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A Symbol for ampere.

Å See: angstrom.

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

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 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.

ABA (Audio Branding Academy) "The Audio Branding Academy was founded by Cornelius Ringe, Kai Bronner and Rainer Hirt in Hamburg in February 2009. It is the first independent institution for acoustic brand communication, aiming at promoting an intentional and responsible use of acoustic stimuli within brand communication.
The Audio Branding Academy is a unique competence center for intersection points of brands, sound and environment and combines a forum, think tank, expert network and education. It hosts the annual Audio Branding Congress and regularly organizes workshops on various audio branding related topics." [From website; hit link]

A Barking Dog The Weblog of Dave Stevens, the founder of LAB and cofounder of ProSoundWeb.

abbreviation 1. The act or product of shortening. 2. A shortened form of a word or phrase used chiefly in writing to represent the complete form, such as Mass. for Massachusetts or USMC for United States Marine Corps. [AHD] Compare with acronym and initialism.

Abe, Yoshiharu (1931 -2013) One of the founders of TEAC, inventor of the Portastudio® and consider the father of personal multitrack recording.

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

A-B powering See: T-powering.

absolute pitch Music. 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. See Siegfried Linkwitz's Acoustic absorption and acoustic resistors; contrast with isolation.

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

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 See: alternating current.

AC Abbreviation for air conditioning. [Some of you may wonder why this entry is included in the pro audio reference. Well it is here because back in 1880 a very early air conditioning system was patented by R. Portner and B. E. J. Eils (US Patent 229,750, Process of and Apparatus for Cooling Air) for use in their brewery, creating the very first air conditioned building for making beer, without which the pro audio industry would collapse.]

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. See Dolby Digital. Competes with DTS Consumer. The terms AC-1 and AC-2 are other versions developed by Dolby for different applications. [See Usage Note under AES3 for transmission gotcha caveats.]

Academy of Ancient Music See: AAM.

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 Music. 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.

accommodation Said to be the most misspelled word in American writing (two "c"s and two "m"s).

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

accumulator Computers. A register or electric circuit in a calculator or computer, in which the results of arithmetical and logical operations are formed. [AHD]

Acousta-Voicette™ Equalizers. 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 calculators Incredible web guide to online acoustic conversion calculators.

acoustic cryocooler See: thermoacoustics.

acoustic cryptanalysis The science of deciphering intelligence from secondary sounds emitted by computers and peripherals. [Fascinating field; hit the link to explore further.]

acoustic diode An apparatus that transmits sound in only one direction. First use is in medical ultrasound imaging equipment. Details of development of acoustic diodes for audible frequencies by Caltech engineers is found here.

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 See: echo canceller.

acoustic enhancement See: EAE.

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 horn See: horn.

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. 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. Additional information available from JBL. 2. Ultrasonography. A lens (often electromagnetic) used to focus or diverge a sound beam.

acoustic lobe See Linkwitz-Riley crossover.

acoustic mirrors See: sound mirrors.

acoustics Hearing; from the Greek akouein: to hear. 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. Music 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]

acoustics and vibration animations Fabulous website created by Prof. Dan Russell illustrating many important acoustic principles.

acoustic-suspension loudspeaker Invented by Edgar Villchur in 1954 and co-developed with Henry Kloss . Hit the link for technical details.

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.

acousto-optics Abbr. AO The science of the interaction of sound and light. A bit of a misnomer since it usually involves ultrasonic frequencies. For the math, see University of Colorado Acoustooptics Lecture.

AC power plugs and sockets Electrical Power. The various plugs and sockets used to connect any country's AC mains and appliances and other electrical equipment. Hit the link to see all the variations and details.

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) Category wiring. 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.

acronym A word formed from the first letters of a name, such as laser for light amplification by stimulated emission of radiation, or by combining initial letters or parts of a series of words, such as radar for radio detecting and ranging. The requirement of forming a word is what distinguishes an acronym from an abbreviation (or initialism as it is also called). Thus modem [modulator-demodulator] is an acronym, and AES [Audio Engineering Society] is an abbreviation or initialism. Compare with portmanteau word [Unsubstantiated rumor has it that the word "acronym" itself is an acronym, created from the phrase "abbreviating by cropping remainders off names to yield meaning" -- but it has never been confirmed.] (Thanks MR.)

