An audio compressor is a sound processing tool designed to reduce the dynamic range of the input audio signal. It functions by "turning down" relatively loud audio signals, and "turning up" softer audio signals. It's main purpose is to smooth the audio signal by eliminating or limiting peaks in the audio response. Because compression onset can be variable, it can also modify the attack and decay characteristics of the audio signal. Compressors are available in a stomp box, in a studio rack format, as well as a software plugin. Compressors increase the apparent sustain of stringed instruments such as guitar and bass by bringing up the level of the instrument as the note decays (gets softer). Unfortunately, this also results in the noise floor being increased as well since a compressor generally does not differentiate between noise and audio signal.

Different types of hardware compressors include tube compressors based on vacuum tubes (such as the legendary LA2A which makes use of an optical VCA that utilizes a photoresistor or "photocell" element to reduce gain) and chip based VCAs (such as the Analog Devices VCA chips or OTAs such as the vernerable CA3080 or LM13600/LM13700 chips). A VCA may also be constructed using a FET as a variable resistance element. The choice of design greatly affects how the compressor sounds and how it responds.

Related signal dynamics processing include limiter and noise gate