Glossary of terms
AAC Advanced Audio Coding.
ABR Auxiliary Bass Radiator.
AC Alternating current.
A/D Analogue-to-digital conversion.
AES/EBU Audio Engineering Society/European Broadcasting Union: typically refers to a standard digital audio interface.
AF Audio Frequency.
AFL After Fade Listen.
AGC Automatic Gain Control.
Aliasing The generation of in-band spurious frequencies caused by using a sampling rate which is inadequate for the chosen frequency range, i.e. it is less than twice the highest frequency present in the signal.
AM Amplitude Modulation.
Amp The unit of electrical current, named in honour of André Marie Ampère (1775–1836).
Anechoic Chamber A highly absorbtive room; the walls, floor and ceiling of which are virtually non-reflective. Used for acoustical measurements of devices such as microphones and loudspeakers.
Antinode The part of a waveform where its propagating medium has maximum velocity and minimum acceleration.
ASW Auditory Source Width.
ATRAC Adaptive Transform Acoustic Coding.
Bandwidth The range of frequencies over which a device will operate; formally, the measurement is taken from the points at which the response is 3 dB down at the frequency extremes compared with mid-frequencies.
BCD Binary Coded Decimal.
Bell Curve A narrow-band EQ curve shaped like a bell when displayed on a frequency response graph.
Bias An ultrasonic frequency added to the audio frequencies sent to an analogue tape recorder’s record head to bias the tape into a more linear part of its operating range.
BPF Band Pass Filter.
CMRR Common Mode Rejection Ratio.
Compansion The complete process of compression during the recording or transmission stages followed by reciprocal expansion during the replay or reception stages.
Compliance ‘Springiness’.
CRC Cyclic Redundancy Check.
Crossover A device in a loudspeaker system for splitting the audio signal into frequency bands which can then be fed to appropriate speaker drive units (e.g. low frequency, mid range, and high frequency). A passive crossover is placed between the power amplifier and the speaker drivers. An active crossover splits the frequencies at line level ahead of the power amplifiers.
CU Compliance unit.
D/A Digital-to-Analogue conversion.
Damping factor In power amplifiers, the impedance of the speaker it is driving divided by the amplifier’s output impedance. It is an indication of the ability of an amplifier to control spurious movements of the speaker cones (particularly at low frequencies) which can be caused by resonances and energy storage in the drivers’ suspension systems.
DASH Digital Audio Stationary Head. An open-reel digital format.
DAT Digital Audio Tape.
dBm Signal level in decibels, referred to one milliwat (0 dBm = 775 mV across 600 ohms).
dBu Signal level in decibels, referred to 0.775 V with an unspecified impedance value (775 mV = 0 dBu).
dBV Signal level in decibels, referred to 1 volt (0 dBV = 1 volt).
dBv The same as dBu, sometimes used in the USA.
DC Direct current.
DCC Digital Compact Cassette.
Decibel (dB) In audio, the unit used to denote the logarithm of the ratio between two quantities, e.g. voltages or power levels. Also used to denote acoustical sound pressure level. Named in honour of Alexander Graham Bell (1847–1922).
De-emphasis Reciprocal treble cut (see also Pre-emphasis) during a replay or reception process.
DI Direct Inject.
Directivity Defines the angle of coverage of a loudspeaker’s output.
Directivity Factor The number which denotes the ratio between a sound source’s output on its axis of maximum radiation and its output if it were perfectly omnidirectional and its total acoustical output were to be evenly spread all around it.
Directivity Index Directivity Factor expressed in dB.
Dispersion See Directivity.
Dither A continuous low-level noise signal added to the programme prior to A/D conversion and quantisation or during signal processing.
DML Distributed-mode Loudspeaker.
Drive unit See Driver.
Driver The component of a speaker system which actually vibrates or ‘drives’ the air, e.g. a speaker cone or tweeter.
DSD Direct Stream Digital.
DSP Digital Signal Processing.
DST Direct Stream Transfer.
DVD Digital Versatile Disc.
EBU European Broadcasting Union.
Eigentone A standing wave in a room which is set up when half the wavelength of a sound or a multiple of it is equal to one of the dimensions of the room (height, width, length).
EIN Equivalent Input Noise.
EQ Equalisation.
FET Field Effect Transistor.
FIR Finite Impulse Response.
FM Frequency Modulation.
FX Effects.
Haas Effect If two sound sources emit similar sounds but one is delayed with respect to the other (up to about 50 mS) the ears perceive the non-delayed sound source to be the louder of the two, and the sound appears to come from a direction close to the non-delayed source, the exact location depending on the amount of delay between them. Beyond the 50 mS time difference, the ears tend to perceive the sounds as coming from two distinct sources.
Harmonics (Also known as Overtones or Partials.) Components of a waveform which are multiples of the fundamental frequency; together with the starting transient they contribute much to the character or tone colour of a sound.
HDTV High Definition Television.
Hertz (Hz) The unit of vibration in cycles per second, named in honour of Heinrich Rudolf Hertz (1857–1894).
HPF High Pass Filter.
HRTF Head Related Transfer Function.
IIR Infinite Impulse Response.
Impedance Measured in ohms, it is a device’s opposition to the flow of AC current. Reactive devices such as loudspeakers, capacitors and inductors exhibit impedances which vary with frequency.
ITD Interaural Time Difference.
LED Light Emitting Diode.
Longitudinal wave A wave in which the to and fro movement of the wave carrier is in the same plane as the wave’s travel. A sound wave is an example of this, the source of sound pushing and pulling in the direction of the wave.
LP Long Playing gramophone record.
LPF Low Pass Filter.
MADI Multichannel Audio Digital Interface.
Masking A psychacoustic phenomenon, whereby quiet sounds in the presence of loud sounds and/or sounds with a similar frequency content will be rendered less audible.
MD MiniDisc.
MDCT Modified Discrete Cosine Transform.
MFM Miller Frequency Modulation.
MIDI Musical Instrument Digital Interface.
MLP Meridian Lossless Packing.
MMC MIDI Machine Control.
MOL Maximum Output Level.
MOSFET Metal Oxide Semiconductor Field Effect Transistor.
MP3 Short for MPEG-1 Layer 3.
MPEG Moving Pictures Expert Group. (‘Empeg’.)
MPX Multiplex.
MS Main (or Middle)-Side.
MTC MIDI Time Code.
NC Noise Criterion.
Node The part of a waveform where its propagating medium has maximum acceleration and minimum velocity.
Noise shaping The technique of reducing noise in perceptually sensitive regions of the audio band at the expense of increasing it above the audio band, or in less sensitive regions, thereby improving the perceived signal-to-noise ratio. It is used in digital systems such as that used in SACD and over-sampling CD players.
NR Noise Reduction. (Also Noise Rating.)
nWb/m Nanowebers per metre. The unit of flux along a magnetic recording medium, named in honour of Wilhelm Eduard Weber (1804–1891).
Nyquist frequency Half the sampling frequency in a digital system.
Ohm The unit of resistance and impedance, named in honour of Georg Simon Ohm (1789–1854).
Overtones See Harmonics.
PA Public Address. (Also Power Amplifier.)
Pad An input attenuator.
PAM Pulse Amplitude Modulation.
Partials See Harmonics.
PCM Pulse Code Modulation.
PD ProDigi. An open-reel digital format.
PDM Pulse Density Modulation.
Phase Two waves of the same frequency are ‘in phase’ when their positive and negative half-cycles coincide exactly in time. For example, two loudspeakers are in phase if, when fed by the same source signal, their cones move backwards and forwards in step with each other, and their acoustical outputs reinforce. If they are out of phase, cancellation of sound results. Electrical signals can similarly be in or out of phase, or in any intermediate relationship.
Phase response A measure of phase lead or lag of signals across the frequency range as they pass through an electrical circuit or device.
Phon A unit denoting the subjective loudness of a sound, the scale derived from research data.
Pink Noise Noise which has equal energy per octave. Its frequency spectrum is therefore flat when the usual logarithmic horizontal frequency scale is used.
Power bandwidth Superficially similar to Bandwidth (qv) for a power amplifier, but it is the range of frequencies over which the amplifier can deliver full power, with −3 dB being allowed at the frequency extremes. A power amplifier’s frequency response is normally wider than its power bandwidth.
PPM Peak Programme Meter.
Pre-emphasis Treble boost applied during a recording or transmission process. See also De-emphasis.
PWM Pulse Width Modulation.
PZM Pressure Zone Microphone.
Q Historically, the ‘quality’ of a tuned radio-frequency receiving circuit. A good sharp tuning which centred on the station of interest, greatly attenuating the unwanted frequencies to either side, was said to be of high quality or Q, and it could be quantified as set out below. In the audio industry it can denote a number of things, including:
(1) the bandwidth or ‘sharpness’ of an EQ curve. The Q is defined as the centre frequency divided by the bandwidth. See Principal EQ bands, Chapter 5.