AC tape bias Tape Recorders. 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 Electronics. 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 designs are more robust and offer greater reliability over passive ones. See Evolution of the DJ Mixer Crossfader by Rane's ace DJ mixer designer, Rick Jeffs, for additional details.

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. See passive crossover and the RaneNote Signal Processing Fundamentals.

active equalizer A variable equalizer requiring a power supply to operate. Available in many different configurations and designs. Favored for low cost, small size, light weight, loading indifference, good isolation (high input and low output impedances), gain availability (signal boosting possible), and line-driving ability. Disliked for increased noise performance, limited dynamic range, reduced reliability, and RFI susceptibility; however, used everywhere. See the RaneNote Operator Adjustable Equalizers.

ActiveX A Microsoft developed software technology released in 1996. ActiveX, formerly called OLE (Object Linking and Embedding), is loosely based on the Component Object Model (COM), but provides substantially different services to developers. An ActiveX component is a unit of executable code (such as an .exe file) that follows the Active X specification for providing objects. This technology allows programmers to assemble reusable software components into applications and services. However, component software development using ActiveX technology should not be confused with Object-Oriented Programming (OOP). OOP is concerned with creating objects, while ActiveX is concerned with making objects work together. Simply stated, ActiveX is a technology that lets a program (the ActiveX component or control) interact with other programs over a network (e.g., the Internet), regardless of the language in which they were written. ActiveX components can do similar things as Java beans, but they are quite different. Java is a programming language, while ActiveX controls can be written in any language (e.g., Visual Basic, C, C++, even Java), Also ActiveX runs in a variety of applications, while Java beans usually run only in Web browsers. ActiveX controls are of concern to the pro audio community, because this is the technology that allows designers of computer-controlled sound systems to create common front-end software control panels that will operate different manufacturer's units, without having to know anything about their internal code or algorithms. Each ActiveX control is made up of properties, values associated with the control which might include such things as level settings and meter readings, and events, which tell the computer something significant has happened, such as a switch closer or clip detection. ActiveX allows the manufacturer to create an object that fully describes a device, while hiding the implementation details, such as protocol from the programmer. By hiding the communication details, there is no longer a need for different manufacturer's devices to agree on protocol. This lack of a protocol standard means that cooperation between manufacturers is not required. It allows each manufacturer to choose the best protocol for their devices.

ACU (Audio Coverage Uniformity) A standard issued by InfoComm performance standards program. Read overview here.

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 (optical digital interface) See ADAT Optical.

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. See data converter bits See the RaneNote Digital Dharma of Audio A/D Converters.

add-in or add-on See: plugin.

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 Electronics. 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 See AAC.

AEC (acoustic echo cancellation) A technique using DSP (analog circuits exist, but DSP solutions are overwhelmingly superior) that filters unwanted signals caused by echoes from the main audio source. Echoes happen in both voice and data conversation, therefore two types of cancellers are encountered: acoustic and line. "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. It is similar to sound system feedback problems (where the sound reinforcement loudspeaker is picked up by the microphone, re-amplified through the loudspeaker, only to be picked up again by the microphone, to be re-amplified, and so on), only made much worse by the additional time delay introduced by the telecommunication link. "Line" echo cancellers are used to suppress electrical echoes caused by the transmission link itself. Such things as non-perfect hybrids, and satellite systems (creating round-trip delays of about 600 ms), contribute to very annoying and disruptive line echoes.

