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Dictionary of Audio Terminology - A


A Symbol for ampere.

Å - Symbol for angstrom. A unit of length equal to one-tenth of a nanometer. Used for measuring the wavelengths of light

A2IM (American Association of Independent Music) - "... serves the Independent music community as a unified voice representing a broad coalition of music labels ... ." [From website]

AAAF (American Academy of Audiology Foundation) - Their mission: "To promote philanthropy in support of research, education, and public awareness in audiology and hearing science." [From website.]

AAC (Advanced Audio Coding) - Shortened name for the MPEG-2 Advanced Audio Coding specification, declared an international standard by MPEG in April 1997; however, now the term is used also to refer to MPEG-4 advanced audio coding. Made most popular by Apple using it for compressing audio CDs for their iPod and iTunes products.

AAM (Academy of Ancient Music) - Founded by British musician Christopher Hogwood in 1973. They perform using instruments that date from the time when the music was composed. "Under Hogwood’s visionary leadership, it established itself as a leading authority on how music was originally performed: this pioneering work had a transformative impact on the world of classical music, and lies at the heart of the AAM’s reputation for musical excellence."

AAM (American Association of Museums) - "AAM’s mission is to enhance the value of museums to their communities through leadership, advocacy, and service." Valuable resource for sound contractors, integrators, etc.

AB - In reference to microphones, a stereo recording technique whereby two microphones are spaced apart (anywhere from about 3 feet to as much as 10 feet) to create a time difference between them that the human brain perceives and translates into stereo localization and imaging. Also called time-difference recording.


A-B powering or T-powering - Named after the German word Tonaderspeisung, It is a special purpose powering system designed for T-power microphones, usually electret or condenser designs. Originally standardized as DIN 45 595.

A-B testing (or A/B testing) - A comparison testing methodology where a first test, A, is compared against a second test, B.

Abffusor® - Registered trademark of RPG Diffusor Systems for their proprietary panel combining absorption and diffusion characteristics.

absolute pitch - The ability to name the pitch of a note, or to sing a named note, without reference to a previously sounded one. It is sometimes called 'perfect pitch.' [Sadie]

absorption - To absorb is to receive (an impulse) without echo or recoil: a fabric that absorbs sound; a bumper that absorbs impact; therefore absorption is the act or process of absorbing. [AHD] The absorption of sound is the process by which sound energy is diminished when passing through a medium or when striking a surface, i.e., sound is attenuated by absorption. [AHD] The physical mechanism is usually the conversion of sound into heat, i.e. sound molecules lose energy upon striking the material's atoms, which become agitated, which we characterized as warmth; thus, absorption is literally the changing of sound energy to heat. A material's ability to absorb sound is quantified by its absorption coefficient, whose value ranges between 0 (total reflection) and 1 (total absorption), and just to keep things interesting, varies with sound frequency and the angle of incidence. Contrast with isolation.

ABX testing (aka ABX double-blind comparator) - A system controller for audio component comparison testing where the listener hears sound-A, sound-B, and sound-X. The listener must make a determination as to whether X is A or B. The subject may go back to A and B as often and for as long as necessary to make a determination. The listener knows that A and B are different and that X is either A or B, so there is always a correct answer. The "double-blind" part comes from neither the tester nor the listener (can be the same) knows what source is A, B or X, only the controller knows, which is downloaded after the test is complete to determine the results. First invented in 1977 by Arnold Krueger and Bern Muller (of the famous Southeastern Michigan Woofer and Tweeter Marching Society or SMWTMS), later refined and marketed by David Clark and his ABX Company. [For complete details see David L. Clark, "High-Resolution Subjective Testing Using a Double-Blind Comparator", J. Audio Eng. Soc., Vol. 30 No. 5, May 1982, pp. 330-338.]

AC or alternating current - An electric current that reverses direction at regularly recurring intervals of time. Contrast: direct current. [IEEE] [Usage Note: Officially the IEEE dictionary is very clear that the abbreviation for alternating current is "ac" not "AC." However most everyone agrees (mags, technical journalists, me, etc.) that when abbreviating alternating current in a standalone sense, that it looks better and reads clearer if you use uppercase, e.g., "The device runs off AC voltage," instead of "The device runs off ac voltage,", particularly if the abbreviation begins or ends a sentence. Imagine a sentence like this: "Ac is a type of generator voltage." Or, "Do you want ac or dc?" Both work better with uppercase. As for Vac vs. VAC, both are seen and accepted even though Vac is the IEEE standard.]

AC-3 (audio coding 3) - Dolby's digital audio data compression algorithm adopted for HDTV transmission and used in DVDs, laserdiscs and CDs for 5.1 multichannel home theater use.

Academy of Ancient Music or AAM - Founded by British musician Christopher Hogwood in 1973. They perform using instruments that date from the time when the music was composed. "Under Hogwood’s visionary leadership, it established itself as a leading authority on how music was originally performed: this pioneering work had a transformative impact on the world of classical music, and lies at the heart of the AAM’s reputation for musical excellence."

Academy curve - The name of the standard mono optical track that has been around since the beginning of sound for film. Standardized in 1938, it has improved (very) slightly over the years. Also known as the N (normal) curve the response is flat 100 Hz-1.6 kHz, and is down 7 dB at 40 Hz, 10 dB at 5 kHz and 18 dB at 8 kHz. This drastic "dumping" of the high-end was to hide the high-frequency "frying" and "crackling" noise inherent in early film sound production. Compare with X curve.

a cappella - Without instrumental accompaniment. [AHD]

Accelerated-SlopeTM - A trademark of Rane Corporation used to describe their family of patented tone control technologies that produce steeper slopes than normal, thus allowing boost/cut of high and low frequencies without disturbing the critical midband frequencies.

accordion - "An instrument in harmony with the sentiments of an assassin." -- Ambrose Bierce.

accumulator - A register or electric circuit in a calculator or computer, in which the results of arithmetical and logical operations are formed. [AHD] Acousta-Voicette™ - Altec Lansing trademarked name for their model 729A graphic equalizer, a two channel, 24-band, cut-only 1/3-octave design introduced in 1971. This was the first commercially available 1/3-octave graphic equalizer.

