Sidechain EQ makes a compressor respond only to sibilant frequencies

De-essing is frequency-conscious gain reduction of sibilance (S/SH/T, typically 5–10 kHz), and the sidechain-EQ technique is how a plain compressor is made sibilance-selective. The sibilant band is boosted in the compressor’s sidechain (its level-detection path) without being boosted in the main signal path; this makes the detector ‘hear’ sibilant energy as much louder, so the compressor triggers specifically when sibilance exceeds the threshold while normal vowels pass through. To set it up: (1) sweep a high-frequency boost in the main signal to find the frequency where the ‘s’ is most overblown; (2) transfer that same EQ setting to the sidechain only; (3) set the threshold so gain reduction occurs on sibilants but not on ordinary speech. A dedicated second compressor reserved for sibilance is often better than the main vocal compressor, whose attack and release may be too slow to catch fast sibilants. Over-de-essing makes the singer lisp, so apply in moderation.

Examples

Sweep-boost the vocal to find the harshest sibilance at ~7 kHz, then boost the de-esser’s sidechain by ~12 dB there and set the threshold so only true esses trigger — e.g. ‘s’ sounds pull ~6 dB of gain reduction while regular speech passes uncompressed.

Assessment

Explain the difference between sidechain EQ and main-signal EQ in a de-esser, and how sidechain EQ makes the compression sibilance-selective. Describe how to find the correct sidechain frequency, and name the perceptual artifact that results from applying too much de-essing.