A compressor automatically turns down the loudest parts of a sound. That makes the gap between quiet and loud moments smaller, so a vocal sits steadily in the mix instead of jumping out on loud words and disappearing on quiet ones. Compression is used on almost every track in modern music.
Note: this is about audio compression in mixing, not file compression like MP3.
What compression does, simply
Imagine someone with their hand on a volume fader. Every time the singer gets too loud, they pull the fader down a little, then push it back up. A compressor does that job automatically and much faster. After it has evened out the level, you can turn the whole track up, and the quiet details become easier to hear.
The controls
| Control | What it does |
|---|---|
| Threshold | The level where the compressor starts working. Anything louder than this gets turned down. |
| Ratio | How much it turns down. At 4:1, a sound 4 dB over the threshold comes out only 1 dB over. |
| Attack | How fast it reacts once the sound passes the threshold, usually in milliseconds. |
| Release | How fast it lets go after the sound drops back below the threshold. |
| Knee | How gently it starts: a soft knee eases in, a hard knee clamps down straight away. |
| Makeup gain (output) | Turns the whole signal back up after compression. |
| Gain reduction meter | Shows how many dB the compressor is turning down at any moment. |
Ratio explained with numbers
Say your threshold is −20 dB and a note peaks at −8 dB, which is 12 dB over.
- At 2:1, it comes out 6 dB over: −14 dB.
- At 4:1, it comes out 3 dB over: −17 dB.
- At 10:1, it comes out 1.2 dB over: −18.8 dB.
Very high ratios, around 10:1 and above, behave like a limiter, which stops the sound going over a level at all. Limiters are common on the master track at the end of mastering.
Attack and release: the part that shapes the sound
- Slow attack lets the first hit of a sound through before the compressor clamps down. On drums, that keeps the punch.
- Fast attack catches the very start of the sound. It controls peaks tightly but can make drums sound flat.
- Fast release lets go quickly and can make a sound feel louder and more energetic, but too fast can cause audible pumping.
- Slow release sounds smoother but can keep a track turned down when it should have recovered.
Some compressors offer an automatic release, which is a safe choice while you are learning. To set release in time with the song, the note lengths from our Delay Calculator are a useful starting point.
Starting settings
These are common starting points, not rules. Always adjust by ear.
| Source | Ratio | Attack | Release | Aim for |
|---|---|---|---|---|
| Vocals | 2:1 to 4:1 | Medium | Medium or auto | 3 to 6 dB of gain reduction on loud words |
| Kick and snare | 4:1 | Slow enough to keep the hit | Fast to medium | Punch, not flattening |
| Bass | 3:1 to 4:1 | Medium | Medium | Even note-to-note level |
| Mix bus | 2:1 or lower | Slow | Auto | 1 to 2 dB, gentle glue |
How to set a compressor, step by step
- Start with a moderate ratio, such as 3:1, and a medium attack and release.
- Lower the threshold until the gain reduction meter moves on the loudest parts only.
- Adjust attack: slower if the sound loses its punch, faster if peaks still poke out.
- Adjust release so the meter returns to zero between phrases or hits.
- Use makeup gain so the compressed track is about as loud as before, then switch the compressor on and off to compare fairly.
Common mistakes
- Too much compression. The track sounds flat and lifeless, and breaths and noise jump up. Back off the threshold.
- Judging louder as better. Match the levels before deciding whether compression helped.
- Compressing everything by default. Some tracks do not need it.
Frequently asked questions
What does a compressor do in music?
It lowers the volume of the loudest parts of a sound, which makes the overall level more even and easier to place in a mix.
What is a good compression ratio for vocals?
A common starting point is 2:1 to 4:1, adjusted so the loudest words get about 3 to 6 dB of gain reduction.
What is the difference between a compressor and a limiter?
A limiter is a compressor with a very high ratio and fast attack. It stops the signal going over a set level, while a compressor only reduces it by a ratio.
Should I compress before or after EQ?
Both are common. Many people cut problem frequencies with EQ first, then compress, then add tone boosts after.
Next, see how compression fits into the bigger picture in mixing vs mastering.