acoustic cryocooler - Thermoacoustic cryocoolers generally have two major sections to their design: an electroacoustic transducer (like a loudspeaker) and a coldhead." "Used for the study of the conversion of acoustic energy -- compression waves in a gas (sound) -- into heat energy and vice versa. Acoustic energy can be harnessed in sealed systems and used to create powerful heat engines, heat pumps, and refrigerators. Thermoacoustic devices use these compression waves to replace mechanical pistons, crankshafts, and valves, reducing the number of moving parts in their design and making them simple, reliable machines.

acoustic distortion - Term coined by Dr. Peter D'Antonio, founder of RPG Diffusor Systems, for the interaction between the room, the loudspeaker, and the listener.

acoustic echo canceller - "Acoustic" echo cancellers are used in teleconferencing applications to suppress the acoustic echoes caused by the microphone/loudspeaker combination at one end picking up the signal from the other end and returning it to the original end.

acoustic enhancement or EAE (electronic acoustic enhancement) (seen shortened to acoustic enhancement and called electronic architecture) - Any of several systems that make use of adding sound energy to a listening space rather than using sound absorbers to improve the quality.

acoustic feedback - The phenomenon where the sound from a loudspeaker is picked up by the microphone feeding it, and re-amplified out the same loudspeaker only to return to the same microphone to be re-amplified again, forming an acoustic loop. Each time the signal becomes larger until the system runs away and rings or feeds back on itself producing the all-too-common scream or squeal found in sound systems. These buildups occur at particular frequencies called feedback frequencies.

acoustic impedance - Technically it is the complex ratio of acoustic pressure to acoustic volume velocity, at a single frequency. Equivalently, it is a frequency response function in which pressure is the output and volume velocity is the input. [Morfey] First described by Webster in 1919.

acoustic lens - (1) In reference to loudspeakers, an acoustic lens focuses sound in much the same way that an optical lens focuses light. Snell's law describes the refraction of sound as it passes through an interface between two materials of differing sound speed. A high frequency loudspeaker mechanical acoustic lens provides the appropriate apparatus to spread a single point sound source into a parallel wave front. (2) In reference to ultrasonography, a lens (often electromagnetic) used to focus or diverge a sound beam.

acoustic lobe - An acoustic lobe results when both drivers operate together reproducing the crossover frequency band, and in the Butterworth case it exhibits severe peaking and is not on-axis (it tilts toward the lagging driver).

acoustic mirrors or sound mirrors - Giant parabolic reflector concrete structures used as an early warning system to detect enemy aircraft approaching Great Britain in the 1920s and 1930s before the advent of radar.

acoustics - The study of sound. (1) Of or relating to sound, the sense of hearing, or the science of sound. (2) (a) Designed to carry sound or to aid in hearing. (b) Designed to absorb or control sound: acoustic tile. (3) In reference to musical instrumentation, (a) Of or being an instrument that does not produce or enhance sound electronically: an acoustic guitar; an acoustic bass. (b) Being a performance that features such instruments: opened the show with an acoustic set. [AHD]

acoustic treatments - There are only three classic (physical) tools available for the acoustician to treat a room: absorbers, reflectors and diffusers. Absorbers attenuated sound; reflectors redirect sound, and diffusers (hopefully) uniformly distribute sound. Or put another way, these tools change the temporal, spectra and spatial qualities of the sound. Additionally, with today's advanced digital audio tools, all of these elements can be electronically manipulated. [Rane Corporation]

acoustooptics Abbr. AO - The science of the interaction of sound and light. A bit of a misnomer since it usually involves ultrasonic frequencies.

AC power plugs and sockets - The various plugs and sockets used to connect any country's AC mains and appliances and other electrical equipment.

acquisition time - The time required for a sample-and-hold (S/H) circuit to capture an input analog value; specifically, the time for the S/H output to approximately equal its input.

ACR (attenuation to crosstalk ratio) - The ratio of attenuation and crosstalk in a cable, i.e., a measure of the difference between the received signal magnitude vs. the leaked crosstalk signal.

AC tape bias Tape - First applied by Dr. Walter Weber at Siemens in the early '40s to ferric-oxide tapes base on previous work in the '20s used on wire recorders.

active component - A component requiring power to operate, e.g. a transistor. Contrast with passive.

active crossfader - A device found in DJ mixers used to crossfade between two music sources. An active design uses the potentiometer to send a control voltage to some type of voltage-controlled device that controls the audio, while in a passive design the audio appears on the potentiometer itself.

active crossover - A loudspeaker crossover requiring a power supply to operate. Usually rack-mounted as a separate unit, active crossovers require individual power amplifiers for each output frequency band. Available in configurations known as stereo 2-way, mono 3-way, and so on. A stereo 2-way crossover is a two-channel unit that divides the incoming signal into two segments, labeled Low and High outputs (biamped). A mono 3-way unit is a single channel device with three outputs, labeled Low, Mid and High (triamped). In this case, the user sets two frequencies: the Low-to-Mid, and the Mid-to-High crossover points. Up to stereo 5-way configurations exist for very elaborate systems. [Rane Corporation]

active equalizer - A variable equalizer requiring a power supply to operate.

ACU (Audio Coverage Uniformity) - A standard issued by InfoComm performance standards program.

adaptive delta modulation (ADM) - A variation of delta modulation in which the step size may vary from sample to sample.

ADAT (Alesis Digital Audio Tape) - Digital tape recording system developed by Alesis, and since licensed to Fostex & Panasonic, putting 8-tracks of 16-bit, 44.1 kHz digital audio on S-VHS tape.

ADAT ODI - ADAT Optical Digital Interface

ADAT Optical - Alesis's proprietary multichannel optical (fiber optic) digital interface specification for their family of ADAT modular digital multitrack recorders. This standard describes transmission of 8-channels of digital audio data through a single fiber optic cable.

ADC (or A/D, analog-to-digital converter) - The electronic component which converts the instantaneous value of an analog input signal to a digital word (represented as a binary number) for digital signal processing. The ADC is the first link in the digital chain of signal processing.

add-in or add-on or plugin or plug-in - In reference to software, an accessory program that extends the capabilities of an existing application. [AHD] First developed in the mid '70s; back in the days of the Univac mainframe computer.

ADE (Amsterdam Dance Event) - International conference and festival for electronic music.

ADJA (American Disc Jockey Association) - An organization of professional disc jockeys that promotes ethical behavior, industry standards and continuing education for its members.

admittance - The reciprocal of impedance.

ADPCM (adaptive differential pulse code modulation) - A very fast data compression algorithm based on the differences occurring between two samples.

