Sidechain Compression

Sidechain compression makes a compressor respond to a different signal than the one it is processing, so one track controls the volume of another. The classic use in post is ducking: a voiceover or dialogue track feeds the sidechain of a compressor on the music bus, so whenever someone speaks the music automatically dips, then rises again in the gaps. This keeps narration clear without an editor riding music faders by hand. A mixer sets the threshold so only the voice triggers it, then tunes attack and release so the dip feels smooth rather than pumping. The same trick tames competing elements, like ducking ambience under an effect. Set well, it is invisible and keeps the most important sound on top; set badly, it breathes audibly.