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. See the RaneNote Interfacing AES3 and S/PDIF. [Usage Note for AC-3 and DTS Transmission: The question comes up of whether a unit that passes AES3 will automatically pass AC-3 and DTS. Two industry experts answer "maybe" as follows:

Tom Holman warns: " 'Automatically' is the operative word here. You have to pass all the bits unaltered and it will work. We have found things with hidden embedded sample rate converters set to 1:1 conversion that changed bits and thus cause AC-3 bit streams to fail for instance. DC blockage filters in DSP code do it, anything that alters the bit stream can do it. It took a very long time to develop a bit identical path through a production chain to make our test CDs, available from Hollywood Edge. We applied a Prism Sound bit test pattern signal as a track, and it failed at nearly every stage but one in getting from computer to DAT to CD-R to workstation to CD-R to mastering station to CD replication plant. Those steps all failed initially, but the one that didn't is the one all the musicians suspect: the actual manufacturing of the discs was a perfect replication of the bits sent them.
Nobody owns the trademark Digital, thus it can be genius or junk.  Hard to tell which some times."

And Rich Cabot elaborates: "There are several issues about how hardware operates that will affect whether it passes AC-3. These interact with the design of the AC-3 decoder.

First, any buffer/storage device which uses a sample rate converter to eliminate jitter on the received signal would pass PCM but completely screw up any coded audio. There are many receiver chips with built in SRCs. Assuming this is not the problem, there are some more subtle issues that must be considered.

There are several flavors of AC-3 over AES/SPDIF (they are described in IEC-61937). The consumer version, which is the most common, puts the AC-3 signal in both channels of the SPDIF. The professional formats allow this two channel packing but also allow AC-3 in either channel and PCM in the other. All three versions pack the AC-3 as 16 bit words so they can pass through consumer interfaces that might not pass all 24 bits.

In all versions, the interface status bits are supposed to indicate that it is a coded signal. Also, the validity bit has been hijacked and now indicates that it is a coded signal.

If someone builds a device that passes the audio bits but changes status bits and/or the validity bit there could be problems. The decoder design affects what happens. Many decoders do not attempt to decode the signal unless both the validity bits and the status bits indicate that it is coded. Others will decode regardless of the status bit content but still look at the validity bits. I don't know for sure, but some may ignore both status and validity information and only look at the stream.

In applications where one interface channel contains PCM and the other contains coded audio the status and validity bits for each channel must be set independently to reflect what the channel is carrying. The decoder is supposed to monitor the channels independently and adjust appropriately. To my knowledge consumer decoders do not do this and there are no consumer devices which produce these split channel formats. However, professional decoders do and some professional installations do carry a linear version of the signal in one channel to simplify monitoring and identification.

If a buffer/storage device read only one channel of status bits and reproduced them in both output channels it would cause problems with these professional applications. If the buffer/storage device read only one validity bit or combined the two channels of validity bits it would also cause problems.

If the product is a simple analog/digital buffer to TOSLINK device I don't see why it wouldn't pass AC-3.  However, if the receiver does any reclocking of the signal it would not. Also, if they use a different optical interface that packs the bits into a synchronous optical fiber link might would not pass AC-3, depending on the status/validity bit handling."]

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

AES/EBU interface See: AES3.

aetherphone Musical Instrument. Alternate name for the theremin.

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

A-filter See: weighting filters.

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. Got it? Compare with PFL and APL.

A-format See: soundfield microphone.

AGC (automatic gain control), aka ALC (automatic level control) Signal Processing. 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. For example, on deluxe professional DJ mixers the gain adjusts automatically when the DJ switches sources between records, CD, or MP3 files. Not only do signal levels differ greatly between different source technologies but also between any two examples of the same technology, e.g., between CDs, or between MP3 files, etc.

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 absorption See: absorption.

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),

air raid siren See Chrysler Air Raid Siren.

A Law Telecommunications. 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.

album covers See Steinweiss, Alex.


aleatoric Music. Using or consisting of sounds to be chosen by the performer or left to chance; indeterminate.  From aleatory meaning dependent on chance, luck, or an uncertain outcome. Of or characterized by gambling. [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 Audio Signal Processing. 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. See Nyquist frequency. Also see RaneNote Digital Dharma of Audio A/D Converters.