ADR (automatic dialog replacement) - Film postproduction term used to indicate the act and location where dialogue that is not taped during production or that needs to be redone is recorded and synchronized to the picture. Usually the name of the room where this occurs, containing a studio with a screen, TV monitors, microphones, control area, console and loudspeakers.

Advanced Audio Coding abbreviated (AAC) - Shortened name for the MPEG-2 Advanced Audio Coding specification, declared an international standard by MPEG in April 1997; however, now the term is used also to refer to MPEG-4 advanced audio coding.

AES (Audio Engineering Society) - Founded in 1948, the largest professional organization for electronic engineers and all others actively involved in audio engineering. Primarily concerned with education and standardization.

AES17 low-pass filter - The common name given to the low-pass filter defined by AES17-1998 AES standard method for digital audio engineering -- Measurement of digital audio equipment, used to limit the measuring bandwidth. The rather daunting specifications call for a filter with a passband response of 10 Hz to 20 kHz, ±0.1 dB and a stopband attenuation greater than 60 dB at 24 kHz.

AES3 interface - (The interface formerly known as AES/EBU). The serial transmission format standardized for professional digital audio signals (AES3-1992 AES Recommended Practice for Digital Audio Engineering - Serial transmission format for two-channel linearly represented digital audio data). A specification using time division multiplex for data, and balanced line drivers to transmit two channels of digital audio data on a single twisted-pair cable using 3-pin (XLR) connectors. Issued as ANSI S4.40-1985 by the American National Standards Institute. In addition, information document AES-3id is available describing the transmission of AES3 formatted data by unbalanced coaxial cable. Transmission by fiber optic cable is under discussion. The consumer version is referred to as S/PDIF.

AES3-MIC - Any microphone having a digital output that conforms to the AES42 Digital Microphone Interface standard.

AES/EBU interface - (AES3 replaces AES/EBU) - The serial transmission format standardized for professional digital audio signals (AES3-1992 AES Recommended Practice for Digital Audio Engineering - Serial transmission format for two-channel linearly represented digital audio data). A specification using time division multiplex for data, and balanced line drivers to transmit two channels of digital audio data on a single twisted-pair cable using 3-pin (XLR) connectors. Issued as ANSI S4.40-1985 by the American National Standards Institute. In addition, information document AES-3id is available describing the transmission of AES3 formatted data by unbalanced coaxial cable

aetherphone - Alternate name for the theremin. Considered the first electronic musical instrument, invented in 1919 by Russian born Lev Sergeivitch Termen, which he anglicized to Leon Theremin. The theremin is unique in that it is the only musical instrument played without being touched.

AF (audio frequencies) Standard abbreviation for the accepted normal range of human audible frequencies being 20 Hz to 20 kHz.

A-filter - A-weighting (not official but commonly written as dBA) The A-curve is a wide bandpass filter centered at 2.5 kHz, with ~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz, therefore it tends to heavily roll-off the low end, with a more modest effect on high frequencies. It is the inverse of the 30-phon (or 30 dB-SPL) equal-loudness curve of Fletcher-Munson.

AFL - Abbreviation for after fade listen, a term used on recording consoles and mixers, referring to a signal taken after the main channel fader; hence this sampling point tracks the main fader level. Also referred to as post fade solo, but since PFL already meant pre fade, AFL was adopted to prevent confusion.

A-format or A-format soundfield microphone - A specialized microphone array comprised of four cardiod or supercardiod microphones: three to measure left-right, front-back, up-down sound pressure levels and another that measures overall sound pressure level.

AGC (automatic gain control), aka ALC (automatic level control) - A circuit or algorithm that varies gain as a function of the input signal amplitude. Commonly found in pro audio applications where you want to automatically adjust the gain of different sound sources in order to maintain a constant loudness level at the output.

AIA (American Institute of Architects) - The premier organization for architects and those working with architects (sound contractors, integrators, etc.).

AIFF (audio interchange file format) - Defined by Apple Computer in 1988, it provides a standard for storing monaural and multichannel sampled sounds at a variety of sample rates and widths.

air motion transformer (AMT) - Midrange tweeter invented by Dr. Oskar Heil, which operates on a different principle than both dynamic and electrostatic drivers. Known also as the AVT (air velocity transformer),

A Law - The PCM coding and companding standard used in Europe and in areas outside of North American influence. [Newton] Contrast with Mu Law used in North America and Japan.

ALC (automatic level control) aka AGC (automatic gain control) - A circuit or algorithm that varies gain as a function of the input signal amplitude. Commonly found in pro audio applications where you want to automatically adjust the gain of different sound sources in order to maintain a constant loudness level at the output.

aleatoric - Using or consisting of sounds to be chosen by the performer or left to chance; indeterminate. [AHD]

algorithm - A structured set of instructions and operations tailored to accomplish a signal processing task. For example, a fast Fourier transform (FFT), or a finite impulse response (FIR) filter are common DSP algorithms.

algorithmic reverb - Digital simulation of reverberation based on algorithms. Contrast with convolution reverb.

aliasing - The problem of unwanted frequencies created when sampling a signal of a frequency higher than half the sampling rate.

Allison Effect - Describes how room boundaries and loudspeaker power output interact. Specifically, the name for the destructive interference pattern that develops when a radiator is located one-quarter wavelength away from a reflective surface.

all-pass filter - A filter that provides only phase shift or phase delay without appreciably changing the magnitude characteristic.

ALMA (American Loudspeaker Manufacturers Association) - Founded in 1964, an international trade association for companies that design, manufacture, sell, and/or test loudspeakers, loudspeaker components and loudspeaker systems.

alnico (al[uminum] + ni[ckle] + co[balt] ) - Any of several hard, strong alloys of iron, aluminum, nickel, cobalt and sometimes copper, niobium, or tantalum, used to make strong permanent magnets [found in loudspeakers]. [AHD]

alternating current Abbr. AC or ac - An electric current that reverses direction at regularly recurring intervals of time. Contrast: direct current. [IEEE]

AM (amplitude modulation) - (1) The encoding of a carrier wave by variation of its amplitude in accordance with an input signal. (2) A broadcast system that uses amplitude modulation. [AHD]

ambience - (1) In reference to acoustics, a perceptual sense of space [Blesser]. The acoustic qualities of a listening space [White]. (2) In reference to psychoacoustics, the special atmosphere or mood created by a particular environment; also spelled ambiance [AHD]. Contrast with reverberation.

ambient noise compensator or leveler - A dynamic processor that maintains (or "levels") the amount of one audio signal based upon the level of a second audio signal. Normally, the second signal is from an ambient noise sensing microphone.

