What is a sidechain?

A sidechain is a second input to a processor, such as a compressor, that decides when the processor acts, while the processor changes a different signal.

The definition, from the manual

Ableton's Live manual puts it plainly: "Normally, the signal being compressed and the input source that triggers the compressor are the same signal." Turn on the sidechain and you pick another source "to act as the compressor's trigger, instead of the signal that is actually being compressed." It adds that "the sidechain audio is only a trigger for the compressor and is never actually heard." (Ableton)

So there are two signals:

• The main input: what you hear, and what gets turned down.
• The sidechain input: what the compressor listens to when it decides how much to turn down.

Input, not effect

"Sidechain" is often used to mean the pumping sound you get when a kick ducks a bass or a pad. That sound is just one result. The sidechain is the routing. The same routing can duck a guitar under a vocal, or trigger only on part of the spectrum: Ableton's sidechain EQ makes the compressor "triggered by a specific band of frequencies, instead of a complete signal." (Ableton)

Two practical notes from the same manual: with Dry/Wet at 100%, "the compressor is triggered entirely by the sidechain source," and "automatic Makeup is not available when using external sidechain." (Ableton)

For the hands-on version, see our guide to sidechain compression.

What we think

Calling the pumping effect "sidechain" makes beginners think it is a genre trick. Learning it as a routing, a key that tells one processor to listen to another track, is more useful: once that clicks, the quiet uses (a vocal that cuts through, a bass that only moves where the kick sits) do more for most mixes than the obvious pump.