Glossary of Audio Terms

"A" weighted measurements use a filter to emphasize noise at mid frequencies where the ear is most sensitive at low sound pressure levels. The drawback is that it also de-emphasizes noise at lower frequencies such as AC hum. Strong magnetic fields, produced by large power supply transformers, make hum a common type of noise in power amplifiers. Unweighted noise specifications provide a more critical indication of AC hum.

The shape of an "A" weighting filter is precisely defined by ASA (Acoustical Society of America) and ANSI to mimic a characteristic of human hearing. The goal is to obtain measurements that correlate well with the subjective perception of noise. The "A" curve is a wide bandpass filter with maximum response at 2.5 kHz, with -20 dB attenuation at 100 Hz and -10 dB attenuation at 20 kHz, so it rolls off the low end heavily and the high frequencies more gently. It is an approximate inverse of the 30-phon (or 30 dB SPL) equal-loudness curve determined by the pioneers of spectral psychoacoustics Fletcher and Munson.

While this is can be a useful and descriptive way of measuring noise, it has drawbacks. As mentioned above, "A" weighting can disguise a serious hum problem. Likewise, it can conceal significant high frequency and ultrasonic noise. For example, if a poorly designed digital processing unit's output produces significant artifacts from the clock frequency and its harmonics and sub-harmonics, those noise signals might not themselves be very audible to people, yet they could strongly affect downstream processors (particularly dynamics processors such as limiters), amplifiers, etc., with audible results.

[Editorial Note: Low-cost audio equipment often list an "A" weighted noise spec; not because it correlates well with our hearing, but because it helps "hide" nasty low frequency hum components that make for bad noise specs. Sometimes "A" weighting can "improve" a noise spec by 10 dB! Words to the wise: always wonder what a manufacturer is hiding when they use only "A" weighting]

See: Ampere

To receive an impulse without recoil or echo. In regard to sound, it is process by which sound energy is decreased when it passes through or strikes a surface. A material's ability to absorb sound can be gauged by its absorption coefficient.

See: Alternating Current

The interaction between the room, the loudspeaker, and the listener. This term was coined by Dr. Peter D'Antonio

There are three classic tools available to an acoustician to treat a room. Absorbers attenuate sound. Reflectors attenuate sound. And finally, diffusers distribute sound in a uniform fashion.

A frequency divider that is based on integrated circuits, discreet transistors, digital signal processing or tubes. An active crossover is impedance buffered, gives a consistent and accurate transition regardless of load and is always located before the power amplifiers in a system.
See: Crossover

An electronic device that consists of a Radio Frequency (RF) signal splitter, preceded by an RF amplifier that compensates for the RF loss of the splitter.

Abbreviation for Attack, Decay, Sustain, and Release, the four parameters found on a basic envelope generator; it can also be called a transient generator. The Attack, Decay, and Release parameters are rate or time controls, they effect the time it takes for a change to take place. When a sound is triggered, the envelope generator will rise to its full level at the rate set by the attack parameter, after reaching its peak level it will begin to fall at the rate set by the decay parameter until it reaches the level set by the sustain control. The sustain value indicates a level. The envelope will remain at the sustain level until the key is released when it will return to zero at the rate set by the release parameter.

See: Automatic Gain Control

A procedure or formula for solving a problem. In FM synthesis, the term refers to the various sound producing structures employed. Also used in effects processors, particularly reverb, to artificially simulate acoustic spaces and other effects.

An electrical flow that swings between a positive and negative voltage relative to ground. Abbr.: AC

The acoustic qualities of a listening space

A unit of electric current, equal to a flow of one coulomb (6.24196 x 10^18 electrons) per second, and it is the amount of current that will flow through a 1-ohm resistance across a 1-volt electrical source. [Named after André-Marie Ampère (1775-1836), a French physicist and mathematician who formulated Ampere's law, a mathematical description of the magnetic field produced by a current-carrying conductor.] Abbr.: A

An amplifier is an electronic device that increases the voltage, current or power of a signal.

Term used to describes the instantaneous magnitude of an oscillating quantity such within a signal or waveform.

Commonly used slang for Amperes
See: Ampere

A room intended for acoustical measurement. This room is designed to minimize internal reflections by using absorbent material.

Attenuation is a general term that refers to any reduction in the strength of a signal, whether digital or analog. It is a natural consequence of signal transmission over long cable runs, or it may be purposely achieved with circuits to prevent overload and distortion in an audio signal path.
See: Attenuate

See: Digital Audio Data Compression

A circuit or algorithm that adjusts gain as a function of the input signal amplitude.