Ambisonics - A British-developed surround sound system designed to reproduce a true three-dimensional sound field.

AME (Association for Manufacturing Excellence) - "A not-for-profit organization founded in 1985 dedicated to helping companies with continuous improvement and their pursuit of excellence."

AMI-C (Automotive Multimedia Interface Collaboration) - "An organization of motor vehicle manufacturers worldwide created to facilitate the development, promotion and standardization of electronic gateways to connect automotive multimedia, telematics and other electronic devices to their motor vehicles."

AMOLED (active-matrix OLED) - Refers to the technology behind the addressing of pixels in OLED displays.

AMP (Audio Music Partnership) - An alliance of industry partners that develop, manufacture, and support products and services that interoperate with the Microsoft platforms.

AMPAS (Academy of Motion Picture Arts & Sciences) - Created in 1927, a professional honorary organization composed of over 6,000 motion picture craftsmen and women.

ampere Abbr. I, also A - (1) A unit of electric current in the International standard meter-kilogram-second (mks) system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2E-7 newtons per meter of length. (2) A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere. (After André Marie Ampère.) [AHD]

Ampère, André Marie - (b. 1775-1836) French physicist and mathematician who formulated Ampère's law, a mathematical description of the magnetic field produced by a current-carrying conductor. [AHD]

amp head also head amp - In reference to electronics, (1) A pre-preamplifier or simply a preamplifier. A very low noise, high gain audio preamp used to boost signal levels from very low sources such as moving coil phono cartridges, some acoustic pick-ups, etc. (2) Slang for headphone amplifier. (3) In reference to music instrumentation, a guitar amplifier without speakers that usually sits on top of and forms the "head" of a loudspeaker stack, classically comprised of two cabinets consisting of four 10" or 12" drivers each.

amplifier - An electronic device used to increase an electrical signal. The signal may be voltage, current or both (power). Preamplifier is the name applied to the first amplifier in the audio chain, accepting inputs from microphones, or other transducers, and low output sources (CD players, tape recorders, turntables, etc.). The preamplifier increases the input signals from mic-level, for instance, to line-level. Power amplifier is the name applied to the last amplifier in the audio chain, used to increase the line-level signals to whatever is necessary to drive the loudspeakers to the loudness required.

amplifier classes - Audio power amplifiers were originally classified according to the relationship between the output voltage swing and the input voltage swing; thus it was primarily the design of the output stage that defined each class. See below:

Class A - Operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have "bottom side" transistors but these are operated as fixed current sources, not amplifying devices. Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning you draw about 5 times as much power from the source as you deliver to the load.) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion.

Class B - Operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.

Class AB - Operation is the intermediate case. Here both devices are allowed to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demand s. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.

Class AB1 & AB2 - Subdivisions of Class AB developed for vacuum tube design. These subsets primarily describe grid current behavior: Class AB1 has no current flowing into the grid of the tube, and Class AB2 has some current flowing into the grid. Class AB1 operates closer to Class A, while Class AB2 operates closer to Class B. Most bipolar solid-state amplifiers would be classified as Class AB2, while power JFET designs mimic Class AB1.

Class AB plus B - Design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.

Class BD - Invented by Robert B. Herbert in 1971 U.S. patent 3,585,517 and improved on by Neil Edward Walker as disclosed in his 1971 U.S. patent 3,629,616. Both patents are concerned with improving original class D design efficiencies by using various bridge connections and cancellation techniques. And most recently more improvements are claimed by inventors James C. Strickland & Carlos A. Castrejon in their U.S. patent 6,097,249 assigned to Rockford Corporation in 2000 for their Fosgate-brand automotive amplifier.

Class C - Use is restricted to the broadcast industry for radio frequency (RF) transmission. Its operation is characterized by turning on one device at a time for less than one half cycle. In essence, each output device is pulsed-on for some percentage of the half cycle, instead of operating continuously for the entire half cycle. This makes for an extremely efficient design capable of enormous output power. It is the magic of RF tuned circuits (flywheel effect) that overcomes the distortion create d by class C pulsed operation.

Class D - Operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle (Sampling Theorem). Theoretically since the output devices are either completely on or completely off they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the load, so the power dissipated by the device is zero (found by multiplying the voltage across the device [zero] times the current flowing through the device [big], so 0 x big = 0); and when the device is off, the voltage is large, but the current is zero so you get the same answer. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.

Class E - Operation involves amplifiers designed for rectangular input pulses, not sinusoidal audio waveforms. The output load is a tuned circuit, with the output voltage resembling a damped single pulse. Normally Class E employs a single transistor driven to act as a switch.

The following terms, while generally agreed upon, are not official classifications:



Class F - Also known by such terms as "biharmonic," "polyharmonic," "Class DC," "single-ended Class D," "High-efficiency Class C," and "multiresonator." Another example of a tuned power amplifier, whereby the load is a tuned resonant circuit. One of the differences here is the circuit is tuned for one or more harmonic frequencies as well as the carrier frequency.

Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks [thus the nickname rail-switcher]. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is common for pro audio designs.

Class H - Operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices [thus the nickname rail-tracker or tracking power amplifier]. The efficiency of class H is comparable to class G designs.

Class I - Operation invented and named by Gerald R. Stanley for amplifiers based on his patent U.S. 5,657,219 covering opposed current converters.

Class J - Operation is the category/name suggested by Gerald R. Stanley for amplifiers that combine class B and class D where converters act in parallel to drive the load.

