This article is about a process that intentionally reduces the dynamic range of audio signals. For similar reductions caused by circuit imperfections, see Gain compression. For processes that reduce the size of digital audio files, see Audio compression (data).
A rack of audio compressors in a recording studio. From top to bottom: Retro Instruments/Gates STA level; Spectra Sonic; Dbx 162; Dbx 165; Empirical Labs Distressor; Smart Research C2; Chandler Limited TG1; Daking FET (91579); and Altec 436c.
A dedicated electronic hardware unit or audio software that applies compression is called a compressor. In the 2000s, compressors became available as software plugins that run in digital audio workstation software. In recorded and live music, compression parameters may be adjusted to change the way they affect sounds. Compression and limiting are identical in process but different in degree and perceived effect. A limiter is a compressor with a high ratio and, generally, a short attack time.
Downward compression reduces loud sounds over a certain threshold while quiet sounds remain unaffected. A limiter is an extreme type of downward compression. Upward compression increases the loudness of sounds below a certain threshold while leaving louder sounds unaffected. Both downward and upward compression reduce the dynamic range of an audio signal.
An expander increases the dynamic range of the audio signal. Expanders are generally used to make quiet sounds even quieter by reducing the level of an audio signal that falls below a set threshold level. A noise gate is a type of expander.