What is Sidechain compression?
Sidechain compression, also known as ducking, is an audio effect commonly used in radio and pop music, especially dance music. In sidechain compression, the presence of one audio signal reduces the level of another one. In radio, this can typically be achieved by lowering (ducking) the volume of a secondary audio track when the primary track starts and lifting the volume again when the primary track is finished.
A typical use of this effect in a daily radio production routine is for creating a voice-over. A professional speaker reading the translation dubbs and ducks the foreign language original sound. Ducking becomes active as soon as the translation starts. In music, producers apply the ducking effect in more sophisticated ways where a signal’s volume is delicately lowered by another signal’s presence. Ducking here works through the use of a “side chain” gate. In other words, one track is made quieter (the ducked track) whenever another (the ducking track) gets louder. This may be done with a gate with its ducking function engaged or by a dedicated ducker.
Kick drum trigger
Sidechain compression is actually a very general term. However, it has come to be associated with a very specific type of usage. If you feed a regular 4/4 kick drum into the sidechain input of a compressor and use a reasonably high ratio and low threshold along with a fast attack and a medium-to-long release, the sound will duck when the kick drum is present, because the kick drum will be triggering the compressor to reduce levels every time it sounds. The combined effect makes whatever you compress sound like it is pumping or sucking. If you apply this compression to the whole track, the effect can be pretty intense.
You can use this effect to take advantage of one natural phenomenon. It gives the impression that the track is louder than it actually is. The human ear, like any kind of listening device or recorder, has limits to how loud the sound can be before it will distort and overload. When you reach these limits, your ears just compress (or limit) any sound louder than their own inbuilt threshold.
In this case of kick-drum-heavy club music, the kick drum will most likely trigger our ears built-in limiter with its huge bass energy levels. When the sound of the kick drum dies away, everything else will seem to get louder for a second until the kick drum comes again. And this is exactly the effect we been describing with sidechain compression. So if we set the sidechain compression up in the right way and on the whole mix, it can have the same effect on the sound, as listening to it extremely loud would.
Make a bigger mix
Our ears hear the effect and naturally make a connection between the “overly loud” effect and the “sidechain compression” effect. It gives the impression that we are listening to the track really loud, but at the same time quietly. Of course, nothing will ever give the same impression as listening on a huge club PA system would. However, it is a useful side-effect of the sidechain compression process that can work in our favor to help make our mixes sound somewhat bigger.
Additional resources & source texts