Class S - First invented in 1932, this technique is used for both amplification and amplitude modulation. Similar to Class D except the rectangular PWM voltage waveform is applied to a low-pass filter that allows only the slowly varying dc or average voltage component to appear across the load.



amplifier dummy load - Modeling a real world loudspeaker for power amplifier testing purposes has been studied for years, resulting in many circuit possibilities.

amplitude - (1) Greatness of size; magnitude. (2) In reference to physics, the maximum absolute value of a periodically varying quantity. (3) In reference to mathematics, (a) The maximum absolute value of a periodic curve measured along its vertical axis. (b) The angle made with the positive horizontal axis by the vector representation of a complex number. (4) In reference to electronics, the maximum absolute value reached by a voltage or current waveform. [AHD] amplitude-frequency response - Connotes amplitude-frequency response and quantifies a device's maximum and minimum frequency for full-output response. The electrical passband of an audio device. The measure of any audio device's ability to respond to a sine wave program, and therefore is a complex function measuring gain and phase shift (see phasor). It is used to express variation of gain, loss, amplification, or attenuation as a function of frequency, normally referred to a standard 1 kHz reference point.

amplitude modulation abbr. AM - (1) The encoding of a carrier wave by variation of its amplitude in accordance with an input signal. (2) A broadcast system that uses amplitude modulation. [AHD]

AMT (air motion transformer) - Midrange tweeter invented by Dr. Oskar Heil, which operates on a different principle than both dynamic and electrostatic drivers. Known also as the AVT (air velocity transformer).

anacrusis or upbeat - An unaccented beat or beats that occur before the first beat of a measure. Also called pickup.

analog - A real world physical quantity or data characterized by being continuously variable (rather than making discrete jumps), and can be as precise as the available measuring technique.

AND - A Boolean logical operator that returns a true value only if both operands are true; a form of multiplication.

anechoic - Literally, without echo, used to describe specially designed rooms, anechoic chambers, built to emulate a free sound field, by absorbing practically all the sound field.

angstrom Abbr. Å - A unit of length equal to one-tenth of a nanometer -- used for measuring the wavelengths of light.

anisotropic - (1) Not isotropic. (2) Having properties that differ according to the direction of measurement. [AHD]

annealed - (1) To subject (glass or metal) to a process of heating and slow cooling in order to toughen and reduce brittleness. (2) To strengthen or harden. [AHD]

anode - (1) A positively charged electrode. (2) In a vacuum tube, it is the plate electrode. (3) In a forward-biased semiconductor diode it is the positive terminal. Contrast with cathode.

ANSI (pronounced "ann-see") (American National Standards Institute) - A private organization that develops and publishes standards for voluntary use in the U.S.A.

Antheil, George - (b. 1900-1959) US Composer, specializing in film music, who described himself as "America's bad boy of music."

anti-aliasing filter - A low-pass filter used at the input of digital audio converters to attenuate frequencies above the half-sampling frequency to prevent aliasing.

anti-imaging filter - A low-pass filter used at the output of digital audio converters to attenuate frequencies above the half-sampling frequency to eliminate image spectra present at multiples of the sampling frequency.

antiquing - "The act of processing modern audio files to make them appear to have originated from historic technology. It is the inverse of restoring old recordings."

antiskating also anti-skating - A control mechanism on a phonograph designed to compensate for the natural tendency of a pivoted tone arm to pull toward the center. [AHD] That is, it keeps the stylus centered in the groove.

AO – acycronym for acoustooptics, which is the science of the interaction of sound and light. A bit of a misnomer since it usually involves ultrasonic frequencies.

AoE (Audio over Ethernet) - Many systems exist using Ethernet for the transport of digital audio.

AOIP (audio over Internet Protocol) - A system for audio file sharing/transport that converts analog or digital audio into Ethernet packers for distribution over a LAN or Ethernet switches.

AP (access point) - A device that connects a wireless network to a wired LAN.

APA (Audio Publishers Association) - The online resource center designed for audiobook listeners and industry professionals.

APD (auto power down) - Term used to describe equipment with a built-in feature that automatically reduces the power draw after a prescribed amount of time.

API (application program interface) - Protocols for creating software applications.

APL (after processing listen) - A mixing console term and feature that allows post-processing monitoring of the signal. Compare with AFL and PFL.

APPA (Association of Higher Education Facilities Officers) - The abbreviation, APPA, comes from their early name: Association of Physical Plant Administrators of Universities and Colleges, which was changed in 1991.

apparent power - The result of multiplying the rms value of the voltage by the rms value of the current in an electronic circuit. It is expressed in watts (W) for resistive loads and in volt-amperes (VA) for reactive loads. It's the amount of power the casual observer thinks is available (hence, apparent), but because of power factor may not be -- the real power is usually less.

appoggiatura - An embellishing note, usually one step above or below the note it precedes and indicated by a small note or special sign. [AHD] A melodic tone.

A&R (artists and repertory) - Historically the record industry term for the department or person that acts as the go-between the artist and the record label. Their job is to select and sign the performers to the label, decide what songs they will record, and select who will work with the artists in the production arranging and performance of the material for the recording of master tapes. These details vary a lot from label to label.

architectural columns or line arrays - A vertical line (or linear) configuration for large venue multi-cabinet loudspeaker systems creating tight (and steerable) beamwidth coverage (degrees of arc for the propagating sound wave, vertically and horizontally). Favored for their controlled directivity that reduces room reflections and produces less reverberation and improved sound intelligibility, as well as reducing the sound that bleeds back onto the performers. The three most popular configurations are (a) uniform array: typically 2-8 boxes arranged in a flat straight line popular in smaller venues and usually tilted downward above the audience; (b) constant splay array: forms a smooth arc by tilting each box the same amount (pitch) resulting in a wider beamwidth popular in concert hall settings, particularly those with balconies; (c) progressive splay array: combines both previous examples by starting out with a straight flat array that gradually creates an arc at the lower end, forming the letter "J" like shape. Popular for large arenas and concert settings. Individual and unique variations are offered by all major loudspeaker companies.

Archos Jukebox Multimedia - Developed by Archos and released in 2002, it is recognized as the first handheld MP3/MP4 media player, combining an audio player, image viewer and video player.

arithmetic - The mathematics of integers, rational numbers, real numbers, or complex numbers under addition, subtraction, multiplication, and division. [AHD]

arithmetric progression - A sequence, such as the positive odd integers 1, 3, 5, 7, . . . , in which each term after the first is formed by adding a constant to the preceding term. [AHD]

ARM (advanced RISC machines) - The name for a microprocessor group formed from Acorn, backed by Apple, VLSI Technology and Nippon Investment and Finance, in 1990. Acorn Computer was the parent company set up by Dr. Hermann Hauser and Dr. Chris Curry in 1979 to make personal computers, but now enjoys its biggest success selling intellectual property around their proprietary RISC computer, called ARM, which originally stood for Acorn RISC Machines.

armonica - Name for Benjamin Franklin's improvement on the Glass harmonic in 1761.

