Compression

By definition, compression reduces the dynamic range (the difference between the quietest and loudest parts) of the signal. It can be applied as an effect so that the sound is obviously compressed, or applied in subtler ways to make the compression as transparent as possible.  Both approaches are equally valid – it’s mostly just a matter of taste.  Broadly speaking, the harder a sound is compressed, the more obvious the compression becomes.  Compression and EQ are the two most important and most used tools for a mixer.

Compression in detail:

•  Threshold – The level that sets where the compression will start to be applied.

•  Ratio - The amount of compression applied once the signal passes the threshold.  For example, a compression of 4:1 would mean that for every 4db of change past the threshold, only 1db will occur.  At a much higher ratio such as 50:1, compression is known as “limiting” because the user is limiting the level to a predetermined amount set by the threshold even though limiting will actually occur at much lower ratios.  At very high ratios, the term ‘brick wall limiting’ is often used to describe the effect, as if the sound has hit a virtual brick wall of compression.

•  Attack – The time it take for compression to be applied once the signal passes the threshold.

•  Release - The time it takes for the compression to stop once the sound drops below threshold.

•  Knee - Often hard or soft, this determines the slope of the compression – hard, like a straight edge, or soft for a more gentle slope.

•  Make Up - When compression is applied it can then reduce the overall volume of the original signal. In that case, make up gain is applied.  Some units do this automatically.

This may all sound very complicated, but after some hands-on experience you’ll see that it’s actually quite simple.  Compression is really something that your ears need to judge. Don’t worry too much about what the numbers say.  There are no rules for the uses of compression.  Buss compression is often applied either at mix busses (such as entire drum sub-mixes), or at the master section during the mix to even out the sound.  In popular culture, the term “loudness wars” was coined to describe the over-use of compression in mixing and mastering.  This was during a period (and in some cases, it still happens) where mastering engineers were forced by record companies to over-use compression in order to try and make songs sound louder during radio play.  In many cases, mastering engineers were given the task of mastering tracks to get the highest possible volume from a track (often against their own professional judgment) or record company execs would threaten to take their mastering work elsewhere.  As you get used to compression you’ll start to notice how much it can affect the sound in both good and bad ways.  On this website, I’ll be showing you some tricks with compression that will really help get your mix to pop, and have that “pro” sound.