noise gate

A useful noise gate (ducker) subpatch for rejecting unwanted sounds

A "ducker" is a system that turns a signal down to 0 when it's below a given threshold. This is also frequently called a "noise gate" in audio engineering, because it acts as a doorway that closes out unwanted low-level ambient noise and lets through only the louder, more desired signal. It's useful for suppressing unwanted low-level audio, such as in a cell phone transmission when the user is not talking, or, more to the point for musical purposes, as in a microphone signal when the musician is not playing.