Armstrong, Edwin Howard - (b. 1890-1954) American radio engineer and inventor of regenerative feedback, FM (frequency modulation) and the superheterodyne receiver.

Arnold, H. D. American - Engineer who invented and patented the thermophone, while working at Western Electric. He is also credited with introducing a vacuum to the triode tube, thus perfecting its performance.

ARRL (American Radio Relay League) - The national association for amateur radio.

articulated line arrays - A vertical line (or linear) configuration for large venue multi-cabinet loudspeaker systems creating tight (and steerable) beamwidth coverage (degrees of arc for the propagating sound wave, vertically and horizontally). Favored for their controlled directivity that reduces room reflections and produces less reverberation and improved sound intelligibility, as well as reducing the sound that bleeds back onto the performers. The three most popular configurations are (a) uniform array: typically 2-8 boxes arranged in a flat straight line popular in smaller venues and usually tilted downward above the audience; (b) constant splay array: forms a smooth arc by tilting each box the same amount (pitch) resulting in a wider beamwidth popular in concert hall settings, particularly those with balconies; (c) progressive splay array: combines both previous examples by starting out with a straight flat array that gradually creates an arc at the lower end, forming the letter "J" like shape.

artifact Audio - Anything added, due to technical limitations, that was not in the original signal.

ASA (Acoustical Society of America) - Founded in 1929, the oldest organization for scientist and professional acousticians and others engaged in acoustical design, research and education.

ASCII (pronounced "ask-ee") (American Standard Code for Information Interchange) - An ANSI standard data transmission code consisting of seven information bits, used to code 128 letters, numbers, and special characters. Many systems now use an 8-bit binary code, called ASCII-8, in which 256 symbols are represented (for example, IBM's "extended ASCII").

ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) - An international organization organized for the purpose of advancing the arts and sciences of heating, ventilation, air-conditioning and refrigeration for the public's benefit through research, standards writing, continuing education and publications.

ASIC (application-specific integrated circuit) - A large-scale integrated circuit whose function is determined by the final mask layer for a particular application or group of applications; for example, an IC that does all the functions of a modem.

ASIO (pronounced "az-ee-o") (audio stream input/output) - Originally a multichannel audio transfer protocol developed by Steinberg in 1997, for audio/MIDI sequencing applications, allowing access to the multichannel capabilities of sound cards. Today it is a standard driver protocol for digital audio and computer sound cards.

ASPEC (adaptive spectral perceptual entropy coding) - A bit rate reduction standard for high quality audio. Jointly developed by AT&T Bell Labs, Thomson, the Fraunhofer Society and CNET. Characterized by high degrees of compression to allow audio transmission on ISDN.

asperity noise - Noise caused by microscopic imperfections in the oxide coating of magnetic tape. Heard as a low frequency rumble similar to rocks banging together.

ASTC (Association of Science-Technology Centers) - "An organization of science centers and museums dedicated to furthering public engagement with science among increasingly diverse audiences."

asymmetrical (non-reciprocal) response - Term used to describe the comparative shapes of the boost/cut curves for variable equalizers. The cut curves do not mirror the boost curves, but instead are quite narrow, intended to act as notch filters.

asynchronous - A transmission process where the signal is transmitted without any fixed timing relationship between one word and the next (and the timing relationship is recovered from the data stream).

ATA (American Telemedicine Association) - "The leading resource and advocate promoting access to medical care for consumers and health professionals via telecommunications technology."

A-taper or audio taper - Usually 15% resistance at the 50% rotation point.

ATM (asynchronous transfer mode) networking - An extremely fast networking technology already found on many disk editors (Avid, Sonic Solutions, Studio Audio, etc.) and predicted to infiltrate homes within the coming decade. ATM specifies the protocol (i.e., the order and sequence) of the digital information on the network, but not the physical means of transmission (e.g., fiber optic, twisted-pair, etc.). The protocol controls how the entire network is run and maintained.

atmospheric pressure - Pressure caused by the weight of the atmosphere. At sea level it has a mean value of one atmosphere but reduces with increasing altitude. [AHD] See 0 dB-SPL.

atofarad Abbr. aF - A prefix for 10-18 farads, as in 1 ppm of a 1 pF measurement (10-6 x 10-12).

ATRAC (Adaptive Transform Acoustic Coding) - Very early (1992) Sony proprietary audio data compression technique using psychoacoustic principles to convert standard CD quality audio (16-bit, 44.1 kHz) into a file one-fifth the original size.

attack - The beginning or manner of beginning a piece, passage, or tone. [AHD] A measure of how long it takes for the beginning to peak. Contrast with decay. Audio Compressors. How fast the gain is turned down once the signal exceeds the threshold setting. Contrast with release.

attenuation to crosstalk ratio abbr. ACR - The ratio of attenuation and crosstalk in a cable, i.e., a measure of the difference between the received signal magnitude vs. the leaked crosstalk signal.

attenuator or attenuator pad - A passive network that reduces the voltage (or power; see usage note under gain) level of a signal with negligible distortion, but with insertion loss. Often a purely resistive network, although any combination of inductors, resistors and capacitors are possible, a pad may also provide impedance matching. [Compare with fader and crossfader. More details available in an excellent article by Rick Chinn, "Pads 101" appearing in the Syn-Aud-Con Newsletter, Vol. 32, No. 2 Spring 2004, pp. 8-11.] Pads are referred to by the topology of the network formed, with the two most common being an L-pad and a T-pad:

L-pad - A two-leg network shaped like an inverted, backward letter "L". It usually consists of two resistors that are fixed or adjustable. A true variable L-pad consists of two variable potentiometers that are ganged (tied) together. The ganged sections work to provide either a constant input or a constant output impedance regardless of the attenuation setting. Since modern analog audio electronic circuits consist of stages characterized by very high input and very low output impedances, the term is now broaden to include all L-shaped networks without the requirement of providing constant impedance to the source or load. Volume and level controls are common examples.

Balanced L-pad (or U-pad) - A balanced version of the above L-pad, the following is for general purpose audio, recommended by the IEC, exact and nearest 1% values shown.

T-pad - A three-leg network shaped like the letter "T". It usually consists of three resistors that are fixed or adjustable. A true variable T-pad consists of two or three variable potentiometers that are ganged (tied) together. The ganged sections work to provide either a constant input or a constant output impedance regardless of the attenuation setting. Since modern analog audio electronic circuits consist of stages characterized by very high input and very low output impedances, the term is now broaden to include all T-shaped networks without the requirement of providing constant impedance to the source or load.

