The parameters
- Threshold: the level beyond which the compressor acts.
- Ratio: how much it reduces. 2:1 gentle, 4:1 vocals, 8:1–10:1 firm control, above 20:1 limiting.
- Attack: reaction speed. Fast (0.1–1 ms) squashes transients; slow (10–30 ms) lets them through and adds punch.
- Release: release time (50–300 ms typical). It should be tied to the tempo: at 120 BPM a quarter note lasts 500 ms; a release that finishes before the next hit makes the track “breathe” in time.
- Knee: soft or hard.
- Make-up gain: recovers the lost volume.
3–6 dB of reduction on a vocal is normal; 1–2 dB on a bus.
Types and characters
- VCA (SSL bus style): precise, “glues” drums and mixes.
- FET (1176 style): fast and aggressive, rap vocals, snares.
- Optical (LA-2A style): soft and slow, vocals and basses.
- Vari-mu: round, buses and mastering.
Techniques
- Parallel (New York): heavily compressed drums blended with the original: punch without losing the transients.
- In series: two light compressors instead of one heavy one (FET for the peaks, optical for the level).
- Sidechain: the 808 or pad ducks when the kick plays; in EDM the “pumping” becomes an effect.
- Bus compression: 2:1, attack 30 ms, auto release, 1–2 dB of reduction.
Example in a beat in A minor
Rap vocal: FET 4:1, attack 3 ms, release 80 ms, 4 dB of reduction; then optical for 2 dB. 808: sidechain from the kick with 3–4 dB of reduction and a release of 60–100 ms.
Real-world example
The drum sound of “In the Air Tonight” by Phil Collins (1981) comes from a heavily compressed studio talkback microphone with a noise gate: a technical accident that became one of the most imitated sounds of the 1980s.