker combinations in which an additional 'sense' connection was required. The EP series of connectors is similar but has a rugged die-cast metal shell rather than plastic; these types are fully compatible with the AP series.

View AP connector (AP5-12) image

APA

An abbreviation for the Audio Publishers Association, a web-based resource for audio professionals and for accessing audiobooks.

APD

An abbreviation for 'auto power down', a feature of some equipment that enables it to automatically enter a state of very low or even zero power consumption, under prescribed conditions such as lack of an input signal for a specified period of time. Often this state will be a 'standby' state, from which the equipment will automatically 're-awaken' under appropriate conditions.

Apparent power

See VA.

Application, App

A computer program, or suite of programs, that may be installed on a computer, laptop, smartphone or tablet to enable it to perform a particular task or fulfil a particular function. The original term for such programs on computers and laptops was 'application', but this changed to 'app' with the popular use of the abbreviated form for smartphones and tablets. See also Plug-in, SAC, DAW, Operating system and Platform (2).

Apron

The part of the stage between the house tabs and the front edge of the stage.

Armoured cable

Cable that has built-in protection against mechanical damage, in the form of earthed metal wires, usually of steel, incorporated beneath the cable's sheath.

Array

A group of speakers positioned close together. In some cases, the intention is for the dispersion pattern of the individual speakers (often of an identical type) to interact, so as to produce a different overall dispersion pattern. In these cases, the relative level and phase of the speakers' drive signals must be appropriately controlled. See also Line array, CSA (2), Cluster, Stack and Trapezoidal.

Artefact

A subjectively identifiable disruption of a signal, most often caused as an unwanted side-effect of processing performed on the signal. The term is most frequently applied to minor disruptions that are apparent as unwanted additions to the signal. In the case of digital audio, see also Aliasing.

Articulated array

See Line array.

Articulation (1)

A measure of the degree to which a PA system is able to accurately reproduce the required programme material, when set up in a particular space. The term is most often used in the context of speech reproduction, where it refers to the degree of intelligibility achieved by the system. See also ALCONS and Clarity.

Articulation (2)

The facility of a line array to be set up in a variety of different curved shapes − i.e. to be articulated. See Line array.

Artist, Artiste

A general term for any type of performer, or for a group of performers. See also Talent.

Art-Net

A DMX-over-Ethernet technology that allows up to 32,768 DMX universes to be controlled via a single network cable. Compare sACN and ShowNet.

ASCII

An abbreviation for 'American standard code for information interchange'. A standardised scheme for the representation of alphabetic, numeric, symbolic and control characters in binary form. It originally used 7 bits per character (sometimes with the addition of a parity bit), but this was later extended to 8 bits to allow the representation of more symbols and of alphabetic characters having accents, while still using just one byte per character.

ASIO

An abbreviation for 'audio stream input/output', an audio sequencing protocol developed by Steinberg (USA) in 1997. See also MIDI.

ASM

An abbreviation for 'assistant stage manager'. See also SM and DSM.

ASPEC

An abbreviation for 'adaptive spectral perceptual entropy coding', a data compression scheme to allow high quality digital audio to be streamed over digital telephone lines.

Aspect ratio

The relationship between the horizontal and vertical dimensions of a display screen. The original format for video image displays (including analogue TV transmission) and for computer displays and data projection screens (see VGA) was 4:3, which represents a horizontal to vertical ratio of 1.33:1. The wide-screen format now adopted for video (including digital TV transmission) is 16:9, a ratio of 1.78:1. Other aspect ratios are also used, for example 5:4 (e.g. SXGA) and 16:10 (e.g. WXGA). Much wider formats are commonly used for professionally produced films. For a more complete list of common video resolutions and their aspect ratios, see VGA. See also HDTV.

Assign switches

See Routing.

Assistive listening

Facilities provided to enable people with a hearing disability to be included in activities from which they would otherwise be excluded, or partially excluded, because of that disability. Sometimes referred to as 'hearing enhancement'. In the UK, such facilities may be required by the Equalities Act and/or by Part M of the Building Regulations. They are often provided by an induction loop system, but in some cases may alternatively or additionally be provided by other means such as infra-red or radio systems, or by speech-to-text displays or sign language interpretation. See also BS 8300.

