DESCRIPTION:
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An overview of the basics of audio compression. Learn how a compressor works, what the compressor controls are (threshold, attack, releases, ratio, knee) and how to use compression in a mix.

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Excerpt from http://theproaudiofiles.com/dynamics-processing

** Shape **

Instead of thinking of a compressor as compressing, think of it as something that changes the shape of a sound. If you start listening for "shape," the mysteries of compression will reveal themselves to you, and fairly quickly.

It may help to think of shape in terms of a sound's envelope: it's attack, decay, sustain, and release.

Setting a compressor is like setting a mold for the signal to fit into:
- Threshold determines at what amplitude the compressor starts working.
- Ratio is how hard it's going to work.
- Attack is another way of saying how sharp will the transient sound be.
- Release is how much tail or sustain you want to emphasize.

**Transients**

A transient is a very fast signal — in other words the "attack" of the signal. Drums have transient attacks. Strings have gradually rising attacks. So the attack control on the compressor is really like saying: how much emphasis on the attack of the signal do you want?

- Do you want the attack to be really rounded out and diminished? Set the attack fast.

- Do you want the attack to be prominent and stick out? Set the attack slower.

Of course, this works directly in conjunction with the threshold. Try it yourself, set the threshold low, and the attack short. Suddenly, the attack sound of your snare is gone. Set the threshold low and the attack long. Suddenly the punch of your kick is very round and bouncy. Set the threshold high and the attack short. Now the snare is a little fatter and rounder, and not quite as spikey (but possibly a little duller). Set the threshold high and the attack long — the change is hardly noticeable, the attack is just a little bit "rounder."

**Maximum Punch**

There is a thin line between a transient sound, and a sustained sound. A sound that holds for any noticeable amount of time is sustaining. A sound that moves by too quickly to register as it's own moment is transient. But transients can vary in length. A transient can be half a millisecond or it could also be ten milliseconds; they won't sound the same. A big factor in punch is how long that transient exists. A quick transient sounds "spikey," but a long transient sounds "punchy." You want to find the point that makes the transient exist as long as possible before "flattening out" or becoming a sustained sound. Only your ear can tell you where that point is.

Good samples are already shaped to have that kind of impact — and any additional compression may actually soften that. Of course, punch has a lot to do with frequency as well, but that's for another article.

Now what about the release? The release is super elusive. It determines how long it takes for the compressor to let go. If the release is too short for the signal you are going to get a disjointed sounding shape which usually results in distortion. If it's too long, your signal never really returns to its natural shape, and you generally lose tone (or you just get permanent drive on the compressor's output, giving the whole signal a new bit of tone). So the idea is to find a point that emphasizes the sustain (which is where most of the signals tone lives) properly.

Lastly, when the attack and release are set in a way that seem to argue — the compression can become very audible. You either hear the decent or the ascent of the signal level. This is called pumping. It's generally annoying, but can sometimes be used an effect. If audibly desired, consider the rhythm of the release time, and ask yourself if it's groove is complimenting the song.

**Conclusion**

So, rather than thinking of a compressor as something that effects the "level" of a signal. Think of a compressor as something that effects shape. Why? Because level can be controlled with the volume fader more accurately and transparently. A fader doesn't really control shape, unless you are being extremely meticulous. Conversely, compression will always effect the shape of the sound it is working on.: