Compressors confuse people for a simple reason: when one is set well, you barely notice it. You don't hear a compressor so much as you notice that the vocal stopped jumping out and disappearing, or that the drums suddenly feel tighter.
Below is how each control works, what to listen for, and settings that make a good starting point. Treat the numbers as a place to begin, not a recipe.
What a compressor does
A compressor reduces dynamic range, the distance between the loud and quiet parts of a sound. When the signal gets louder than a level you choose, the compressor turns it down automatically. Afterward you usually raise the whole thing with makeup gain, so the quiet details end up louder relative to the peaks.
Think of it as a very fast hand on a fader. A good engineer could ride a vocal fader through a whole song by hand, and some still do. A compressor does a version of that continuously, reacting in milliseconds.
Threshold
The threshold is the level where compression starts. Anything below it passes untouched; anything above it gets reduced. Lower the threshold and more of the signal is affected. Some compressors (many 1176-style and LA-2A-style designs) have no threshold control at all: you drive the input harder instead, which has the same effect.
Ratio
The ratio decides how strongly the signal is reduced once it crosses the threshold. With a 4:1 ratio, a signal that goes 8 dB over the threshold comes out only 2 dB over. At 2:1, the same signal comes out 4 dB over.
| Ratio | Input 12 dB over threshold | Output over threshold | Gain reduction | Feel |
|---|---|---|---|---|
| 1.5:1 | +12 dB | +8 dB | 4 dB | Gentle glue, mastering |
| 2:1 | +12 dB | +6 dB | 6 dB | Buses, acoustic sources |
| 4:1 | +12 dB | +3 dB | 9 dB | Vocals, bass, drums |
| 8:1 | +12 dB | +1.5 dB | 10.5 dB | Heavy control |
| 20:1 and up | +12 dB | +0.6 dB or less | ~11.4 dB | Limiting |
Attack: the control that shapes punch
Attack is how fast the compressor reacts after the signal crosses the threshold. It changes the character of the sound more than any other setting.
A slow attack (20–40 ms) lets the first part of a hit, the transient, through before the compressor clamps down. On drums this usually sounds punchier: the snap stays and the body gets controlled. A fast attack (under 5 ms, down to fractions of a millisecond on FET designs) catches transients too, which smooths and flattens the sound. Great for a spiky vocal, less great if you want a snare to crack.
One technical detail that confuses people: attack and release values aren't standardized. Many compressors define attack as the time to reach about 63% of the target gain reduction (one time constant), others use different points on the curve. A "10 ms" setting on two different plugins can sound quite different, so set attack by ear, not by copying numbers between plugins.
Release: how the compressor lets go
Release is how quickly the compressor recovers once the signal falls back below the threshold. Too fast and you get distortion, especially on bass, because the gain starts following individual waveform cycles. A 50 Hz note has a 20 ms cycle, so a release in that range can literally modulate the wave. Too slow and the compressor never recovers between notes, so the track sits permanently turned down.
A good way to set it: watch the gain reduction meter and make it "breathe" with the song, returning close to zero before the next hit or phrase. Some designs use program-dependent release, which recovers quickly from short peaks and slowly after sustained loud passages. Optical compressors behave this way naturally, and many plugins offer an auto release mode that imitates it.
Knee and makeup gain
The knee controls how gradually compression begins around the threshold. A hard knee applies the full ratio the moment the threshold is crossed. A soft knee eases in over a range of several dB, which sounds smoother and less obvious.
Makeup gain raises the output after compression. Set it so the compressed signal is about as loud as the bypassed one. If you don't, you'll compare a louder version with a quieter one and almost always pick the louder one, even when the compression made it worse.
Reading the gain reduction meter
There's no correct number, but these ranges are typical:
- Lead vocal: 3–6 dB on louder phrases, more in dense pop and rap
- Bass: 3–6 dB
- Drum bus: 2–4 dB
- Mix bus: 1–2 dB, sometimes less
If one compressor is doing 10 dB or more of constant reduction, it'll usually sound squashed. Splitting the job between two compressors normally sounds better.
Studio Tip
If you can't hear what a compressor is doing, exaggerate it. Set 10:1, pull the threshold down until you see 10–15 dB of gain reduction, then move attack and release. You'll clearly hear the punch disappear with fast attack and the pumping with fast release. Once you hear it, back off to sensible settings: the same character will be there, just subtler.
