What does a compressor do?
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.
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 on danmurtagh.com, and its guide to attack time.
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