What is a sidechain?
A sidechain is a detection and control signal path that an effect uses to decide when to act. Separately from the main audio being processed, it supplies the answer to what triggers the effect. In computer-based production the word usually refers to sidechain compression, but the same idea applies to gates, expanders, and filters.
The processed sound and the detected sound
For example, if you put a compressor on the bass and send the kick to its sidechain input, the bass is compressed at the moment the kick sounds. What you hear is the processed bass, while the kick is the detection signal that decides the behavior. The sound sent to the sidechain is not mixed into the compressor’s output.
Ducking to make room
Lowering the bass only at the moment its low end overlaps with the kick, or lowering the BGM only while a narration plays, is called ducking. Beyond keeping the level steady, it is also used to create the periodic pumping effect heard in EDM. For setting examples, see How to create ducking in EDM (Japanese).
The basic setup steps
Insert a compressor on the track you want to duck, and select the track that triggers the reaction as its sidechain input. Then set how much it drops with the threshold and the ratio, and shape how it drops and recovers with the attack and the release. Watch the Gain Reduction meter and confirm with both your ears and your eyes that it reacts only at the moments you need.
When it does not work
First check the source selection, whether the send is pre- or post-fader, and whether the destination is correct. Too long a release keeps the level down into the next note, and too short a release can cause wobble or distortion. Shaping the sidechain signal, for instance using the built-in filter to detect only the low end of the kick, also reduces false triggering.
