Expansion and gating reduce unwanted low-level signals by reversing dynamic range compression

An expander works by reducing the level of signals below a threshold — the opposite of a compressor. A gate is a high-ratio expander that effectively silences signals below the threshold. In mixing, expansion and gating control spill (leakage between microphones), background noise, and room ambience. Side-chain equalization of the gate’s level detector is a powerful technique: filtering out the kick drum’s low-frequency content from the snare gate’s detector allows the gate to respond to the snare hit without being fooled into opening for kick drum spill. Parallel gating (blending a gated version with the unprocessed signal) allows precise control over how much spill is present without altering the spill’s dynamic range.

Examples

Hugh Padgham’s famous gated drum sound for Phil Collins’s ‘In the Air Tonight’ used keyed room mics to create the distinctive explosive drum ambience — a serious artistic tool, not just a cheesy 80s effect.

Assessment

Your snare close-mic is triggering its gate every time the kick drum hits. Describe how side-chain EQ on the gate’s detector solves this problem. What risk does a fast gate attack without hold time create?