Asymmetric response

See Equaliser.

Asynchronous

Describes something in which events occur independently of any fixed timing reference, or independently of each other. In asynchronous communications, such as MIDI, and DMX it is the start of transmission of each word of information which occurs asynchronously, requiring this event to be indicated by a 'start bit'. Each such word is followed by one or more 'stop bits' in order to ensure that the start bit of the next word can be unambiguously recognised at the receiving end, even when the next word follows on immediately. Compare Synchronous.

ATC

An abbreviation for 'Absolute time code'.

ATM

An abbreviation for 'asynchronous transfer mode', an extremely fast networking technology such as might be used to interconnect computers in a large studio.

Atmospheric conditions

Atmospheric conditions can impact upon the propagation of sound, for example at outdoor events. At the most basic level, air temperature affects the speed of sound, and relative humidity (RH) levels affect absorption. Unusual temperature gradients such as temperature inversions can impact on sound propagation in unexpected ways, such as enabling the sound from an outdoor event to be clearly heard from an unusually large distance away.

ATRAC

An abbreviation for 'adaptive transform acoustic coding', the scheme used for data c of the digital audio information stored on a Mini Disc. A more recent variation of the scheme is called ATRAC3, and this provides a compression ratio of about 10:1. See also MP3 and Masking.

ATS

An abbreviation for 'automatic transfer switch', usually referring to power distribution equipment that monitors the normal supply (whether from the mains or a local generator) and implements an automatic switch-over to a standby source (usually a standby generator) in the event of a supply failure. More commonly referred to as AMF (automatic mains failure) equipment. Or, ATS may refer to the actual switching device itself. See also UPS and Power conditioner.

Att

An abbreviation for 'attenuator', often used in the labelling of equipment switches.

Attack (1)

The period of time during which a sound builds up from silence to its initial maximum value. The notes produced by many musical instruments have a very short attack time, in comparison with their decay time. For further details see ADSR. See also Percussive and Envelope.

Attack (2)

A control that adjusts the delay before a compressor applies compression, or the rate at which a noise gate opens, after the signal level exceeds the threshold level. Compare Release (1).

Atten

An abbreviation for 'attenuator', often used in the labelling of equipment switches.

Attenuation

The process of reducing the level of a signal. Or, the amount by which a signal is reduced in level. The term may refer to:

• A reduction by the same amount at all relevant frequencies (a device that is designed specifically to provide this function is called an attenuator or a 'pad').
• A reduction by an amount that differs for different frequencies (a function provided by a filter).
• The amount of reduction at a specific frequency of interest, or within a specific frequency band of interest.

Attenuation is measured in decibels. An alternative term for the amount of attenuation given to a signal is the 'loss' or, less formally, 'cut'. Compare Amplification.

Attenuator

A device whose purpose is to reduce ('attenuate') the level of all relevant frequencies contained in a signal by essentially the same amount (as compared to a filter, which is a device that reduces different frequencies in the signal by different amounts). The amount of reduction provided is referred to as the attenuation, or loss. Also called a 'pad'.

Attenuators are typically used when a reduction in signal level is required in order to avoid overload of the equipment to which the signal is being supplied. Stand-alone attenuators can be purchased as 'in-line' plug-in units, for example with an XLR connector at each end.

They can also be constructed using resistors, which may be wired inside suitably-marked connectors. As attenuators may cause an increased circuit impedance, they should be inserted at the load end of the interconnection to avoid loss of high frequencies due to the effect of the cable capacitance. When used in high-power situations, such as between amplifiers and speakers, it is essential to ensure that the resistors have an adequate power rating and are adequately ventilated.

