Expander (audio)
The compressor's mirror image: it turns down what is already quiet, so the gap between soft and loud grows instead of shrinking.
A downward expander at 1:2 takes a signal sitting 10 dB below threshold and drops it to 20 dB below. Hiss, room rumble and bleed from other mics sink away while the instrument itself passes untouched. A noise gate is the same idea taken to the extreme — infinite ratio, everything below threshold switched off.
The advantage over a gate is gentleness: the tail of a snare or a cymbal is allowed to decay rather than being chopped off mid-air. Upward expansion, which lifts the peaks, also exists, but mostly gets used to try to restore dynamics someone else already squashed — a rescue mission that rarely fully succeeds.