Allison Effect Loudspeakers & Acoustics. 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; after Roy F. Allison, American acoustic engineer of AR (Acoustic Research) and Allison Acoustics fame, who first wrote of this effect in his paper, "The Influence of Room Boundaries on Loudspeaker Power Output," J. Aud. Eng. Soc. Vol. 22, p.314 (May 1974), reprinted in Loudspeakers Vol. I, pp.339-345, edited by R.E. Cooke (Audio Engineering Society, 1978).

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] ) Magnetics. 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]

ALS (assisted listening systems) Hearing. See: hearing loop for one example.

alternating current Abbr. AC or ac (See usage note at end) 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.]

aluminum sheath Wiring & Cable. An impervious aluminum or aluminum alloy tube, either smooth or corrugated, which is applied over a cable core to provide mechanical protection. [IEEE]

AM (amplitude modulation) Radio broadcast. 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. Acoustics. A perceptual sense of space [Blesser]. The acoustic qualities of a listening space [White]. 2. Psychoacoustics The special atmosphere or mood created by a particular environment; also spelled ambiance [AHD]. Contrast with reverberation.

ambient The medium (for example, air, gas, liquid, or earth) in which electric equipment operates. [IEEE]

ambient noise compensator See leveler.

Ambisonics A British-developed surround sound system designed to reproduce a true three-dimensional sound field. Based on the late Michael Gerzon's (1945-1996) (Oxford University) famous theoretical foundations, Ambisonics delivers what the ill-fated quadraphonics of the '70s promised but could not. Requiring two or more transmission channels (encoded inputs) and four or more decoded output loudspeakers, it is not a simple system; nor is the problem of reproducing 3-dimensional sound. Yet with only an encoded stereo input pair and just four decoded reproducing channels, Ambisonics accurately reproduces a complete 360-degree horizontal sound field around the listener. With the addition of more input channels and more reproducing loudspeakers, it can develop a true spherical listening shell. As good as it is, a mass market for Ambisonics has never developed due to several factors. First, the actual recording requires a special tetrahedron array of four microphones: three to measure left-right, front-back and up-down sound pressure levels, while the fourth measures the overall pressure level. All these microphones must occupy the same point in space as much as possible. So far, only one manufacturer (first Calrec, bought by AMS, bought by Siemens, sold, now Soundfield) is known to make such an array. Next, a professional Ambisonics encoding unit is required to matrix these four mic signals together to form two or more channels before mastering or broadcast begins. Finally, the consumer must have an Ambisonics decoder, in addition to at least four channels of playback equipment. [The generic term is now soundfield microphone recording .]

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) Electronic Displays. 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.

AMP (Association of Music Parents) A non-profit organization that promotes music education in school systems.

ampacity (ampere capacity) The maximum current-caring capacity of a material. [This is an example of a portmanteau word.]

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

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 (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 See head amp.

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. See amplifier classes.

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. Classification was based on the amount of time the output devices operate during one complete cycle of signal swing. Classes were also defined in terms of output bias current [the amount of current flowing in the output devices with no applied signal]. For discussion purposes (with the exception of class A), assume a simple output stage consisting of two complementary devices (one positive polarity and one negative polarity) using tubes (valves) or any type of transistor (bipolar, MOSFET, JFET, IGFET, IGBT, etc.).

[Historical Notes marked "GRS" provided by Gerald R. Stanley, Senior V.P. of Research, Crown International, Inc., designer of the famous Crown DC-300, inventor of the Crown K Series switchmode amplifier line and holder of over 30 U.S. Patents.]

[GRS on amplifiers: "At first there were no amplifiers as the very thought of amplification had yet to enter the vocabulary of electronics (another word which had yet to be birthed!). The invention of a three-terminaled device (DeForest Audion U.S. patent 841,386 or later triode) was the invention in 1906 of a more sensitive radio detector and not an element for an amplifier.