Attenuators must be suitable for the type of interconnection: balanced or unbalanced, and voltage-matched or impedance-matched. Attenuators for unbalanced voltage-matched interconnections can be simply constructed using just two resistors − one in series with the signal conductor followed by a parallel one to signal earth. However, care must be taken to ensure that the source impedance and load impedance of the interconnection do not have an unexpected effect on the amount of attenuation obtained. Provided that the source impedance is less than 100 ohms and that the load impedance is greater than 10 kilohms, the following resistor values can be used in unbalanced voltage-matched interconnections:

Loss (approx.)	Series R   (kohm)	Parallel R (kohm)
10 dB	2.2	1
20 dB	9.1	1
30 dB	30	1
40 dB	100	1

See also the resistive divider calculator on the Calculations page.

Attenuators for balanced interconnections and for impedance-matched interconnections are rather more complex; email me for details.

Audacity

A free audio editor and recorder for Windows, Mac OS X, GNU/Linux and other operating systems. It may be downloaded from audacity.sourceforge.net (this link opens in a new tab or window).

Audio

In general, relating to that which can be heard, or to audio-frequency sounds. In technical circles, normally refers to a signal of audio frequency, i.e. the term is used as an abbreviation of 'audio signal'. Although such a signal cannot be heard directly, the description 'audio' refers to the fact that it could be heard if converted into sound waves (e.g. by a speaker). Similarly, the term 'audio' may be used to describe any equipment that handles such signals. See also Acoustic.

Audio chain

The signal chain of an audio signal.

Audio group

See Group.

Audio spectrum

The range of frequencies described as 'audio-frequency'. See also Spectrum.

Audio taper

See Taper.

Audio-frequency

Describes something that uses or processes frequencies in the audio range, generally considered to be 20 Hz to 20 kHz. Or (without the hyphen), any such frequency itself. This range is often described as the 'audio spectrum', and is considered to be divided into several bands: bass (or 'LF'), mid-range and treble (or 'HF'). Frequencies below the audio spectrum are referred to as infrasonic, and those above it as ultrasonic. May be abbreviated to 'AF'. Compare Radio-frequency.

Audiofool

See Audiophool.

Audiogram

A graph of an individual person's hearing response, produced during an audiology assessment. It serves to indicate any deficiencies in hearing, as compared to the typical response of a person with normal hearing. See also NIHL, SNHL and Presbycusis.

Audiology

The science of hearing. The term is often used to refer to the branch of medicine concerned with human hearing; specifically the measurement of hearing response, the analysis of defects and the provision of corrective aids. See also NIHL, Audiogram, Dosimeter, SNHL, Presbycusis, Assistive listening, Induction loop, Tinnitus and Acoustic Safety on the Safety page.

Audiophile

A person having an extremely strong passion for the very best possible quality of sound reproduction. (Literally, 'a lover of sound'.) Caution: Equipment that is specifically aimed at the audiophile market is frequently very much more expensive than functionally equivalent equipment typically accepted as being of adequate quality by sound engineering professionals. It would not be unusual to find cables as much as 50 to 200 times as expensive, and other equipment 10 to 50 times as expensive. Of course, the manufacturers and suppliers of such equipment generally claim significantly enhanced subjective results from it, and sometimes give details of the specific design aspects that are claimed to be responsible for these results. See also Hi-Fi, Oxygen-free copper, Directional cable and Subjectivism.

Audiophool

A slang term making ironic play on the term 'audiophile'. As this may be considered a very derogatory term that questions the ability of some individuals to make reasonable judgements, it is strongly recommended that use of the term is avoided. The term effectively questions aspects of the presumed reasoning of some audiophiles, or of others who, while not classifying themselves as audiophiles, are presumed to adopt similar reasoning positions. The term is most commonly used in relation to presumed judgements regarding equipment's subjective performance benefits versus equipment cost, i.e. in critisism of those who are prepared to pay what others see as vastly over-inflated prices for equipment, usually based on their own or others' subjective performance judgements or on manufacturers' performance claims. See also Subjectivism.

Auditorium

A location where an audience gathers to be entertained or educated (e.g. by live music, lectures, a theatrical performance, etc.). Specifically, the area of a building (such as a theatre) where the audience sits or stands, as distinct from other parts of the same building (such as the stage). Commonly referred to as the house. The plural is strictly 'auditoria', but 'auditoriums' is also considered acceptable. See also Front-of-house, Gods and Mosh pit.

