# What does a compressor do?

> A compressor turns a sound down when it passes the threshold, by the ratio, so the loud moments come down. Hear it on drums with the levels matched.

Page: https://mixchops.com/learn/what-does-a-compressor-do

By [Dan Murtagh](https://danmurtagh.com/about) · Updated 18 October 2026

**Short answer:** a compressor turns a sound down whenever it gets louder than a level you set, the threshold, by an amount set by the ratio. The loudest moments come down toward everything else, so the part sounds more even and denser. Attack sets how fast it grabs and release how fast it lets go. Turned back up with make-up gain, the quiet detail sits higher. It’s easy to mistake compression for a change in level, so hear it with the levels matched.

> **Demo** (plays in the browser on this page): The drums from a blues-rock track, dry and through the compressor the Mix Chops drills use. **Heavy** is 12 dB of gain reduction at 8:1 with a 1 ms attack, **Gentle** 4 dB at 3:1 with a 2 ms attack, and **Slow attack** 10 dB at 8:1 with a 60 ms attack, each with the release set for drums. Both sides play at the same loudness, so the compressed one can’t win by being louder. Start with the drums on their own, then hear them with the band.

## The controls

- **Threshold:** the level where it starts working. Anything quieter goes through untouched.
- **Ratio:** how hard it turns down what goes over. At 4:1, a sound 4 dB over the threshold comes out 1 dB over. A high ratio clamps nearly flat, almost like a limiter; a low one just leans on it.
- **Attack:** how quickly it grabs once the level goes over. A fast attack shaves the front of each hit; a slow one lets the front through first, so it snaps.
- **Release:** how quickly it lets go once the level drops back.
- **Gain reduction:** the meter that shows how much it’s turning down, in dB. More gain reduction, less the level is allowed to move.
- **Make-up gain:** turns the whole result back up, which is why compressed sounds tend to come out louder.

To see what each setting does to a waveform, try the [compression visualiser](https://danmurtagh.com/compression-visualiser) on danmurtagh.com, and its [guide to attack time](https://danmurtagh.com/what-does-attack-time-do).

## How to hear it

- **Match the levels first.** Louder sounds better, so a compressor with make-up gain can fool you. With both sides at the same loudness, what’s left is what the compressor did.
- **Listen to the gap** between the loudest hits and the quiet bits around them. The compressed one has the smaller gap: the hits come down toward everything else, so the kit sounds flatter and denser, and the room and cymbals can swell up after each hit.
- **Listen to the front of each note.** A fast attack shaves the crack off a hit, so the tail sits higher and it feels softer. A slow attack lets the crack through before it clamps, so it punches.
- **Start on drums.** Compression is easiest to hear on parts with sharp hits. On a vocal or a pad it hides better.

## What it’s for

- **Evening out a part** that jumps around in level, so it sits steadily in the mix.
- **Shaping the hit:** a slow attack for more punch, a fast one to round it off.
- **Bringing up detail:** with the peaks down and make-up gain on, the quiet parts of a performance come up.

## Questions

### Why does compression make things louder?

It doesn’t on its own: it turns the loudest parts down. Make-up gain then turns the whole sound back up, and with the peaks lower it can go up further, so it ends up louder. That’s why compression is easiest to fool yourself with, and why the demo matches the levels.

### How much gain reduction is normal?

It depends on the part and the job. A few dB is subtle and can be hard to hear; 10 dB or more is heavy and obvious on drums. Compressed or Dry starts at 12 dB so you can hear it clearly, and comes down to 4.

### What’s the difference between a compressor and a limiter?

A limiter is a compressor with a very high ratio and a fast attack: nothing gets much past the threshold. A compressor at a lower ratio only leans on the loud moments.

### Why can’t I hear compression?

Usually because the change is small, the part has soft transients, or the louder version is winning. Match the levels, start on drums with plenty of gain reduction, and listen to the hits against the gaps between them.

## Stop guessing. *Start hearing.*

Compressed or Dry plays one part of a song two ways, matched for loudness: one dry, one compressed. Pick the compressed one. It starts with 12 dB of gain reduction on the drums and ends with 4. How Much and Attack come next: which one is working harder, and which one lets the front of the note through.

[Train it in Compressed or Dry](https://mixchops.com/compression/dry)

Then: [How Much](https://mixchops.com/compression/amount) · [Attack](https://mixchops.com/compression/attack) · Free, in your browser, no account

## More in Learn

- [What is EQ?](https://mixchops.com/learn/what-is-eq): What an equaliser does, heard on a mix
- [What each frequency sounds like](https://mixchops.com/learn/what-frequencies-sound-like): Nine bands, 63 Hz to 16 kHz
- [Parametric EQ](https://mixchops.com/learn/parametric-eq): Frequency, gain and Q
- [High-pass and low-pass filters](https://mixchops.com/learn/high-pass-low-pass-filters): Including the high-pass on a vocal
- [Shelving EQ](https://mixchops.com/learn/shelving-eq): High shelf, low shelf, and a bell
- [Mix problems you can hear](https://mixchops.com/learn/mix-problems): Muddy, boxy, harsh, sibilant, dull
- [Mixing on headphones](https://mixchops.com/learn/mixing-on-headphones): What they hide, and how to check
- [How to train your ears for EQ](https://mixchops.com/learn/train-your-ears-for-eq): A routine, and the bands people mix up
- [Critical listening](https://mixchops.com/learn/critical-listening): What it is, and how to train it
- [How to find resonant frequencies](https://mixchops.com/learn/find-resonant-frequencies): Boost, sweep, cut
- [Subtractive EQ](https://mixchops.com/learn/subtractive-eq): Cut first, and how to practise it
- [How to fix a muddy mix](https://mixchops.com/learn/fix-a-muddy-mix): Find the part, then cut
- [How to build a static mix](https://mixchops.com/learn/build-a-static-mix): Drums first, vocal last
- [All of Learn](https://mixchops.com/learn): Every explainer and guide, and what it trains