Sidechain compression is a special variant of regular channel compression. A normal compressor adjusts the output level of the audio based on the input level. Sidechain compression, however, adjusts the output level of the audio based on the level of a different audio channel.
This means that the volume of a channel reacts to the volume of another channel. The audio that the compressor is reacting to is often referred to as the ‘key’ or the ‘sidechain’.
There are two common uses for this:
- Kick drum ducking. This technique uses the kick drum for the sidechain signal. It’s set up so that the compressed channel (usually the bass) is briefly turned down when the kick drum is sounding. It was originally used to make the kick drum bigger – by reducing the level of some other tracks (usually the bassline), the kick punches through the mix with relatively more presence and power. It’s most commonly used to compress the bassline (either bass synth or bass guitar), but is also used to compress synth pads, vocals or even other drum and percussion tracks. It’s become a recognisable and characteristic sound in a lot of electronic dance music.
- Vocal ducking. This technique uses the main vocal channel as the sidechain audio. It’s set up so that the compressed channel is turned down when the main vocal is sounding. It was originally used in radio broadcast so that the music would be automatically turned down when the announcer or DJ started speaking. It can useful when mixing a song that contains a prominent foreground part (such as a guitar or vocal harmonies) that should be pushed to the background when the lead vocals come in. Ideally, however, this situation is best avoided by careful composition and arrangement.
In day-to-day mixing, there’s usually not much need to use sidechain compression unless you’re aiming to create a certain effect such as a pumping bassline for a dance song.