glossary
A sidechain filter changes the signal a dynamics processor listens to.
A sidechain filter shapes the detector signal that controls a compressor, limiter, gate, or de-esser. It does not necessarily EQ the audible signal directly.
High-pass sidechain filters are common because low-end energy can trigger compression harder than the rest of the mix suggests.
On a mix bus, filtering low bass out of the detector can reduce unwanted pumping. On vocals, a narrow detector emphasis can make de-essing respond mostly to sibilance.
The goal is not to hide dynamics but to make the processor react to the problem you actually hear.
A sidechain filter can make compression feel more natural, but it can also let real peaks through if pushed too far.
Watch gain reduction, peak level, and loudness together so the smoother motion does not create new headroom problems.