Sidechain keying drives a compressor's level detection from a different signal than the one being compressed

By default a compressor detects level from its own input and applies gain reduction to that same signal. With sidechain (or ‘key’) routing, the level-detection input is taken from a different source: the audio being processed and the signal driving gain reduction are decoupled, so one sound can control the dynamics of another. ‘Sidechain’ is short for side signal chain — a parallel control path that tells the compressor how to react without itself being heard. When the sidechain source gets loud, the compressor turns the processed track down; as the source falls away, the level rides back up. A common misconception is that the sidechain signal is heard or processed — it is not; it only feeds the detector. The same routing appears on other threshold-based dynamics processors such as gates and dynamic EQs, and is the mechanism behind both ducking and rhythmic pumping.

Examples

Podcast mix: voiceover on channel A, music on channel B; key channel B’s compressor from channel A so the music ducks automatically whenever speech is present. Dance mix: a kick drum keys the compressor on the bass so the bass ducks on every kick hit, freeing space in the low end.

Assessment

Given a bass and a kick competing in the low end, say which track gets the compressor, which becomes the sidechain, and what the listener hears when the kick hits. Then identify the sidechain control path versus the main signal path, and explain why the sidechain signal is not itself audible.