What is compression?

Compression in plain English

Picture an engineer with a hand on the vocal fader. Every time the singer belts a line they pull it down a little, and when the singer drops back to a whisper they push it up again. A compressor does the pulling-down half of that job on its own, thousands of times a second, and it never reacts late because it got distracted.

It listens to the level of whatever you put through it. While the level stays under the threshold, nothing happens at all. Once the sound goes over, the compressor turns it down, and the ratio decides how hard. Two more controls, attack and release, set how quickly it reacts and how quickly it lets go.

Because only the loud parts get turned down, the distance between the loudest and quietest moments gets smaller. Engineers call that distance dynamic range, which is why compressors are known as dynamics processors. Everything else on this page is about how fast that happens and what it does to the sound when you push it.

Why use compression?

Mostly to keep things audible. A vocal that sits comfortably in the verse can vanish under a loud chorus and then leap out on one shouted word; with a compressor evening out those jumps, you don't have to ride the fader through every line of the song.

A compressor can also change the shape of a sound. Every drum hit starts with a transient, a spike a few milliseconds long. Set a slow attack and that spike escapes before the compressor reacts, so the crack of the snare stands out against the ring that follows. Set a fast attack with a fast release and you get the reverse: the hit flattens and the room sound behind it comes up.

On a group of instruments, like a drum bus or a full mix, light compression makes the parts move together, because they all get turned down by the same amount at the same moments. People call that glue. It's subtle on its own and obvious when you take it away.

Then there's loudness. Lowering the peaks leaves room to turn the whole track up, and that's the reason to be most careful with, since a track squeezed too hard stops sounding like people playing in a room.

Reading the lab

The Whole loop picture shows the loop from start to finish. The pale shape behind is the original and the pink shape in front is what comes out of the compressor, so wherever the pink is smaller than the pale, the sound is being turned down. Along the bottom, the amber lane shows gain reduction. The deeper it dips, the harder the compressor is working at that moment, and the Turning down readout above the picture gives you the exact figure as the loop plays.

The Transfer curve tab draws the compressor as one line. Input level runs along the bottom and output level up the side, both in dBFS. Below the threshold the line runs at 45 degrees, meaning what goes in comes out unchanged; above it the line bends flatter, and the bend gets sharper as you raise the ratio. While the loop plays, a white dot rides the curve at the current level.

The One hit up close tab zooms in on 160 ms around the loudest hit, which is the best place to see what the attack control does. Next to the Turning down readout, Loudest peak out shows the highest level coming out of the compressor, makeup included.

Switch to Original to hear the loop without the compressor. The lab sets the original to the same loudness as the compressed version when you do, so you're comparing shape with shape. That one feature fixes the most common way beginners fool themselves, which is covered further down.

The loops come from two Sounds Like You packs, but you can use any sound. Press Your own sound and pick a file from your computer, and it loops through the lab in place of ours. Nothing leaves your device, and your sound is still there when you open the next lesson.

The controls

Most compressors have the same core controls, whether it's the stock compressor in Ableton Live, Logic's Compressor, FL Studio's Fruity Limiter in compressor mode or a hardware unit. The names change a little between them; the ideas don't.

Threshold

The threshold is the level where compression begins, measured in dBFS. With the threshold at −20 dB the compressor ignores anything quieter than −20 dB. A snare hit peaking at −6 dB is 14 dB over, so it gets turned down; a hi-hat at −24 dB passes straight through.

Lowering the threshold puts more of the sound over the line, so more of it gets compressed. If the amber lane in the lab is completely flat, the threshold is above everything in the loop and the compressor has nothing to work on.

Ratio

The ratio sets how hard a sound over the threshold gets turned down. At 2:1, a peak 10 dB over comes out 5 dB over. Raise it to 4:1 and that same peak comes out just 2.5 dB above the threshold, and at 10:1 it's only 1 dB.

Ratios between 1.5:1 and 3:1 sound gentle, which is why they turn up on mix buses and acoustic instruments. Around 4:1 to 6:1 gives firm control for vocals, bass and drums. Past 10:1 a compressor starts behaving like a limiter, letting very little past the threshold.

Attack

Attack is how quickly the compressor clamps down once a sound crosses the threshold. Most compressors offer somewhere between 0.1 ms and 100 ms, and on drums this is the control that changes the feel the most.

With a 20 or 30 ms attack the transient is mostly over before the compressor has fully reacted. The spike comes through at close to full height while the body of the hit gets turned down, so the drum sounds sharper and closer. Drop the attack under 1 ms and the compressor catches the spike itself, which makes the same drum softer and further away. Neither is wrong. They're two different sounds.

Release

Release is how quickly the compressor lets go after the level falls back under the threshold. Short releases of 30 to 80 ms recover in time for the next hit and keep a part lively. Long releases of 300 ms or more hold the gain down for longer and sound smoother, and if the release is longer than the gap between hits the compressor never fully lets go.

Be careful with very short releases on bass. A 55 Hz note takes about 18 ms per cycle, so a release shorter than that can start following the waveform up and down, and you'll hear it as a buzzy distortion. Pick Bass in the lab and pull the release under 20 ms to hear it.

When the release recovers in time with the beat, the whole track starts to breathe in and out. That's called pumping, and it can be a mistake or the entire point: it's a big part of the sound of French house records like Daft Punk's “One More Time”.

