What compression is and how to use it in a mix

What a compressor actually does

A compressor is a volume knob that moves on its own. You tell it a level, and whenever the sound goes over that level, it turns the sound down. When the sound drops back under the level, it lets go. Everything else is about how much it turns down and how fast it moves.

In a mix you mostly use it to make a part steadier. A singer gets louder on the big notes and quieter on the soft words. A bass player hits some strings harder than others. Each of those jumps makes the part hard to place, because if you set the fader for the loud bits, the quiet bits get lost, and if you set it for the quiet bits, the loud bits poke out. A compressor closes that gap, so one fader position works for the whole song.

Once the loud parts are turned down, you can turn the whole part back up. That is makeup gain. The peaks end up where they were, and everything under them is now louder. This is why compression can make a part sound fuller and closer, and also why it can fool you, which comes up again below.

Compression also changes the shape of each note or hit. Depending on how fast it reacts, it can let the front of a drum hit through and then hold the tail back, which makes the hit sound punchier. Or it can grab the front and soften it, which makes a sharp part sound smoother. So a compressor is a tone tool as well as a level tool.

Threshold, ratio, attack and release in plain words

The threshold is the level where the compressor starts working. Anything quieter passes through untouched. The lower you set it, the more of the sound goes over it and gets turned down. Set it last, by watching the gain reduction meter, because the right threshold depends on how loud your own track is.

The ratio sets how hard it turns things down. At 2:1, a sound that goes 4 dB over the threshold comes out only 2 dB over. At 4:1, the same sound comes out 1 dB over. Low ratios sound gentle and natural. High ratios, from about 8:1 up, hold the peaks almost flat and start to act like a limiter.

The attack is how fast the compressor reacts once the sound goes over the threshold. A fast attack, a few milliseconds or less, catches the very front of each note. A slow attack, 10 to 30 ms, lets that front edge through before the compressor clamps down. That front edge is where the punch of a drum or the pluck of a bass lives, so the attack is often the most important setting on the whole device.

The release is how fast it lets go after the sound drops back under the threshold. Too fast and the volume flutters, which you hear as distortion on low notes. Too slow and the compressor is still holding the next note down when it arrives, so everything sounds flat. A good release lets the gain reduction meter fall back toward zero just before the next note or hit.

The knee sets how gently the compressor eases in. A hard knee starts working all at once at the threshold. A soft knee starts a little before it and blends in, which is harder to hear. Gain reduction is how much the compressor is turning down right now, and it is the number to watch while you set everything else.

How to hear compression

Good compression is meant to be hard to hear, so at first it is. The trick is to make it obvious, learn what it sounds like, and then back off.

Pull the threshold right down and set the ratio high, so the meter shows 10 dB or more of gain reduction. Now play with the attack and listen to the front of each note get sharper or softer. Play with the release and listen to the sound pump and breathe. Once you know what to listen for, raise the threshold until the meter moves only a few dB on the loudest parts.

Always compare at the same loudness. A compressor with makeup gain makes the part louder, and louder almost always sounds better for a few seconds. Switch the device off and on, and match the level with its output or a Utility, so you judge only the change in shape and steadiness. If the part only sounds better when it is louder, take the compressor off and turn up the fader.

Listen in the mix, not in solo. A vocal can sound squashed alone and sit perfectly against the beat.

When not to compress

Not every part needs a compressor. A synth pad that plays at one level, a sample that is already squashed, or a distorted guitar that the amp has already flattened may not need any. Adding one anyway costs you punch and brings up the noise under the sound.

Fix big level jumps before you compress. If one verse of a vocal is much quieter than the rest, split the clip or draw clip gain so the parts arrive at a similar level. A compressor working hard to lift one quiet section will squash the loud sections too.

Stop when the meter moves more than you planned. Most parts in a mix need a few dB of gain reduction, not ten. When in doubt, use two gentle stages instead of one hard one.

Compressor and Glue Compressor in Live

Compressor is the main tool, and it ships with every edition of Live 12, from Lite up. It gives you Peak or RMS detection, a soft or hard knee, auto release, a sidechain with its own EQ and a Dry/Wet knob. The sidechain EQ is how you build a de-esser in Live. The free kit's Vocal Chain does exactly that, with one Compressor set up as a de-esser and a second one leveling the phrases.

Glue Compressor comes with Standard and Suite. It has fewer controls, with only three ratios and set steps for attack and release, and its sound is built for buses. It is the usual choice on a drum bus or a mix bus, where a little gain reduction pulls the parts together. It also has a Dry/Wet knob, so it can do parallel compression on one track.

If you are in Lite or Intro, you lose nothing important. Compressor covers every job on these guides, and each guide gives exact Compressor settings. The kit's Club Test Master rack uses Compressor too, at 2:1 with a 30 ms attack.