VCA vs FET vs Optical vs Vari-Mu
Most plugin compressors are modeled, loosely or closely, on a few hardware topologies. The labels describe the gain-control element inside the circuit, and each one has a typical behavior:
| Type | How gain is controlled | Speed | Sound | Typical uses |
|---|---|---|---|---|
| VCA | Voltage-controlled amplifier chip | Fast and precise, fully adjustable | Clean, punchy, predictable | Drum bus, mix bus, anything |
| FET | Field-effect transistor (1176 style) | Very fast; attack can be well under 1 ms | Aggressive, colored, exciting | Vocals, drums, room mics, bass |
| Optical | Light source and photocell (LA-2A style) | Slow-ish attack, program-dependent two-stage release | Smooth, gentle, musical | Vocals, bass, acoustic instruments |
| Vari-Mu | Tube with variable gain | Moderate; ratio rises as you drive harder | Warm, thick, soft knee | Mix bus glue, mastering |
Your DAW's stock compressor can cover all of these jobs. The character differences are real, but small compared to setting the controls well.
Starting settings by source
| Source | Ratio | Attack | Release | Gain reduction |
|---|---|---|---|---|
| Lead vocal | 3:1–4:1 | 5–15 ms | 50–100 ms or auto | 3–6 dB |
| Kick | 4:1 | 10–30 ms | 50–100 ms | 3–5 dB |
| Snare | 4:1 | 5–20 ms | 80–150 ms | 3–6 dB |
| Bass guitar / synth bass | 4:1 | 10–30 ms | 100–200 ms | 3–6 dB |
| Acoustic guitar | 2:1–3:1 | 10–20 ms | 100 ms or auto | 2–4 dB |
| Drum bus | 4:1 | 10–30 ms | Auto or timed to tempo | 2–4 dB |
| Mix bus | 2:1 | 10–30 ms | Auto or 100–300 ms | 1–2 dB |
Always adjust by ear. A slow ballad and a fast trap beat need very different release times, even on the same kind of source.
Serial compression
Instead of one compressor working hard, use two working lightly. On vocals the classic pattern is a fast FET-style compressor first, catching only the loudest peaks (2–3 dB), followed by a slower optical-style one doing another 2–3 dB of general leveling. Two stages of 3 dB usually sound more natural than one stage of 6 dB, because each compressor stays in its comfortable range.
Parallel (New York) compression
Parallel compression means blending a heavily compressed copy of a sound with the untouched original. You get the density and sustain of heavy compression while the dry signal keeps the natural transients. It's a classic on drums, often called New York compression, and works on vocals and bass too.
Send the track to an aux bus, put a compressor there with a high ratio (8:1 or more), fast attack and 10–20 dB of gain reduction, then fade the bus in under the original until you feel extra weight without hearing obvious squashing. Many compressors also have a mix knob that does the same thing inside the plugin.
Signal Chain Example: parallel drum bus
- Kick, snare, toms, overheads → Drum Bus (VCA, 4:1, 30 ms attack, auto release, 2 dB GR)
- Drum Bus send → Parallel Crush aux (FET, 20:1, fastest attack, 100 ms release, 15 dB GR)
- On the crush aux: HPF 60 Hz, gentle high shelf −2 dB at 8 kHz to tame cymbal splash
- Blend the crush aux about 10–15 dB below the main drum bus
Sidechain compression
A sidechain lets one signal control the compressor on another. The common example is the kick briefly turning down the bass on every hit, so they don't pile up in the low end. Push it further and you get the pumping effect heard in a lot of dance music. Many compressors also offer a sidechain high-pass filter, which stops low frequencies from triggering too much gain reduction on a full mix. The kick, bass and 808 guide goes deeper on ducking.
Common compression mistakes
- Compressing because a track "should" have a compressor. If it already sits well, leave it.
- Comparing without level-matching, and so always choosing more compression.
- Fast attack on drums, then wondering where the punch went.
- Very fast release on bass, causing grainy distortion.
- Heavy mix bus compression early in the mix, then mixing into a squashed sound you can't undo.
- Ignoring the sidechain filter on the mix bus, so the kick pumps the whole song.