Knee

The knee decides how the compressor behaves right around the threshold. A hard knee (0 dB) switches compression on at one exact level. A soft knee fades it in across a range, so a 12 dB knee at a −20 dB threshold starts working gently at −26 dB and reaches the full ratio by −14 dB. Soft knees are harder to hear working, which suits vocals and mix buses, while a hard knee grabs drums more firmly. In the lab the knee is the shaded band on the transfer curve.

Makeup gain

A compressor only ever turns things down, so whatever comes out is quieter than what went in. Makeup gain fixes that. It's plain volume, added after the compression, and Logic's Compressor calls its automatic version Auto Gain while Ableton's Compressor has a Makeup button that does the same job.

Aim to match the loudness of the uncompressed sound and no more. Add extra and the compressed version will win every comparison simply because it's louder, which teaches you nothing about whether the compression helped.

Mix and parallel compression

The mix control, sometimes labelled dry/wet, blends the compressed sound with the original. At 100% you hear only the compressed version. Set heavy compression and pull the mix down to 30 or 40% and you have parallel compression: the original keeps its transients on top, while the squashed copy underneath lifts the quiet detail. You'll also see it called New York compression, after the engineers who used it on drums in the 1970s and 80s.

Older compressors don't have a mix knob, so the same thing is done with a send to a separate bus. The parallel compression lesson covers both ways.

Reading a gain reduction meter

The gain reduction meter shows how far the compressor is turning the sound down right now. Most read downwards from zero, so a meter sitting at −4 means the sound is 4 dB quieter than it would be with the compressor switched off. In the lab it's the Turning down readout above the picture.

The movement tells you more than the number does. A meter that kicks on each hit and returns to zero in between is working with the rhythm. One that never makes it back to zero means the threshold is so low, or the release so long, that the compressor is always on, which flattens everything it touches.

As a rough guide, 1 to 3 dB of gain reduction is subtle, 3 to 6 dB is clear control, and anything past 10 dB is a deliberate effect.

How to set a compressor, step by step

This order works on almost any source and is the method the lesson in the lab follows.

1. Start from neutral settingsA 4:1 ratio, a 10 ms attack and a 100 ms release are middle-of-the-road values that let you hear what the threshold is doing before you shape anything.
2. Lower the thresholdBring it down until the meter shows 3 to 6 dB of gain reduction on the loudest moments. On a vocal, that's usually the biggest words in the chorus.
3. Shape the front of each note with the attackListen to the very start of the hits. Slow it down for more punch, speed it up for tighter control.
4. Set the release by watching the meterYou want it to recover before the next hit or phrase. If it never returns to zero, shorten the release or raise the threshold.
5. Match the level with makeup gainTurn it up until the compressed and bypassed versions sound equally loud.
6. Compare, then decideSwitch it off and on. If you can't say what improved, ease the threshold back up or take the compressor out.

Starting settings for common sounds

These are places to begin, chosen so the compressor is clearly doing something. Your ears and the Original and Compressed switch decide where you end up.

Common compression mistakes

Judging it by volume

Make something a decibel louder and most people will say it sounds better, which is how a compressor with too much makeup gain wins every quick comparison. Always compare at matched loudness, either with the Original and Compressed switch in this lab or by setting makeup gain carefully in your DAW.

Compressing everything by habit

A lot of samples, including most one-shots in sample packs, were compressed when they were made. Another compressor on top often does nothing useful. If the level of a part doesn't jump around and its hits already sound the way you want, leave it alone.

Taking the snap off drums you wanted punchy

If a drum sounds smaller after compression, the attack is almost always too fast. Slow it down to 20 ms or more and listen to the front of the hit come back.

Too much on the mix bus

Two decibels of gain reduction on a whole mix is plenty. Beyond that, every loud kick drags the vocal and the pads down with it, and the track starts to pump without you meaning it to.

Training your ears to hear compression

Compression is hard to hear at first because it changes shape more than tone. The quiz under the lab helps: it picks hidden settings, covers the controls and matches the levels, then asks whether the attack is fast or slow, or how high the ratio is. Eight rounds take about three minutes, and your best score is saved in your browser so you can see it improve.

Types of compressor

Hardware compressors are grouped by the part that does the turning down, and plug-ins copy their behaviour. Each type has its own speed and character, which is why engineers reach for different ones on different sources.

VCA

A voltage-controlled amplifier does the work. VCA compressors are fast, precise and fairly clean, and the SSL bus compressor and the dbx 160 are the classic examples. They're a common choice on drum buses and mix buses.

FET

In a FET compressor, a field-effect transistor does the turning down. The Universal Audio 1176 is the usual example, with an attack as fast as 20 microseconds and an aggressive, forward sound that suits vocals, drums and bass.

Optical

A light shines on a light-sensitive cell, and the brighter the input the more the cell turns the sound down. The cell responds at its own pace, so optical compressors feel smooth and slightly slow. The Teletronix LA-2A has just two main knobs, gain and peak reduction, and is a standard choice on vocals and bass.

Vari-mu

Valves change their own gain as the control voltage changes. Vari-mu compressors such as the Fairchild 670 and the Manley Variable Mu are slow and gentle and add some warmth, and they're mostly used across whole mixes and in mastering.

Compression, limiting, clipping and saturation

These four get confused because they can all lower peaks. A limiter is a compressor pushed to the extreme, with a ratio so high and an attack so fast that nothing gets past its ceiling. A clipper doesn't turn the level down at all: it cuts off the top of the waveform, which is instant and adds distortion, and you can hear exactly how that differs in soft clipping vs hard clipping vs limiting. Saturation rounds the peaks off gently and adds harmonics, so it compresses a little as a side effect.

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