Basic Concept

Compression is best thought of like an automatic volume control, smoothing out the volume and reducing the dynamic range of the signal. Compression is used extensively in mixing for a number of reasons:

• By smoothing the volume of an instrument, you can sit it more consistently within a mix
• To control the dynamics of a piece
• To alter the transient properties of an instrument (i.e. the attack and sustain)
• To bring out quieter parts of the sound of an instrument.
• To increase the average volume(RMS) of a signal

Compressors may appear a little confusing at first but are in actual fact very simple.
Sound that exceeds a certain threshold volume level is reduced by a given ratio. For example everything above –20dB is reduced by a ratio of 5:1.
The below diagram shows how compression is typically represented:

Make up gain

Because the compressor reduces the level of the signal we use makeup gain to compensate for the loss. For example if the gain reduction is around 3dB at the peaks, you should boost the makeup gain by 3dB.

Something to be mindful of is that by boosting the overall signal using the make-up you are also boosting the noise floor!

Below is an example of the process of compression then make-up gain

Attack & Release

The attack control determine how long it takes the compressor to activate once the threshold level is exceeded. The release control determines how long it takes for the compressor to deactivate after the level goes back below the threshold.
Attack is important to maintain the transients of instruments, particularly when dealing with drums. If the attack time is too short, you will also hear distortion. Used carefully, the attack control can be used to emphasise the punch of drums.

The release control determines how long the compression takes to de-activate after the signal falls back below the threshold.
The release control serves a few purposes when I’m mixing, the longer the release, the smoother the compression and ultimately the less effect it has. Once you are comfortable with the sound of compression you can tune the release control to make instruments ‘breath’ with the music.

So when do I use a compressor?

There are different schools of thought upon compression, currently mixing trends favor quite heavy amounts of compression in most popular music genres such as Rock, Pop, R’n’B Dance & Hip Hop.
The drums and bass instruments are the first main instruments usually to be compressed; this helps to obtain a solid backbone to a mix, however can be used on most instruments. The hard bit is learning objectively when and when not to use it and how to use it for each instrument.

In summary you should not be afraid to experiment with compression, the only way to learn any new tool is to use it. Try compressing groups of instruments on busses, try compressors before eq, after eq, before reverb, after reverb, try compressing a compressed signal, limit a compressed signal. In time I've found my ears have naturally become a lot more sensitive to the sound of compression and where an instrument needs it.

© Nathan Boddy 2006 unauthorised copying or duplication prohibited.

Go to compression part 2

Go back to articles