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Glossary of PA Terms - A

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The glossary pages provide definitions for over 1900 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

A * A & E specifications * A cappella * A-B pair * A-B test * A/D convertor * A-gauge jack * A-taper * A-type jack * A-weighting * A/D * A3D * AAC * ABS * Absolute time code * Absorption * ABTT * AC * AC-3 * Academy curve * Accelerated-Slope * Acceptance angle * Acoustic * Acoustic feedback * 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 * Admittance * ADPCM * ADSR * ADT * ADU * Advanced audio coding * AES * AES/EBU * AES10 * AES24 * AES3 * AES3-ID * AES3-IE * AETTI * AF * AFILS * AFL * AGC * 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 * Ambisonics * American terminology * American thread * AMI-C * Amp * Ampere * Ampere-hour * Amplification * Amplifier * Amplifier sensitivity * Amplitude * Amplitude modulation * Analog * Analogue * Analogue to digital conversion * Anechoic * Angle of acceptance * 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 * APA * Apparent power * Application * Apron * Array * Artefact * Articulated array * ASCII * ASIO * ASM * ASPEC * Aspect ratio * Assign switches * Assistive listening * Asymmetric response * Asynchronous * ATC * ATM * ATRAC * Att * Attack * Atten * Attenuation * Attenuator * Audio * Audio chain * Audio group * Audio spectrum * Audio taper * Audio-frequency * Audiophile * Auditorium * Auditory filter * Aureal 3D * Autoformer * Automatic double-tracking * Automatic gain control * Automatic mic mixer * Automation * Autotransformer * Aux * Aux send * Auxiliary * AV * AVCD * AVD * Average power * AVO * AWG * Ax * Axes * Axial * 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.

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.)

A-B test
A test in which two makes or models of similar equipment 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.

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.

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
An abbreviation for 'Absolute time code'.

Absolute time code
Timing information that is carried by the subcode of a digital bit-stream. Primarily used by digital tape recording systems such as DAT, to enable any frame of recorded data to be identified by its time from the start of the tape, which is recorded 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' 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.

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.

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.

Acceptance angle
See Pick-up angle.

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 Audio.

Acoustic feedback
See Feedback.

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 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 and Haas effect.

Acoustics (2)
The science of sound. In particular, the study of the effect of the properties of spaces on the way that sound is heard within those spaces, and of how such properties may be modified to improve (subjectively) the way that sound is heard (see Acoustics (1)).

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
Describes something containing circuitry that requires a source of electrical power in order to operate. An example would be an active DI box. The opposite of active is passive.

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. Particularly popular as cross-faders on DJ consoles.

Active DI box
See DI box.

Active pick-up
See Pick-up.

Active monitor
See Powered monitor.

Active speaker
See Powered speaker.

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 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.

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.

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.

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. 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.

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 samples 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 (less 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.

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. Their website is: http://www.aetti.org.uk. See also ABTT.

AF
An abbreviation for 'audio-frequency'.

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. See also PFL and Solo.

AGC
An abbreviation for 'automatic gain control'.

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 means to perform some operation, often a series of mathematical computations, towards completing a desired process.

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, half the sampling rate used.

Any frequencies higher than the Nyquist frequency 'fold around' that value, producing alias frequencies that are below the original ones and which are therefore often more noticeable. The folding process effectively removes any intelligibility from the aliased parts of the signal, and makes 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 sampling operation.

For 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). So, the higher the original 'out of bounds' frequency, the lower its alias. See Analogue to digital conversion.

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.

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.

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.

* These terms are generally well understood in the UK.

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.

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

Amp
An abbreviation for either amplifier or ampere.

Ampere
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
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.

Amplification
The process of increasing the level of a signal. The equipment or circuitry that performs this function is an amplifier. The amount by which the level is increased is termed the gain. 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). This is also known as a 'gain stage'.

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.

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 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 for 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. (Compact disc digital audio uses 16-bit samples for 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 Acquisition time, Over and Oversampling. Compare Digital to analogue convertor.

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.

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 (i.e. positive or negative) to those of some reference signal carrying the same information. (They may, additionally, be of different level.)

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). (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 situation is 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
A mounting device for a microphone, specifically designed to reduce the transfer of vibration from the mounting point to the microphone.

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

Apparent power
See VA.

Application
A computer program, or suite of programs, that may be installed on a computer to enable it to perform a particular task. See also Plug-in and DAW.

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

Array
A group of speakers positioned close together. See also Line array, 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.

Articulated array
See Line array.

ASCII
An abbreviation for 'American standard code for information interchange'. A 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, whilst 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.

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, and the usual format for computer displays, is 4:3 (see, for example, VGA). The wide-screen format for digital video is 16:9. Other aspect ratios are also used, for example SXGA is 5:4. See also HDTV.

Assign switches
See Routing.

Assistive listening
Improvement of audio reproduction for people with a hearing disability, often provided by an induction loop system, but alternatively by other means such as infra-red or radio systems. See also DDA.

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.

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.

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 Envelope.

Attack (2)
A control that adjusts the rate at which a compressor applies compression, 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.

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:

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

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.

Audiophile
A person having an extremely strong passion for the very best possible quality of sound reproduction. (Literally, 'a lover of sound'.) See also Hi-Fi, Oxygen-free copper, Directional cable and 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.

Auditory filter
In psychoacoustics, one of the group of filters by means of which the ear distinguishes 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'.

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.

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, because part of the winding is shared by both sides. 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 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.

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 power
See Power.

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.

AWG
An abbreviation for 'American wire gauge', a scheme for specifying the thickness of the conductors of a cable. In this 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.

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 a component or other part, this usually means "along the length" of the component. Compare Radial.

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. Compare Zenith.

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This page last updated 10-Nov-2009.