A Compressed Guide To Audio Compression

A compressor is arguably the most vital plug-in processor in an audio producer’s toolbox (after reverb, that is) – but is also one of the most difficult to fully understand, and almost certainly the most difficult to use properly. There are a few things you should know before you start down the compression trail…

Limiting Use Of Compression

A lot of producers put compressors in a mix as a matter of habit – however, a compressor (like any effect or processing) should only really be used if it makes the track (or the mix) sound better. It also helps if you know what sound you’re looking for and how to achieve it. Many great articles have been written about compression in the past; I’ve listed a couple of these at the end of this post, so I won’t go into too much detail here.

A limiter is basically a particular type of compressor – one that has an infinite compression ratio. What this means is that no signal gets past the level you set as the threshold (brick-wall limiter). So if you set your limiter with a threshold of -5 dB, then there will be no waveform present above -5 dB. This means that you can then raise the average overall loudness of the track by about 5 dB without clipping the signal.

Standard compression is generally used to smooth out a performance, bringing down the loud parts and bringing up the quiet ones. Before compressors were invented, this was done by hand – you literally had to keep your hand on the mixer’s volume fader and move it up and down as the track got softer and louder. This is known as ‘gain riding’, and requires a steady hand and a good knowledge of what’s going to happen next.

In the case of an electronic compressor, instead of having a human react to the mix, you can set the attack time in milliseconds – this indicates how quickly the compressor should bring the levels down once a sound gets too loud.

You define ‘too loud’ by using the threshold dial, and you set ‘how much to bring the fader down’ using the compression ratio dial.

For some very useful explanations of everything to do with compression, you can read this article by Paul White. This is a fairly definitive introduction to the subject, and the fact that it was written in 1996 illustrates how the underlying principles of such waveform manipulation have not changed in the past decade (or ever, in fact). Another compression and limiting guide can be found at The Recording Project, which also provides some mp3 illustrations of compression in action.