AAC | Advanced Audio Coding. |
ABR | Auxiliary Bass Radiator. |
AC | Alternating Current. |
A/D | Analog-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 honor 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. |
ASK | Amplitude Shift Keying. |
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 analog tape recorder’s record head to bias the tape into a more linear part of its operating range. |
BPF | Band Pass Filter. |
CD | Compact Disc. |
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-Analog 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 1 milliwatt (0 dBm = 775mV 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. |
DCLD | Downmix Channel Level Difference. |
DCP | Digital Cinema Project. |
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 honor of Alexander Graham Bell (1847–1922). |
De-emphasis | Reciprocal treble cut (see also Pre-emphasis) during a replay or reception process. |
DHCP | Dynamic Host Configuration Protocol. |
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 program prior to A/D conversion and quantization 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 | Equalization. |
FET | Field Effect Transistor. |
FIR | Finite Impulse Response. |
FLAC | Free Lossless Audio Coding. |
FM | Frequency Modulation. |
FSK | Frequency Shift Keying. |
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 color of a sound. |
HDTV | High Definition Television. |
Hertz (Hz) | The unit of vibration in cycles per second, named in honor of Heinrich Rudolf Hertz (1857–1894). |
HPF | High Pass Filter. |
HRTF | Head Related Transfer Function. |
ICS | Internet Connection Sharing. |
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. |
IOC | Inter-Object cross Coherence. |
IP | Internet Protocol. |
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. |
MXF | Material Exchange Format. |
NAT | Network Address Translation. |
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 honor of Wilhelm Eduard Weber (1804–1891). |
Nyquist frequency | Half the sampling frequency in a digital system. |
OCA | Open Control Architecture. |
Ohm | The unit of resistance and impedance, named in honor of Georg Simon Ohm (1789–1854). |
OLD | Object Level Difference. |
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. |
PFL | Pre-Fade Listen. |
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 centered 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 center 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). |
QPSK | Quadrature Phase Shift Keying. |
Quantization | After sampling a waveform, the quantization 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 Independent 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 analog 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. |
SAOC | Spatial Audio Object Coding. |
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 | Synchronization. |
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 honor of Alessandro Volta (1745–1827). |
VTR | Video Tape Recorder. |
VU | Volume Unit. |
WAP | Wireless Application Protocol. |
Watt | The unit of electrical power, named in honor of James Watt (1736–1819). |
WFS | Wave Field Synthesis. |
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. |
WMA | Windows Media Audio. |
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 three-pin microphone and balanced line XLR-3. |