Feeding a frequency-shaped copy into a compressor's detector makes it react only to those frequencies

The sidechain detector reacts to whatever signal it is fed, which need not be the signal being processed. If you EQ a copy of the signal to emphasize a narrow frequency band and route that copy into the compressor’s detector, the compressor ‘listens’ only to that band but still applies its gain reduction to the full main signal. Gain reduction therefore triggers only when energy appears in the emphasized band, leaving other passages untouched. This is the structural blueprint of the de-esser — historically the first sidechain application, invented by cinema sound designer Douglas Shearer to suppress sibilance in dialogue.

Examples

A de-esser boosts ~6–12 kHz on a copy of a vocal and feeds it to the detector of a compressor on the main vocal; gain reduction fires only on sibilant ‘ess’ sounds and leaves the rest of the vocal alone. Shearer’s original split the dialogue into a compressor path and an EQ path, routing the EQ’d path into the detector.

Assessment

Describe the routing that makes a compressor de-ess a vocal, identifying which path is the sidechain and which is the main signal, and explain why non-sibilant passages pass through untouched.