A compressor is an automatic volume control. It watches the level of the signal and, whenever the level goes above a set point, turns the gain down by an amount that depends on how far above it went. Loud parts get quieter, quiet parts are untouched, so the gap between them, the dynamic range, shrinks.
Everything below is measured in decibels. The amount of gain the compressor removes at any instant is the gain reduction, written GR and always zero or positive: a GR of 6 dB means the output is 6 dB lower than the input at that moment.
The static curve: threshold and ratio
If you hold a steady tone at one level and note what comes out, you get the compressor’s transfer curve. It is the blade in the figure: input level along the bench, output level up the post. It has two straight segments.
- Below the threshold the output equals the input. The compressor does nothing.
- Above the threshold every ratio decibels at the input produce 1 dB at the output.
As a formula, for an input level above the threshold:
output = threshold + (input − threshold) / ratio
gain reduction = input − outputA worked example with the figure’s starting point of a −24 dB threshold and 4:1 ratio, fed a −10 dB signal: the input is 14 dB over the threshold, 14 ÷ 4 = 3.5 dB over at the output, so the output is −24 + 3.5 = −20.5 dB and the gain reduction is 10.5 dB. Drag the input level slider and the marble, the dial and the readout all follow the same arithmetic.
The ratio sets how hard the compressor pushes back:
| Ratio | Effect above the threshold |
|---|---|
| 1:1 | Nothing. The curve is the straight 1:1 wedge. |
| 2:1 | Gentle. 10 dB over becomes 5 dB over. |
| 4:1 | Firm and common for vocals, drums and bass. |
| 10:1 | Strong. 10 dB over becomes 1 dB over. |
| ∞:1 | Limiting: the output never rises above the threshold. |
Move the ratio slider to its last position and it reads ∞:1. The blade goes flat above the threshold. A real limiter is a compressor in this regime, usually with a very fast attack and often a short look-ahead delay so it can react before a peak arrives; the flat blade describes only its steady-state behaviour. Ratios of about 10:1 and above are in practice treated as limiting.
Knee
A hard knee switches from 1:1 to the full ratio at exactly the threshold, which produces a visible corner in the curve. A soft knee blends the two over a range of decibels centred on the threshold: compression starts gently below the threshold and reaches the full ratio only above it.
With a 10 dB knee, a −20 dB threshold and a 4:1 ratio, compression begins at −25 dB and reaches the full 4:1 slope at −15 dB. Exactly at the threshold the gain reduction is about 0.9 dB where a hard knee would give 0 dB. The figure uses a quadratic blend across the knee, the usual textbook formulation. Soft knees make the onset of compression harder to hear, which suits gentle glue on a mix bus. Hard knees suit cases where you want a clear, deliberate clamp on peaks.
Attack and release: how the gain moves
The static curve says where the gain should be. It says nothing about how quickly the compressor gets there. That is the job of the attack and release controls, which smooth the gain reduction over time.
- Attack is how fast the gain reduction increases when the signal goes above the threshold.
- Release is how fast the gain reduction lets go when the signal falls back below it.
In the common one-pole model each is a time constant: the gain reduction covers about 63% of the remaining distance in one time constant and about 90% in 2.3 of them. A 10 ms attack is therefore around 90% closed after roughly 23 ms. Manufacturers define these times differently (some measure 10% to 90%, some to a fixed number of decibels), so the same number on two compressors does not sound identical.
The scope on the right of the figure shows a quiet passage, a loud burst and the quiet passage again. The dim line is the input, the bright line is the output. Raise the attack time and the front of the burst pokes through before the gain comes down: that overshoot is the transient, and letting it through is why slower attacks keep drums punchy. Raise the release time and the output sits below the input for a while after the burst ends, because the gain is still recovering.
Practical starting points, as rules of thumb rather than laws:
- A fast attack (about 1 to 5 ms) catches peaks but also softens the initial click of drums, plucks and consonants.
- A slow attack (about 20 to 50 ms) lets the transient through and compresses the body that follows.
- A short release brings the level back quickly and sounds lively, or even breathy, on sustained material.
- A long release sounds smooth, but if it has not recovered before the next loud event the compressor never fully lets go.
Many compressors also offer an auto-release that adapts to the material, and a choice of peak or RMS detection. A peak detector reacts to instantaneous maxima. An RMS detector reacts to averaged energy, which tracks perceived loudness more closely and is gentler on transients.
Makeup gain and the gain-reduction meter
Compression removes level, so the output is quieter than the input. Makeup gain is a fixed gain stage after the compressor that adds the lost level back. The figure shows the signal before makeup, so you can read the gain reduction directly.
A good starting point is to set makeup so that peak level or perceived loudness matches the bypassed signal. This matters for judging the result: a louder version nearly always sounds better, so compare with levels matched rather than trusting the compressed version just because it is louder. See the gain staging article for where makeup sits in the signal chain and how it affects headroom downstream.
The gain-reduction meter is the most useful readout on a compressor, and it is the dial in the figure. It shows how many decibels are being removed right now. A few decibels of reduction on the loudest moments is light glue. Ten or more decibels, or a meter that barely leaves the stop, means heavy compression.
Common problems
Pumping. The level of the whole signal audibly rises and falls with the loud events. It happens when a loud element, often a kick or bass note, triggers gain reduction that also turns down everything else, and the release then brings the rest back up. Slow the release so it spans the gap between hits, raise the attack so the transient does not trigger as much, or filter the bass out of the detector with a sidechain high-pass. Used on purpose, with a fast release, the same effect is the “ducking” heard in a lot of dance music.
Over-compression. Too low a threshold, too high a ratio, or several compressors in series leaves the gain reduction meter permanently pinned. The result is a flat, lifeless sound: transients are squashed, the noise floor and room sound are lifted along with the quiet parts, and every part sits at the same level with no sense of movement. If the meter never returns to zero, the compressor is doing the arrangement’s work for it.
Distortion from very fast times. An attack and release of well under a millisecond can follow individual waveform cycles of low frequencies, which reshapes the wave and adds distortion. Slow one of the two times slightly, or use a longer RMS window.
Judging by ear while the level changes. Match the loudness of the processed and bypassed signal before deciding anything, as described above.
A sensible workflow: set the ratio first, lower the threshold until the meter shows the amount of reduction you want on the loudest parts, set attack and release by ear on the material, then add makeup gain to match the bypassed level. Compression also interacts with tone, so an EQ before or after it, as covered in the EQ basics article, changes what the detector responds to.