Learn — By WATMM Desk —
What is a compressor? The one-page reference
A compressor is an audio processor that turns a signal down when it gets louder than a level you set, so the gap between loud and quiet parts shrinks. Here are its six controls in plain English and when to leave it alone. Sources checked on 27 Oct 2026.
Key facts
- A compressor reduces gain for signals above a user-set threshold (Ableton Live manual).
- At a 4:1 ratio, a signal 4 dB over the threshold comes out only 1 dB over it.
- A limiter is a compressor with a high ratio and, generally, a fast attack time.
A compressor is an audio processor that automatically turns a signal down when it rises above a level you choose, which narrows the distance between the loud and quiet parts of a performance. Ableton's manual puts it in one line: a compressor "reduces gain for signals above a user-settable threshold" (Ableton Live manual).
That is the whole idea. Everything else is detail about how much, how fast and how smoothly.
The six controls
| Control | What it does | Source |
|---|---|---|
| Threshold | The level where compression begins; below it the signal passes unchanged | Ableton, Wikipedia |
| Ratio | How hard it turns down. At 4:1, a signal 4 dB over the threshold comes out 1 dB over. A ratio of 1 means no compression | Wikipedia, Ableton |
| Attack | How long it takes to reach full gain reduction once the signal crosses the threshold | Ableton |
| Release | How long it takes to let the gain back up once the level falls | Wikipedia |
| Knee | Whether the change at the threshold is abrupt (hard) or gradual (soft) | Wikipedia |
| Makeup gain | A fixed boost at the output to win back the level compression took away | Wikipedia |
Many compressors also let you choose how they listen. In Ableton's Compressor, Peak mode reacts to short peaks, while RMS is "closer to how people actually perceive loudness" (Ableton).
Related terms
- Limiter: a compressor with a high ratio and, generally, a fast attack (Wikipedia). It is what sits last on most masters; see mastering at home: the basics.
- Upward compression: the less common cousin, which raises quiet sounds below a threshold instead of lowering loud ones (Wikipedia).
- Sidechain: normally the signal being compressed also triggers the compressor; a sidechain lets a different signal do the triggering (Ableton). Full walkthroughs: what is sidechain and sidechain compression.
- Loudness war: the name given to the push to make records ever louder, largely with compression and limiting (Wikipedia).
What we think
Most beginner mixes that sound "small" are not under-compressed; they are compressed without a reason. Our rule: name the job before you load the plugin. "Keep this vocal level so every word reads" is a job. "Make it sound pro" is not. If you can't name the job, bypass it.
Two habits help more than any preset. First, match the output level when you A/B, because a louder version almost always sounds better and makeup gain will fool you. Second, start with slow attack and moderate ratio; a fast attack shaves the front of every note, which is often the part that gave the sound its character. And since streaming services turn loud masters down anyway (see streaming loudness targets), squeezing a mix for level alone buys you less than it used to. That last point is our reading of how normalisation works, not a rule from any one service.
Shape tone first with EQ, then reach for a compressor when the problem is level over time.
Sources
- Live Audio Effect Reference: Compressor, Ableton (primary source). Accessed 27 Oct 2026.
- Dynamic range compression, Wikipedia. Accessed 27 Oct 2026.
Researched with AI assistance, written and checked by WATMM Desk.