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:
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.
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.
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).
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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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