Expanders and Gates

How downward expansion and gating reduce level below a threshold, and why hold and hysteresis exist.

Related

CompressorSidechainDrum Kit

The question

A gate is not a mute switch. It and the expander do one job, lowering gain below a threshold, and how far and how fast decide whether it cleans noise or ruins tails.

How it works

Downward expansion keeps reducing level below the threshold by a ratio; at 2 to 1 every 1 dB lower becomes 2 dB lower. A gate pushes the ratio to its limit and becomes a two-state switch. Hold keeps it open through short gaps, and hysteresis sets different open and close thresholds to stop chatter. A high-pass in the sidechain keeps bass from reopening it.

Settings and what you hear

A range of minus 40 dB is already near silence. Put the threshold just above the noise floor, with attack 1 to 5 ms, release 100 to 500 ms and hold 50 to 100 ms. Too fast an attack clicks; too short a release chops the decay of a drum.

What to listen for

Gate a drum room microphone and hear the cymbal tail vanish all at once, then lengthen hold and hear some of it return. With bass and drums recorded together, the gate opens and closes with the kick.

Common misconceptions

That a gate removes noise without a trace, when room tone vanishes in the gaps; that gate and compressor are opposites, though both can share one detector; that hold is optional.

Sources

• Giannoulis, Massberg and Reiss, Digital Dynamic Range Compressor Design, JAES 2012 — expander and gate topologies checked, not quoted
• Udo Zölzer, DAFX — Digital Audio Effects, dynamics chapter — background checked, not quoted

Used for fact-checking only; all prose is original.