By 1912 the triode had become both a vacuum tube and an amplifier (multiple names can be attached to this collective achievement). The oscillator also dates to 1912 giving proof to the saying "When you set out to make an amplifier you get an oscillator and when you attempt to make an oscillator you get an amplifier."]

[GRS on amplifier classes: "Originally it was adequate to distinguish amplifier classes only by the conduction angles of the control elements (tubes or valves). More recently it has been necessary to add distinctions that relate to topology, degrees of conduction and control methods to be able to determine class."]

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. [Much mystique and confusion surrounds the term class A. Many mistakenly think it means circuitry comprised of discrete components (as opposed to integrated circuits). Such is not the case. A great many integrated circuits incorporate class A designs, while just as many discrete component circuits do not use class A designs.]

[GRS Historical Note: "Class A - The most basic of operating modes saw both single-ended and push-pull embodiments by 1913. The first known use of push-pull appears in a patent of E.F.W. Alexanderson of GE U.S. 1,173,079 filed in 1913. While Alexanderson would have been aware of other levels of biasing his push-pull stage, such as classes B and C, he would have only been able to produce a useful result with a tuned stage such as a transmitter where resonant filtering would have managed the distortion problem. Negative feedback is not understood in 1913 to be able to cope with distortion problems."]

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.

[GRS Historical Note: "Class B - This class has no obvious inventor, but it does have its master and perfector. Loy Barton working for RCA developed tube designs and biasing methods to manage the open loop distortion of class B push-pull power stages. His IRE paper in 1931 titled "High Output Power from Relatively Small Tubes" is a landmark in the history of class B. Technically he only used class AB but the distinction was not in the language. Class AB is a later and probably unnecessary class fabrication."]

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.

[GRS Historical Note: "Class AB+B is a term that I'd coined and is intended to be very descriptive but is not truly worthy of its own class. The Crown DC-300 was the first to use this mode of operation in 1968."]

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.

[GRS comments: "This is a class designation that would best be forgotten. It has been applied to multiple modulation schemes on a class D derived full-bridge. This is perhaps the most reinvented class design in recent history with "filter-less amplifiers" and other such things. An interleave of two class D full-bridge is what we actually have here, and it is a good improvement to an interleave of one class D full-bridge. However an interleave of four is actually possible on a full-bridge if one uses Class I design."]

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%.

[Historical note: the original use of the term "Class D" referred to switching amplifiers that employed a resonant circuit at the output to remove the harmonics of the switching frequency. Today's use is much closer to the original "Class S" designs.]

[GRS Historical Note: "Class D is a subset of all possible switch-mode amplifier topologies that is typified by use of the half-bridge (totem-pole) output stage that has two interconnected switches operating in time alternation. The paradigm is that of Loy Barton's class B, but uses the statistics of conduction angle to produce amplification (PWM). There are many subclasses within class D that describe the origins of the modulation. Class D is at least as old as 1954 when Bright patented a solid-state full-bridge servo amplifier U.S. 2,821,639."]

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. See References Krauss, et al. for complete details.

[GRS Historical Note: "Classes E and F are distinguished by their resonant topology and not conduction angle else we would class them with C. A good reference to these is found in the many patents of Nathan Sokal. Also class S which is very old (1929-1930) has similar applications (resonant RF)."]

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.

[Historical note: Hitachi is credited with popularizing class G designs with their 1977 Dynaharmony HMA 8300 power amplifier, however it is shown much older by GRS: "Class G - I have been searching for the proper inventor of this class, but have not been able to find a reference older than 1965 when I first encountered it in a college text "Handbook of Basic Transistor Circuits and Measurements" by Thornton et al., SEEC vol. 7. The method is introduced without references or fanfare. One is led to believe that it was common knowledge in 1965 and earlier. This is not the first known use of extended quasi-linear methods (beyond class B), as there is a dual found in Fisher U.S. 2,379,513 from 1942."]

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.

