compressor

A compressor is a processor that automatically reduces the amplitude of an audio signal when it exceeds a set threshold. Unlike a limiter, which acts as a hard ceiling, a compressor applies a ratio: if you set a 4:1 ratio and the signal crosses the threshold by 4 dB, the output exceeds it by only 1 dB. This controlled reduction tames dynamic range, making quiet passages audible and loud ones manageable without manual fader riding.

The core parameters are threshold (the level at which compression begins), ratio (the amount of gain reduction applied), attack time (how fast the compressor responds to peaks, typically 1 to 100 milliseconds), and release time (how quickly it stops compressing after the signal falls below threshold, usually 50 milliseconds to several seconds). A makeup gain control restores overall level after compression. These settings interact: a fast attack catches transients but can sound unnatural; a slow attack preserves punch; a short release can cause pumping if the material is heavily compressed.

Types and applications

Tube compressors use vacuum tubes in the gain-reduction path and are prized for smooth, musical coloration; solid-state designs offer transparency and reliability. VCA (voltage-controlled amplifier) compressors, common in mixing consoles and outboard gear, respond quickly and can be linked across multiple channels. Variable-mu designs, based on tube technology, have an exponential response curve that sounds gentle at low levels of compression. In broadcast and live sound, compressors prevent peaks from distorting limiters or clipping converters. In recording studios, they shape vocal tone, control bass dynamics, and glue instrumental tracks.

The term compression itself reflects the physics: the device compresses the distance between quiet and loud parts of a waveform. It originated in early broadcast and telephony, where dynamic range had to be controlled to fit within transmission constraints. Modern digital compressors emulate these hardware behaviors, though software implementations can change parameters sample-by-sample, enabling lookahead and behaviors impossible with analog circuits.