What a compressor does
A compressor reduces level when the input exceeds a threshold. The ratio determines how strongly level above the threshold is reduced. At 2:1, every 2 dB above the threshold becomes about 1 dB at the output before makeup gain. Compression is not simply “making audio louder.” It first makes peaks smaller; the resulting headroom can then be used to raise the overall level. The audible result depends on timing, detector behavior and the material.
Threshold, ratio and knee
A lower threshold catches more of the performance. A higher ratio applies stronger control once the signal crosses it. The knee determines how gradually the ratio is introduced around the threshold. A soft knee often sounds smooth on vocals, bass or mix buses, while a hard knee can provide decisive peak control. Settings cannot be copied mechanically because input level changes how much gain reduction occurs.
Attack and release
Attack controls how quickly gain reduction begins. A fast attack catches transients but can remove punch and dull consonants. A slower attack lets the initial impact through while controlling the body. Release determines how quickly the compressor recovers. Too fast can create distortion or chattering; too slow can keep the next phrase suppressed. Set timing in context and watch whether the gain reduction returns naturally between events.
Makeup gain and loudness bias
Because compression lowers peaks and often average level, makeup gain restores output. This creates a common comparison trap: the processed signal sounds better simply because it is louder. Match output loudness before judging tone, dynamics and artifacts. Automatic makeup gain can hide how much processing is occurring, so manual gain is often safer. Check the output for clipping and True Peak overs after raising level.
Practical uses and common mistakes
Vocals and speech may need moderate compression to keep words intelligible. Bass can benefit from level control, while drums may use slower attack for punch. Bus compression can gently glue a mix when gain reduction is small. Common mistakes include compressing only because a preset says so, using very fast attack on everything, choosing release without listening to rhythm, and applying heavy makeup gain. If the source is already consistent, compression may not be needed.
How to set a compressor by ear
Start with the ratio and timing at moderate values, lower the threshold until gain reduction is clearly audible, then adjust attack and release to preserve the desired transients and movement. Back off the threshold or ratio to the minimum that solves the problem. Match output loudness and bypass frequently. Analyze peaks, LUFS and dynamic range before and after. Save the uncompressed original so the decision remains reversible.