[Historical note: Soundcraftsmen is credited with popularizing class H designs with their 1977 Vari-proportional MA5002 power amplifier, designed by Soundcraftsmen's Chief Engineer and Vice President, Paul Rolfes. However like class H above GRS finds precedence: "Class H - The apparent inventor of class-H in full-blown multi-level form was Manuel Kramer of NASA in 1964 U.S. patent 3,319,175. Class H optimally applied to a full-bridge was invented in 1987 (Stanley) U.S. 4,788,452. Classes G and H are all members of a class of amplifiers that has articulated rail voltages to improve the efficiency of class B power stages. Examples are available of tracking using binarily weighted segments, (Stanley) U.S. 5,045,990. Continuously variable tracking with switch-mode PWM appears to have been first done by Hamada in 1976 U.S. 4,054,843. The ultimate rail tracker using interleaved technology is found in (Stanley) U.S. 5,513,094. Only with interleave is the converter fast enough to meet the needs of full-bandwidth audio and yet have low switching losses."]

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.

[GRS explains: The "I" of the class is short for "interleave" as this is the only four-quadrant converter known that uses two switches yet has an interleave number of 2 in the terminology of interleave. When used with fixed-frequency natural two-sided PWM it forms a theoretically optimum converter having the least unnecessary/undesirable PWM spectra. A good reference is found in the IEEE Transactions on Power Electronics Vol. 14, No. 2, March 1999, pages 372-380."]

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.

[GRS elaborates: "There are serious problems with the power efficiency of these products when processing fast signals into arbitrary loads. The class B stage is used to actively remove the ripple of the class D stage and other distortion problems that plague class D. No solution is offered for the MOSFET CSOA (current safe operating area) problem of class D. To solve that problem it would be necessary to parallel a class I and class B amplifier but this would be without merit as the class I amplifier generally does not need the class B amplifier to meet fidelity requirements."]

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. Essentially this is what is termed "Class D" today. See References Krauss for details.

[Final GRS Amplifier Historical Note: "All of our amplifier classes have thrived under a very important invention, without which most would have floundered. That invention is, of course, negative feedback. Harold Black in 1927 changed our world forever while riding to work on the Lackawanna Ferry. (See U.S. patent 2,102,671.) Harold Black did not stop there however, he also in 1953 wrote the text "Modulation Theory" which we today use to understand the fundamentals of PWM. In 1935, Terman, in his now famous "Fundamentals of Radio" handbook, wrote that it was good that class B was only used in places like radio stations as there needed to be an engineer on duty full time to keep the bias tweaked to where the distortion was acceptable. Thanks go to Harold Black for changing all that and leading us into the next century of amplification."]

amplifier dummy load Modeling a real world loudspeaker for power amplifier testing purposes has been studied for years, resulting in many circuit possibilities. An article compiled and edited by Tomi Engdahl entitled "Speaker Impedance" is an excellent summary of the results. He gives a complete (and complex) solution to the loudspeaker dummy load question. However you can get excellent results with a simplified version developed by electronics engineer Michael Rollins appearing below. The series resistor and inductor model the loudspeaker voice coil's DC resistance and inductance, while the parallel inductor and capacitor simulate the mechanical components of suspension compliance and cone mass respectively. The values shown work well for most power amplifier measurements.

amplifier dummy load

RS = 6 ohms (aluminum body power resistor bolted to heatsink; power rating twice max testing watts)
LS = 0.33 mH (air core inductor; wire sized for max current)
LP = 20 mH (air core inductor; wire sized for max current)
CP = 1000 µF (100 V, or maximum expected peak voltage; paralleling two 500 µF caps may be smaller, cheaper)

amplitude 1. Greatness of size; magnitude. 2. Physics. The maximum absolute value of a periodically varying quantity. 3. 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. Electronics. The maximum absolute value reached by a voltage or current waveform. [AHD]

amplitude-frequency response See frequency response.

amplitude modulation See: AM.

AMT (air motion transformer)  See: air motion transformer.

anacrusis See upbeat.

anagram A word or phrase formed by reordering the letters of another word or phrase, such as satin to stain. [AHD]

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.