Auditory filter

In psychoacoustics, one of the group of filters by means of which the ear may be considered to distinguish different frequencies of sound. There are many of these filters, effectively behaving as a set of overlapping bandpass filters. The bandwidth of each filter is not the same, but depends upon its centre frequency − see Critical band. See also SNHL.

Aureal 3D

A proprietary surround-sound technology, first developed by Crystal River Engineering and sometimes referred to as 'A3D'.

Auto-ducking

See Ducking.

Autoformer

An American term for an autotransformer.

Automatic double-tracking

See ADT.

Automatic gain control

A facility that automatically adjusts the gain (or attenuation) being applied to a signal by an item of equipment, so as to maintain a constant average level and/or to avoid a maximum level being exceeded. This behaviour is essentially the same as compression, usually pre-set with a very short attack time and a very long release time.

The facility is sometimes encountered as a switchable facility of portable audio recording equipment, where it is provided to assist in best use of the available dynamic range of the recording medium. Commonly abbreviated to AGC.

Automatic mic mixer

A microphone mixer that automatically fades down microphones that are not being used, to reduce unwanted pickup and feedback. Used mostly for conferences with a large number of microphones, where manual control would not be practicable.

Automation

See Mixer automation.

Autotransformer

A transformer-like device that, unlike a true transformer, does not give galvanic isolation between its input and its output sides, because part of the winding is shared by both sides in order to reduce the cost, size and weight of the device (as compared to an equivalent transformer). Therefore, if one side is connected to a voltage that is considered dangerous, then the other side must also be considered dangerous − even if its voltage is meant to be lower. The American term for this device is 'autoformer'.

Aux

An abbreviation for 'auxiliary'.

Aux master

A control that sets the overall level of an auxiliary (Aux) mix.

Aux send

See Auxiliary.

Auxiliary

In general, this term means 'extra', or 'additional', and is often abbreviated to 'Aux'. It usually refers to the facility of a mixer that enables the creation of additional mixes, independent of the main mix, and provides separate output(s) for these mixes. Each channel of the mixer will usually have the facility to feed its signal into several auxiliary mixes, which may be pre-fade (typically for sending to stage monitors), post-fade (typically for sending to effects units), or switchable between these two types. The level of the channel's signal within each auxiliary mix is set by the channel's respective 'Aux Send' control.

The overall level of each auxiliary mix is adjusted using 'Aux Send Master' controls, and the mixed signals are available at 'Aux Send Outputs'. Other facilities that may be provided in the auxiliary master section include inserts, AFL, metering and matrixing.

AV

An abbreviation for 'audio-visual'. Any system or equipment, or connection between them, that handles both sound and picture information. Or, any equipment that, although it handles only sound or picture information, would typically be used as part of a larger system that involves both. See also AVL and Multimedia.

AVCD

An abbreviation for 'advanced video compact disc'. A standard for the recording of digital video information on a CD, giving an improved picture quality as compared to the VCD standard, but inferior to the DVD standard.

AVD

An abbreviation for 'Advanced Video Disc', an alternative standard to the DVD, proposed by the Japanese.

Average level

A means of specifying the level of an audio signal that is indicative of its perceived relative loudness. For example, the level indicated by a VU meter. Or, describes a signal that is at its nominal level.

Average power

A means of specifying a power level that varies with time, for example an audio power level. The method is to specify the average of the instantaneous values of power over a complete cycle (in the case of a repetitive waveform) or, more generally, over an interval that is long enough to yield an approximately repeatable value.

In the case of electrical power, multiplying an RMS voltage by an in-phase RMS current gives an average power value. (For this reason, average power is often referred to as 'RMS power', however this is incorrect terminology). Note that if the voltage and current waveforms are not in phase (i.e. non-unity power factor). then it is also necessary to multiply by the power factor.

