Compression is the most misunderstood tool in mixing. Beginners either ignore it entirely or crank every knob until the track sounds squashed. Neither approach works.
This guide covers every parameter on a standard compressor — what it does, how to hear it, and what to set it to when you're starting out. By the end, you'll understand compression well enough to use it confidently on any track.
What Does Compression Actually Do?
A compressor automatically turns down the loudest parts of an audio signal. That's it. The purpose is to reduce dynamic range — the gap between the quietest and loudest moments in a performance — so everything sits at a more consistent level in the mix.
Why does that matter? Because in a real performance, a singer might sing softly in one line and very loudly in the next. A drummer hits some snare hits harder than others. Without compression, these loud hits jump out of the mix, and the quiet moments get buried. A compressor smooths this out automatically.
Compression also shapes the feel and punch of a sound. A well-placed attack setting can make a snare hit harder. A slow attack lets transients punch through before clamping down. Understanding how to use this is what separates a good mix engineer from a great one.
Threshold — When the Compressor Kicks In
Set in dBFS (decibels full scale). When the signal exceeds this level, the compressor activates. Everything below the threshold passes through unaffected.
Typical range: -40 dBFS to 0 dBFS
If your threshold is too high (close to 0 dBFS), the compressor barely touches the signal. If it's too low (very negative), it's compressing nearly all the time. The goal is to set it so the compressor catches the loud peaks while leaving the quieter parts alone.
How to set it: Watch your gain reduction meter. You want to see 3–6 dB of gain reduction on a typical pop/rock vocal. Start at -18 dBFS and move it until you get that range.
Ratio — How Hard It Clamps Down
Expressed as X:1. At 4:1, for every 4 dB that the signal goes above the threshold, only 1 dB gets through. At 2:1, it's gentler. At 10:1 or higher (called "limiting"), it's a hard ceiling.
Common ratios:
- 2:1 — 3:1: Gentle, transparent compression. Good for mix bus, acoustic instruments.
- 4:1 — 6:1: Standard vocal and instrument compression. Audible but musical.
- 8:1 — 10:1: Heavy compression. Useful on bass, parallel drum compression.
- ∞:1 (Limiting): Nothing exceeds the threshold. Used on mastering limiters.
For beginners, start at 4:1 on most tracks and adjust from there.
Attack — How Fast the Compressor Reacts
Measured in milliseconds (ms). A fast attack (1–5 ms) catches transients immediately and tames them. A slow attack (20–100 ms) lets the initial transient of a sound punch through before compression kicks in.
Why this matters: The "snap" of a snare drum or the "click" of a kick drum is in the first few milliseconds of the sound. If you use a very fast attack, the compressor kills this transient and the drum loses its punch. If you use a slow attack, the transient comes through — then the compressor smooths the body of the sound.
Common starting points:
- Vocals: 10–20 ms (let the consonants through)
- Drums/snare: 5–30 ms depending on desired punch
- Bass: 30–60 ms (preserve the initial attack)
- Full mix bus: 30–60 ms
Release — How Fast the Compressor Lets Go
Measured in milliseconds or seconds. A fast release (20–50 ms) makes the compressor breathe quickly — you'll sometimes hear a "pumping" effect where the volume surges back between notes. A slow release (200 ms–1 s) is smoother and more transparent.
The pumping effect is generally unwanted on vocals and full mixes. But on sidechain compression in electronic music, it's an intentional artistic effect.
Common starting points:
- Vocals: 60–120 ms
- Drums: 50–80 ms for punch, 200–400 ms for smooth
- Bass: 100–300 ms
- "Auto" mode on many compressors works well if you're unsure
Knee — Hard or Soft?
A hard knee means the compressor switches on instantly at the threshold — full ratio kicks in immediately. This sounds more aggressive and punchy.
A soft knee gradually increases the ratio as the signal approaches and crosses the threshold. This sounds smoother and more transparent — the compression "sneaks up" on the signal.
For most beginners, soft knee is more forgiving and sounds more natural on vocals. Hard knee is useful on drums when you want a defined "clamp."
Makeup Gain — Bringing the Level Back Up
When the compressor reduces the peaks of your signal by 4–6 dB, the overall perceived loudness drops. Makeup gain brings it back up. It doesn't undo the compression — it just raises the whole compressed signal so it matches the original volume.
How to set it: Bypass your compressor. Note the output level. Re-engage it. Use makeup gain to match the bypassed level. Now you can make an honest comparison between compressed and uncompressed — because they're at the same perceived volume.
This is important: without gain-matching, the compressed signal sounds louder, which your brain interprets as "better." Gain-match first, then judge the compression on its actual effect on the sound.
Starting Settings for Common Instruments
Use these as a starting point — every track and recording is different, but these give you a sensible foundation:
| Instrument | Threshold | Ratio | Attack | Release | GR |
|---|---|---|---|---|---|
| Lead Vocal | -18 to -12 | 3:1 – 4:1 | 10–20 ms | 60–120 ms | 3–6 dB |
| Snare | -20 to -10 | 4:1 – 6:1 | 5–15 ms | 50–100 ms | 4–8 dB |
| Kick Drum | -18 to -10 | 4:1 – 6:1 | 20–40 ms | 100–200 ms | 4–6 dB |
| Bass Guitar | -20 to -12 | 4:1 – 6:1 | 30–50 ms | 150–300 ms | 4–8 dB |
| Acoustic Guitar | -20 to -14 | 2:1 – 3:1 | 20–40 ms | 100–200 ms | 2–4 dB |
| Mix Bus | -12 to -6 | 2:1 | 30–60 ms | 200–500 ms | 1–3 dB |
Less is more. If you can hear the compression working, you're probably compressing too hard. Aim for 2–4 dB of gain reduction on most tracks. Save the heavy compression (8+ dB GR) for parallel compression techniques where you blend the compressed signal underneath the dry signal.
Now that you understand compression, put it into practice on a real mix. Read the full vocal mixing guide to see how compression fits into a complete processing chain — alongside EQ, saturation, reverb, and automation.