(2) In a loudspeaker system: at its low-frequency resonant point, Q is the ratio between the speaker’s output level here and its output level over the nominally flat part of its frequency range, expressed as a number. For example, if the output is 3 dB down at a speaker’s LF resonant frequency (which is fairly typical), it has a Q of 0.707 (20 log Q = −3 dB). If it is 6 dB down, it has a Q of 0.5 (20 log Q = −6 dB).
Quantisation After sampling a waveform, the quantisation process assigns a numerical value to each sample according to its amplitude. For example, a 16-bit digital system can assign one of a possible 65 536 values (216) to a particular sample, with no ‘in-between’ values being permitted.
RAID Redundant Array of Inexpensive Disks.
RAM Random Access Memory.
R-DAT Rotary-head Digital Audio Tape (same as DAT).
Resonance This takes place in a system at frequencies where the balance of its moving mass and its compliance gives rise to regions where a relatively small amount of input energy is required to produce vibrations of large amplitude compared with that required at most other frequencies.
RF Radio frequency.
RIAA Recording Industry Association of America.
RMS Root-mean-square. The RMS heating power of a sine wave is 0.707 the value of its peak-to-peak measurement.
ROM Read Only Memory.
SACD Super Audio Compact Disc.
Sampling (1) The process of encoding a signal digitally by registering it as discrete values of level at specified intervals of time (the sampling frequency), in contrast to analogue recording which registers the waveform continuously. (2) The process of recording sounds into a ‘sampler’ which can then be edited and processed in various ways.
SCSI Small Computer Systems Interface. (‘Scuzzy’.)
S-DAT Stationary-head Digital Audio Tape.
Sensitivity For present purposes, sensitivity effectively denotes the efficiency with which a transducer converts electrical energy into acoustic energy (e.g. a loudspeaker) or vice versa (e.g. a microphone).
Shelving Low- or high-frequency boost or cut with a gentle curve up to a ‘shelf’.
Signal-to-noise ratio The ratio in dB between the wanted signal and the unwanted noise in a system.
Sine wave A wave which is made up of one single frequency.
Sinusoidal waveform See Sine wave.
Slew rate The maximum rate of change in volts/microsecond of which a circuit’s output is capable.
SMART System Managed Audio Resource Technique.
SMPTE Society of Motion Pictures and Television Engineers. (‘Simpty’.)
Solo On a mixer, pressing ‘solo’ on a channel routes its post-fade output to the monitor output. It is the same as AFL.
SPDIF Sony-Philips Digital Interface.
SPL Sound pressure level.
SPMIDI Scalable Polyphonic Musical Instrument Digital Interface.
SPP Song Position Pointers.
SR (1) Sound reinforcement. (2) Spectral recording (Dolby).
Standing wave A standing wave is the result of reflections from room boundaries reinforcing each other at certain frequencies to create points where the sound pressure level is very high, and other points where it is very low.
SWR Standing wave ratio.
Sync Synchronisation.
THD Total harmonic distortion.
TOA Time of arrival.
Transducer A device which converts one form of energy into another form of energy. For example, a loudspeaker converts electrical energy into acoustical energy.
Transverse wave A wave in which the device or particles creating it move at right angles to the direction of the wave’s travel, the carrying medium also oscillating at right angles to the wave’s travel. An example is electromagnetic radiation, created by the electrons’ up-and-down motion along the length of a transmitting aerial.
UHF Ultra-high frequency.
UHJ Universal HJ. The Ambisonics surround sound encoding and decoding system, the H and J denoting earlier systems.
VCA Voltage controlled amplifier.
VCO Voltage controlled oscillator.
VHF Very high frequency.
VITC (‘vitcee’) Vertical Interval Time Code.
Volt The unit of electrical pressure, named in honour of Alessandro Volta (1745–1827).
VTR Video tape recorder.
VU Volume Unit.
Watt The unit of electrical power, named in honour of James Watt (1736–1819).
White noise Noise which has equal energy per Hz of frequency. Its frequency spectrum therefore rises by 3 dB per octave when the usual logarithmic horizontal frequency scale is used.
WORM Write Once Read Many times.
XLR Originally a part code for the ITT-Canon company’s professional audio connector, the most familiar of which is the 3-pin microphone and balanced line XLR-3.