Search "what does a compressor do" and you'll mostly get answers about what it's for: "it controls dynamics", "it glues the mix", "it adds punch". Those are outcomes. They don't tell you what's physically happening to your audio, which is why so many people turn the knobs until it sounds different and hope different means better.

This piece explains the mechanism in plain English. No prior mixing knowledge needed, and no maths beyond simple subtraction.

The short answer

A compressor is an automatic volume knob that only turns down, and only when the sound gets loud.

Imagine someone sitting at your mixing desk with a hand on the fader. Every time the singer belts a note, they pull the fader down a little. When the singer goes quiet again, they let it back up. That's all a compressor does, just faster and more consistently than a person could.

The result is that the gap between your loudest and quietest moments gets smaller. Engineers call that gap dynamic range, and a compressor reduces it. Everything else people say about compression (punch, glue, polish) is a side effect of that one change.

The controls, one at a time

Every compressor answers the same four questions for that imaginary hand on the fader: when to act, how hard, how fast to react, and how fast to let go. Then a fifth control tidies up afterwards.

Threshold: when does it start working? The threshold is a volume line. Anything quieter than the line is left completely alone. Anything louder gets turned down. Lower the threshold and more of your audio crosses the line, so more of it gets compressed.

Ratio: how hard does it push back? The ratio decides how much of the overshoot is allowed through. At 4:1, for every 4 dB the sound goes over the threshold, only 1 dB comes out the other side.

A worked example: threshold at -18 dB, ratio 4:1. A snare hit peaks at -10 dB, which is 8 dB over the line. The compressor lets through a quarter of that, 2 dB, so the hit comes out at -16 dB. It has been turned down by 6 dB. That 6 dB is the gain reduction, and most compressors show it on a meter.

Low ratios (1.5:1 to 3:1) are gentle. High ratios (8:1 and up) clamp down hard.

Attack: how quickly does it react? Attack is how long the compressor takes to reach full strength once the sound crosses the threshold, usually measured in milliseconds. A slow attack lets the very first moment of a sound slip through before the squeeze arrives. That first moment, the transient, is the crack of a snare or the pick of a guitar string. Slow attack keeps it; fast attack flattens it.

Release: how quickly does it let go? Release is how long it takes to stop turning down once the sound drops back under the threshold. Too fast and you can hear the volume flutter. Too slow and the compressor is still holding things down when the next quiet passage arrives.

Makeup gain: putting the volume back Because compression only turns things down, the track ends up quieter overall. Makeup gain is a plain volume boost applied afterwards to bring the level back up. This step is where the "quiet parts get louder" effect actually comes from.

What you actually hear

Put those steps together: the loud peaks get pulled down, then the whole track gets pushed back up. The peaks end up roughly where they started, but everything underneath them has risen. Breaths, room sound, the tail of a reverb, the quiet verse before the big chorus.

That's why compressed audio feels closer and more present. The detail that was hiding in the quiet parts is now in the same neighbourhood as the loud parts. On a vocal, it means you stop reaching for the volume every time the singer drops a word. On a whole mix, it means the track holds together instead of lurching between soft and loud.

It also means compression raises things you might not want: hiss, hum, mouth noises, a fridge in the next room. If those appear after compressing, the compressor didn't add them. It turned them up.

When it goes wrong

Most compression problems come from doing too much of a good thing. Three are worth recognising by ear.

• Pumping. The volume audibly swells and dips in time with the loudest element, often the kick drum. Usually a release that's too fast combined with a lot of gain reduction.

• Lost punch. Drums sound like cardboard and guitars lose their bite. The attack is too fast, so the transients are being flattened before you hear them.

• Flat and tiring. Nothing is wrong exactly, but the track never breathes. Everything sits at one level, so nothing feels big because nothing is ever small. Too low a threshold, too high a ratio, or both.

One trap catches almost everyone: compressed audio is usually louder after makeup gain, and our ears hear louder as better. Before deciding a setting has improved things, match the volume of the before and after versions (most compressors have a bypass switch for this). If you only prefer it when it's louder, the compression isn't helping.

Compression, limiting and mastering

A limiter is a compressor taken to the extreme: a very high ratio and a very fast attack. Instead of gently leaning on the peaks, it sets a ceiling nothing is allowed to pass. Limiters are what make a finished track loud enough to sit alongside commercial releases without distorting.

In mastering, compression is used lightly. On a full mix, 1 to 3 dB of gain reduction with a low ratio is typical. The aim is to nudge the elements of the mix into sitting together, not to reshape them. If a master needs heavy compression to sound right, the fix usually belongs back in the mix, on the individual instrument that's jumping out.

Seeing it on your own tracks

The fastest way to learn compression is to watch it happen to music you know. That's the thinking behind SoundSmith, the mastering tool we build at Hearthlight.

When SoundSmith suggests a setting, it tells you why. Its plain-English report describes the change in everyday terms; switch to the technical report and you get the figures instead. You can also adjust the settings yourself and get an explanation of what each one is doing. Your audio is processed on your own device and never uploaded.

There's a free tier, so you can run one of your own tracks through it and compare the result against everything above.