The term is frequently used as an abbreviated form of 'continuous average power', which refers to an average power level that remains constant for an extended duration (which may or may not be stated). This method of stating a power loading of an item of equipment is commonly used to specify the power rating of speakers; a defined signal should be specified, such as a defined sine wave or pink noise. For further information see Power and Continuous average sine wave power.

AVIXA^®

The Audiovisual and Integrated Experience Association, an international trade association representing the audiovisual industry and producer of the InfoComm trade shows. Their website is www.avixa.org (opens in a new tab or window).

AVL

An abbreviation for 'audio, video and lighting'. Typically used to refer to the combined supply or deployment of these three facilities for an event. See also AV.

AVO

A manufacturer of test equipment (a registered trademark), notably multimeters. Commonly used as a slang term for any such tester or meter, regardless of its manufacturer. The name arises from an abbreviation of 'amps, volts and ohms', the three quantities that most multimeters are able to measure. See also Megger.

AVoIP

An abbreviation for 'AV over IP', referring to any scheme enabling single or multiple channels of AV to be carried over any connection using the internet protocol (IP). Such connections are usually set up in advance over a local or global network, but may alternatively be point-to-point connections. See also AoIP and SDVoE.

AWG

An abbreviation for 'American wire gauge', a scheme for specifying the thickness of the conductors of a cable, otherwise known as the cable's gauge. In the AWG scheme, smaller numbers indicate thicker conductors, giving a lower resistance and therefore a greater current-carrying capability. It is a logarithmic scheme − each reduction of 3 in the gauge represents a doubling of cross-sectional area.

The size of cable conductors is of special importance for interconnections between power amplifiers and speakers, because of the high currents that flow in these interconnections and the need to maintain a high damping factor. See Gauge for advice on speaker cable sizes. A table follows for conversion between some AWG sizes and the equivalent approximate cross-sectional area in mm²; the sizes most commonly used for speaker cables are indicated in red.

The table also indicates the series resistance of various AWG sizes of speaker cable; if the conductors get warm in use or are installed in a hot environment (e.g. in proximity to stage lighting), the high temperature figures should be used. Note that resistance applies only at DC − at high audio frequencies the value of the cable's series impedance will be significantly higher than its resistance, because of inductive effects. For metric sizes of cable, see Gauge.

AWG	mm²	Round-trip resistance at room temperature (20 ºC). Ohms per metre cable length	Round-trip resistance at high temperature (70 ºC). Ohms per metre cable length
20	0.519	0.070	0.084
19	0.653	0.055	0.067
18	0.822	0.044	0.053
17	1.04	0.035	0.042
16	1.31	0.028	0.033
15	1.65	0.022	0.026
14	2.08	0.017	0.021
13	2.63	0.014	0.017
12	3.31	0.011	0.013
11	4.17	0.009	0.011
10	5.26	0.007	0.008
9	6.63	0.0056	0.0067
8	8.37	0.0044	0.0052
7	10.5	0.0035	0.0042
6	13.3	0.0028	0.0033
5	16.8	0.0022	0.0026
4	21.1	0.0017	0.0021
3	26.7	0.0014	0.0016
2	33.6	0.0011	0.0013

Ax

A slang term for a musical instrument, especially an electric guitar or bass.

Axes

The plural of axis.

Axial

In the direction of the axis. In the case of an electronic component or other part described as 'axial leaded', this refers to it having leads that extend out of the ends of the component, in the same direction as the length-ways axis of the component. See also the next definition. Compare Radial.

Axial mode

See Mode (2).

Axis

An imaginary line at right-angles to the operative face of a speaker or a microphone, and passing through the centre of that face. The polar response of the speaker or microphone is generally plotted with reference to the axis. The plural of 'axis' is 'axes'. See also On-axis and Off-axis.

Azimuth

The angle of a tape head gap relative to the direction of travel of the tape; its 'side-to-side' tilt. For correct reproduction of treble frequencies, it is essential that the azimuth is adjusted to make this angle as close to 90º as possible. This adjustment is usually carried out with the help of a calibrated test tape. Compare Zenith.

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This page last updated 29-Jan-2021.