ANC (ambient noise compensator) A signal processing circuit that monitors the ambient noise in a space and adjusts the level of the amplified program material as necessary to maintain a desired signal-to-noise ratio. [Different than ANR]

AND Computer Science & Logic. A Boolean logical operator that returns a true value only if both operands are true; a form of multiplication. For example, two series connected switches, A and B, requires both be closed for current to pass, thus it requires switch A AND switch B closed to operate.

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.

anechoic room See: free-field room

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

Animusic Company that creates computer animated music videos that are pure genius and totally entertaining. See: Extraordinaire Instrument de Musique for example.

anisotropic Physics. 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.

ANR (ambient noise reduction) Any of various DSP algorithms that reduce unwanted background noise, for example, HVAC noise. A common signal processing block found in acoustic echo cancellers. [Different than ANC]

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

Among Antheil's early avant-garde pieces, none caused a greater sensation than his Balet mécanique, scored for automobile horns, airplane propeller, fire siren, ten grand pianos, and other instruments. When it was performed at Carnegie Hall in 1924, a concertgoer near the orchestra could stand no more than a few minutes of the racket. Tying his handkerchief to his cane, he raised the white flag in surrender. [Bartlett's Book of Anecdotes]

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 Digital Audio. "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." From the abstract.

antiskating also anti-skating Turntables. 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 See acoustooptics.

AoE (Audio over Ethernet) Networks. 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®),

AOIP (audio over Internet Protocol) Networks. 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) Networks. 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) Electronics. 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) Software. 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. A good resource for audio contractors, integrators, etc., especially for their focus on energy efficient buildings.

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. See power factor .

appoggiatura Music. 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. For a good discussion on how the A&R world is changing see Rap Coalition's Intelligence Program by Wendy Day.

architectural columns See: line arrays.

Archos Jukebox Multimedia See: Jukebox Multimedia.

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 Musical Instrument. Name for Benjamin Franklin's improvement on the Glass harmonic in 1761. Go to William Zeitler's website for fascinating details from one of the few modern players.

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

Armstrong Park New Orleans site named after Louis Armstrong, considered to be the birthplace of jazz.

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 See line arrays.

articulation 1.  The act of vocal expression; utterance or enunciation: an articulation of the group's sentiments. 2a.  The act or manner of producing a speech sound. 2b.  A speech sound, especially a consonant. [AHD]

articulation index Abbr. AI  Speech. A number (ranging from 0 to 1) which is a measure of the intelligibility of speech--the higher the number, the greater the intelligibility. [Harris]

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

aspiration Speech. Expulsion of breath in speech. [AHD] "It occurs in the phoneme "h" in English, or with less duration after the release of an unvoiced consonant, for example, after the "p" in "pie". [Bregman]

ASTC (Association of Science-Technology Centers) "An organization of science centers and museums dedicated to furthering public engagement with science among increasingly diverse audiences." Another valuable resource for audio contractors, integrators, etc.

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." Another valuable resource for audio contractors, integrators, etc.

A-taper See potentiometer.

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 Nanotechnology. 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 Music. 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. Also see: automatic attack & release.

attenuation to crosstalk ratio See ACR.

attenuator or attenuator pad Electronics. 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 Music. 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] [Usage Note: The word "audio" often refers to the electrical signal, as opposed to the acoustic meaning, which is most often referred to as "sound," rather than "audio."]

audio archaeologist A term coined by sound designer Andrew Roth to describe his specialty of capturing sounds that relate to specific time periods.

audio books See Pro Audio Reference Books for books used to create this site.

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. See mix-minus.

audio compression See digital audio data compression.

audio connectors See connectors.

Audio Courses Interesting website, it's an online audio production school but with lots of useful (and free) information.

audio coverage uniformity See: ACU.

audio levels See levels and decibels.

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

Audio magazine (1947-2000) America's first and longest running audio magazine. Its demise after 53 years of continuous publication leaves a huge void in the consumer audio world. Gone is the last great rational voice, lost amidst the pseudoscientific din dominating high-end audio. An audio warrior is dead and we are lessened.

