Gating is a technique used in audio engineering and mixing to remove unwanted sound. A gate is an electronic device or digital plug in that controls and filters sound. It works by monitoring the decibel level of an audio channel. When the decibel level reaches a certain amount, the gate opens, then closes after it drops down past the set threshhold. Something similar is also used by communications radios. The squelch control acts as a type of gate.
Gates work well for filtering sound. For instance, in a studio, the snare, bass drum, and cymbals are all recorded to seperate tracks. One would definitely gate the bass drum track/channel. This way you could eliminate the snare and cymbal background noise. This isolation allows one to use other tools like reverb to change to bass drum sound into exactly what is desired. Also, gates are useful for controling tone and vocals. A gate can eliminate breathing, held 's' sounds and clicks. RadioHead's Exit Music (for a film) is a wonderful example of vocal gating.
The actual functions and control of a gate can be likened to a real gate, hinged so that it opens both ways, with springs on both sides to keep it closed. Imagine the sound to be wind blowing the gate back and forth. Controling how the gate swings open and closed is called envelope control. Gates can be adjusted in many ways, depending largely on how much they cost and how old they are.
Threshhold: The decibel/amplitude level the track has to be at to open the gate.
Frequency: The sound must fall in between a certain frequency range to open the gate. This filters unwanted sound.
Envelope: This set of controls determine how long, how wide, and how fat the gate opens and closes. The gate can even open and close at an exponential rate.
Milliseconds matter. Adjusting the envelope controls by mere milliseconds can give the track a smooth or very clipped edge to it. This is the black art of good gating, to give the desired sound while filtering the un-needed stuff.
thanks to Dr. Barsotti, audio professor at SF State for allowing me to audit his class and learn.