Bridged T-pad - In this configuration, R1 and R2 are fixed to the pad's impedance, while R3 and R4 can be variable.

Balanced T-pad (or H-pad) - R1 and R3 are half the values of the unbalanced T-pad above.

O-pad - Used when the input impedance is much higher than the impedance across the output.



aubades - (1) A song or instrumental composition concerning, accompanying, or evoking daybreak. (2) A poem or song of or about lovers separating at dawn. [AHD]

Audimax - CBS trademark for a broadcast AGC unit invented by Emil Torick in the '50s to replace the transmitter watch engineer.

audio - (1) Of or relating to humanly audible sound, i.e., audio is all the sounds that humans hear (approximately 20 Hz - 20 kHz). (2) (a) Of or relating to the broadcasting or reception of sound. (b) Of or relating to high-fidelity sound reproduction. (Audio traveling through air is vibrations, or cycles of alternating pressure zones. Rarefaction follows each cycle of compression, which produces a wave.) [AHD]

audio bridge - A communications bridge that allows multiple duplex connections over 4-wire telephone connections. Well designed audio bridges do not connect inputs to their own outputs, thus avoiding feedback.

audio compression (digital audio data compression) - Any of several algorithms designed to reduce the number of bits (hence, bandwidth and storage requirements) required for accurate digital audio storage and transmission. Characterized by being "lossless" or "lossy." The audio compression is "lossy" if actual data is lost due to the compression scheme, and "lossless" if it is not. Well-designed algorithms ensure "lost" information is inaudible.

audio connectors - Audio equipment uses many types of connectors.

audio coverage uniformity abbr. ACU - A standard issued by InfoComm performance standards program.

audio levels - Terms used to describe relative audio signal levels, such as:

mic-level - Nominal signal coming directly from a microphone. Very low, in the microvolts, and requires a preamp with at least 60 dB gain before using with any line-level equipment.

line-level - Standard +4 dBu or -10 dBV audio levels.

instrument-level - Nominal signal from musical instruments using electrical pick-ups. Varies widely, from very low mic-levels to quite large line-levels.

audiology - The study of hearing, especially hearing defects and their treatment. [AHD]

Audio magazine - (b. 1947-2000) America's first and longest running audio magazine, 53 years of continuous publication.

audion - Dr. Lee De Forest's name for his 1906 invention of the triode (three-element vacuum tube), building upon Sir John Ambrose Fleming's thermionic diode, based on the Edison effect. De Forest credits his assistant, C.D. Babcock for the name.

Audio over Ethernet abbr. AoE - Many systems exist using Ethernet for the transport of digital audio. Hit the link for details about the various proprietary protocols that include Layer 1 (e.g., Digigram's Ethersound®), Layer 2 (e.g. Cirrus Logic's CobraNet® also AVB: Audio/Video Bridging) and Layer 3 (e.g. Audinate's Dante®).

audion piano - The first vacuum tube instrument in 1915, invented by Dr. Lee De Forest.

audio snake or snake - The nickname for the cable running from the stage of a live performance to the main mixing console, which is usually set-up in the middle or rear of the audience (in spite of being called FOH). It typically contains one shielded pair (STP) of wires for each of the stage microphones. The name comes from the multiconductor cable looking sort of snake-like.

audio taper (aka A-taper) - Usually 15% resistance at the 50% rotation point.

Audio Timeline - A most fascinating audio development timeline created by three esteemed AES members, Jerry Bruck, Al Grundy and Irv Joel, as part of the 50th anniversary of the AES.

auditory filter - Term used to describe the concept of critical bands. Analogous to a bandpass filter with a rounded top ("rounded-exponential" after Patterson and Moore, 1986). The filter is slightly asymmetric, being wider on the low-frequency side.

auditory masking or mask (aka masking - The human hearing phenomenon where the response to one stimulus is reduced in the presence of another, i.e., two sounds arrive but only one sound is heard. Particularly evident when one sound is louder than another, with the result being that we hear the louder sound, even if arriving at a slightly different time. Frequency plays a part: a louder sound heard at one frequency prevents softer sounds near that frequency from being heard. However, not all frequencies mask the same. Mid-band frequencies mask far better than low frequencies, for example.

aulosAncient Egyptian double flute. Also used for single flute; hit the link for all the variations.

aural - Of, relating to, or perceived by the ear. [AHD]

aural architecture - The phrase coined by Blesser & Salter in their book: Spaces Speak, Are You Listening, to describe the complex phenomenon of how humans sense space by listening. Sensing spatial attributes is a nature human ability and the authors conclusively make the point that every environment has an aural architecture.

aural hallucinations or clairaudient - The supposed power to hear things outside the range of normal perception.

Aureal 3D (A3D) - Proprietary 3D sound technology first developed by Crystal River Engineering, which became the advanced technology subsidiary of Aureal Semiconductor.

auricle or pinna - The outer portion of the ear; acts like an audio filter or equalizer and separates sounds coming from the front and rear.

authoring - DVD, CD or CD-ROM. A term used to indicate more than writing, now used to include all the processes necessary (designing, creating & editing) to add information of any sort onto a DVD, CD or CD-ROM primarily providing search and retrieval features.

autoformer - Autoformer is short for autotransformer, or self-transformer, from the definition of auto-. An autotransformer is one that self-magnetizes to produce the transformer voltage, it does this by not having a true secondary, i.e., there is only one winding with one part acting as the primary and the other part acting as the secondary, but there is no second winding, and no air gap, and thus no true isolation between the primary and secondary. Therefore an autotransformer is a transformer in which part of one winding is common to both the primary and the secondary circuits associated with that winding.

automatic attack & release - Automatic attack and release designs typically look at the rate of change of the difference between the threshold and the current signal level (the error signal). If the difference changes quickly, attack and release respond faster to the change. If the rate of change is small, attack and decay are slowed. The result is a relatively fast response to quick changes and a much slower response to slower changes, significantly reducing pumping and distortion. Digital designs do this using DSP algorithms.

automatic gain control abbr. AGC aka ALC (automatic level control) - A circuit or algorithm that varies gain as a function of the input signal amplitude. Commonly found in pro audio applications where you want to automatically adjust the gain of different sound sources in order to maintain a constant loudness level at the output.

automatic mic mixer or automixer - A specialized mixer optimized for solving the problems of multiple live microphones operating together as a system, such as found in boardrooms, classrooms, courtrooms, church systems, etc. An automatic mic mixer controls the live microphones by turning up (on) mics when someone is talking, and turning down (off) mics that are not used, thus it is a voice-activated, real-time process, without an operator, hence, automatic. An automatic mic mixer must adapt to changing background noise conditions. Further it must control the additive effect of multiple mics being on at the same time. If one mic is on at maximum gain, opening up another one may cause acoustic feedback, so an automatic mixer must also control the system gain to prevent feedback or excessive noise pickup. [Rane Corporation]

aux - Nickname for auxiliary jack, found on audio equipment and used as an additional input or output.

aux fed subs, or aux fed subwoofers - A live sound technique becoming popular when subwoofers are used with the FOH system. It is claimed that a properly configured and operated aux fed subwoofer system better maintains gain structure and crossover relationships.

