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The term "audio compression", meaning '''audio level compression''' is used in the [[sound recording]] and live [[sound reinforcement]] fields. In this context, it refers to a process whereby the [[dynamic range]] of an audio signal is reduced.
An audio level compressor reduces the dynamic range of an audio signal by using a variable gain [[amplifier]] to reduce the [[gain]] of the signal if it passes a set threshold. The amount of gain reduction is usually determined by a [[ratio]] control. That is, with a ratio of 4:1, if the input level is 4 [[decibel|dB]] over the threshold, the gain will be reduced so that the output level will only be 1dB over the threshold.
Compressors usually have controls to set how fast the compressor responds to changes in input level, known as ''attack'', and how quickly the compressor returns to no gain reduction once the input level is below the threshold, known as ''release''. The attack of a compressor is always set to at least as slow as the lowest frequency signal in the track, and usually a few times slower. Because the compressor is reducing the gain (or level) of the signal, the ability to add a fixed amount of gain at the output is provided so that an optimum level can be used.
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Some compressors implement '''side-chaining'''. This feature uses the dynamic level of another input to control the compression level of the signal. This is used by [[disk jockey|disk jockeys]] to lower the music volume automatically when speaking, for example (known as ''ducking''). Another use is in music production, to maintain a loud bass track, while still keeping the bass 'out of the way' of the bass drum when the drum hits.
A different, non-linear type of compressor is used to reduce the dynamic range of a signal for transmission, to be expanded afterwards. This reduces the effects of a channel with limited dynamic range. See [[Companding]].
== See also ==
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