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 See: AoE.

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

audio snake See snake.

audio taper See potentiometer.

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.

audio websites A truly astonishing and remarkable list of audio related websites compiled daily by Steve Ekblad. Also see Audio & Hi-Fi Page, an equally astonishing and remarkable list of audio related websites compiled by Tomi Engdahl. And for a refreshingly rational voice on hot audio topics check out Rod Elliott's site, particularly his get-rich-quick scheme for exploiting the gullible regarding burning-in audio cables. [Absolutely brilliant.]

audition Hearing. 1. The sense or power of hearing. 2. The act of hearing. [AHD]

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 Hearing. See: masking.

auditory streaming (Shortened form of "auditory stream segregation" dubbed by Ulric Neisser.) Hearing. Term coined by Albert Bregman in his book Auditory Scene Analysis to describe the hearing phenomenon where we separate a continuous sound stream into discrete words or sounds. This explains, for example, the "cocktail party effect" where we separate out specific sounds (words) important to us from a cacophony of surrounding sound. Another example is that speech is a continuous audio stream, even though we "hear" separate words, yet there are no pauses in the audio stream. Yet another example is where musicians separate and listen to one or two instruments from among many playing at once. [I am vastly oversimplifying here to just give you a top-layer understanding. Hit the link and then obtain Bregman's book for the full treatment.]

aulos Musical Instrument. Ancient Egyptian double flute [See vase photo here.] 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 See: clairaudient.

Aureal 3D (A3D) Proprietary 3D sound technology first developed by Crystal River Engineering, which became the advanced technology subsidiary of Aureal Semiconductor, alas, now defunct. Aureal 3D made many claims. At one time their website stated that "since we can hear sounds three dimensionally in the real world by using two ears, it must be possible to create sounds from two speakers that have the same effect" ... well ... NO ... it's pretty rhetoric, but flawed logic. Our two ears receive sound coming from sources located in every possible direction, and from that information process three-dimensional location -- that is not the problem. The problem is how to make our two ears receive sound from sources located in only two directions, and trick them into hearing three dimensionally -- that is the problem. Aureal claimed to have solved this problem, but didn't stay in business long enough for anyone to find out.

auricle See: pinna.

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 Audio Compressors. 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. See THAT Corporation Design Note 114, Adaptive attack and release rates ... for a description of an automatic scheme for an analog design.

automatic gain control See: AGC.

automatic mic mixer 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 (see NOM). 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. Dan Dugan patented the first automatic mic mixer and is recognized as the father of this technology. A final problem that automatic mixers solve is maintaining a natural ambience from the room. This is especially critical in recording and broadcasting. A good automatic mixer must make rapid and dramatic changes in the gains of the input channels while maintaining the sonic illusion that nothing is happening at all.

automixer See: automatic mic mixer

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. See Tom Young's article at ProSoundweb, "A Detailed Explanation Of The Aux Fed Subwoofer Technique."

AVB (Audio/Video Bridging) Networks. 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. Since this is a non-proprietary, i.e., no licensing fees, networking standard and is backed by giants Cisco and Intel, many pro audio companies are watching it closely. Also see: AoE.

AV Abbreviation for audio- video, referring to systems that contain both.

AVD (advanced video disk) A Chinese proposed alternative to the DVD standard to avoid paying what they consider exorbitant royalties. This threatened standard would apply to DVD-like players sold only in China. Replaced by EVD.

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) See: air motion transformer.

average power See apparent power.

A-weighting See weighting filters.

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 (I have been unsuccessful in finding out what Sharpe's role was). For more detail, see Douglas Brooks' "How to Gauge Traces." Many tables exist on the Internet. The British standard is called SWG standing for Standard wire gauge, also called Imperial wire gauge.

axe Musical instrument, usually a guitar.

axial mode Acoustics. Sound reflecting between two parallel surfaces. Compare tangential mode and oblique mode.

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. 3. The lateral deviation of a projectile or bomb. [AHD]

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

azure noise See noise color.