AVB (Audio/Video Bridging) - Name for the IEEE 802 emerging standard for gigabit Ethernet networks that is finding favor in the live sound industry to transport digital audio between the stage and the FOH console.

AV Abbreviation for audio- video - Referring to sytems that contain both.

AVD (advanced video disk) - A Chinese proposed alternative to the DVD standard.

AVnu Alliance - (pronounced "avenue") From website: "An industry forum dedicated to the advancement of professional-quality audio video by promoting the adoption of the IEEE 802.1 Audio Video Bridging (AVB), and the related IEEE 1722 and IEEE 1733, standards over various networking link-layers."

AVT (air velocity transformer) or air motion transformer - Midrange tweeter invented by Dr. Oskar Heil, which operates on a different principle than both dynamic and electrostatic drivers.

average power -The result of multiplying the rms value of the voltage by the rms value of the current in an electronic circuit. It is expressed in watts (W) for resistive loads and in volt-amperes (VA) for reactive loads. It's the amount of power the casual observer thinks is available (hence, apparent), but because of power factor may not be -- the real power is usually less. [Rane Corporation]

A-weighting - The A-curve is a wide bandpass filter centered at 2.5 kHz, with ~20 dB attenuation at 100 Hz, and ~10 dB attenuation at 20 kHz, therefore it tends to heavily roll-off the low end, with a more modest effect on high frequencies. It is the inverse of the 30-phon (or 30 dB-SPL) equal-loudness curve of Fletcher-Munson.

AWG (American wire gauge) - A specification for non-ferrous (e.g., copper, aluminum, gold, silver, etc.) wire diameter. [Note, for example, that this means that 14 gauge galvanized steel wire & 14 gauge cooper wire have different diameters.] Also known as Brown and Sharp (B&S) wire gauge, after J.R. Brown who devised the system in 1857. The British standard is called SWG standing for Standard wire gauge, also called Imperial wire gauge.

axe - Musical instrument, usually a guitar.

axial mode - Sound reflecting between two parallel surfaces.

AZA (Association of Zoos and Aquariums) - "AZA-accredited zoos and aquariums are leaders in the protection of endangered species." Another resource for audio contractors, integrators, etc.

azimuth - (1) The horizontal angular distance from a reference direction, usually the northern point of the horizon, to the point where a vertical circle through a celestial body intersects the horizon, usually measured clockwise. Sometimes the southern point is used as the reference direction, and the measurement is made clockwise through 360°. (2) The horizontal angle of the observer's bearing in surveying, measured clockwise from a referent direction, as from the north, or from a referent celestial body, usually Polaris. [AHD]

azimuth recording - Magnetic recording trick that reduces crosstalk between adjacent tracks.

azure noise or noise color - People working in pro audio know the terms white noise and pink noise, but few recognize the terms "azure noise" or "red noise," but they are real terms. Noise that is not white is called colored noise and will have more energy at some frequencies than others, analogous to colored light. White noise and pink noise are well defined and known; much less so are the others. White noise is so named because it is analogous to white light in that it contains all audible frequencies distributed uniformly throughout the spectrum. Passing white light through a prism (a form of filtering) breaks it down into a range of colors. Examination shows that red light is characterized by the longer wavelengths of light, i.e., the lower frequency region. Similarly, "pink noise" has higher energy in the low frequencies, hence the somewhat tongue-in-cheek term.

The Federal Standard 1037C Telecommunications: Glossary of Telecommunication Terms defines four noise colors (white, pink, blue & black) and is considered the official source. No official standard could be found for the others. The following list of noise colors is loosely based on a rainbow-prism light analogy, where a prism creates a rainbow effect by separating white light passed through it into a visible spectrum labeled red, orange, yellow, green, blue, indigo, and violet from lowest to highest frequencies. Also shown is the approximate slope of the power density spectrum relative to white noise used as the reference:

red noise also called brown noise or Brownian noise after Robert Brown - -6 dB/oct decreasing density (most amount of low frequency energy or power; used in oceanography; power proportional to 1/frequency-squared); popcorn noise.

pink noise - -3 dB/oct decreasing noise density (but, equal power per octave; 1/f noise or flicker noise; power proportional to 1/frequency).

white noise - 0 dB/oct reference noise with equal power density (equal power per hertz; Johnson noise).

grey noise - A random pink noise within the audible frequency range subjected to inverted A-weighting loudness curve per IEC 61672. It gives the listener the perception that it is equally loud at all frequencies.

blue (or azure) noise - +3 dB/oct increasing noise density (power proportional to frequency).

purple (or violet) noise - +6 dB/oct increasing noise density (power proportional to frequency-squared; most amount of high frequency energy or power).

black noise - silence (zero power density with a few random spikes allowed).

Other noise colors exist for specialized fields like video/photographic/image processing, communications, mathematical chaos theory, etc., but are not found in pro audio circles. [Rane Corporation]





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HistoryOfRecording.com acknowledges the Elsevier, Inc. publication, Audio Engineering know it all, the University of Washington Press publication, The Audio Dictionary, second edition, the Howard W. Sames & Co., Inc. publication, Audio cyclopedia, the Cambridge University Press publication, The Art of Electronics, Rane Corporation (Dennis A. Bohn, CTO), Houghton Mifflin Company publication, The American Heritage Dictionary of the English Language, Fourth Edition, the IEEE publication, IEEE 100: The Authoritative Dictionary of IEEE Standards Terms, Seventh Edition and Wikipedia in the preparation of this Dictionary of Audio Terminology.

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