Compression is the process of reducing the dynamic range between the loudest and quietest parts of an audio signal. This is done by boosting the quieter signals and attenuating the louder signals.

Compression is mainly used on vocals, however can and should be used across the board of mixes, to ensure full control of the final mix.
Compression should be enough to control the mix, and whenever using the effect it should not be obtrusive – this is all how your own ears perceive the sounds however.

Parameters of Compression:

Threshold – How loud the signal is before compression is applied.

Ratio – How much compression is applied. For example, if the compression ratio is set for 6:1, the input signal will have to cross the threshold by 6 dB for the output level to increase by 1dB.

Attack – how quickly the compressor starts to work.

Release – how quickly after the signal drops below the threshold the compressor stops.

Knee – sets how the compressor reacts to signals once the threshold is passed. Hard Knee settings mean it clamps the signal straight away, and Soft Knee means the compression kicks in more gently as the signal goes further past the threshold.

Make-Up Gain – allows you to boost the compressed signal, as compression often reduces the signal significantly.

Output – allows you to boost or decrease the level of the signal output from the compressor.

Other Types of Compression:

Side-Chain Compression – Compressor uses the input signal to determine how strongly the compressor will reduce the gain on it’s output. Can be used by DJs for Ducking (E.G. Lowering the volume quickly for when they talk!)

Multi-band Compression – Compressor that allows you to change the frequency bands, ensuring you can change the compression levels of certain frequency bands rather than the full bandwidth. Multi-band compression is good for mastering.