A noise gate is a hardware device or plugin that can be set to automatically mute the signal during parts of the audio track where the instrument is not being played, and un-mute again when the instrument is being played. Some noise gates are able to close partially thereby reducing the level rather than muting the signal altogether.
Picture a farmer opening the gate to let the cows through and closing it again to keep the sheep out. The main difference of course is that unlike the farmer you can set the gate to open and close automatically as needed and you have further control over how long it stays open for and the speed at which it opens and shuts.
You may have recorded a great guitar part but during the parts of the song where you are not playing the guitar amp hums. A noise gate is an effective solution as it will simply shut during these parts and open again when trigged by the guitar playing again.
A noise gate can also be used as an effect to deliberately open and shut on an instrument to create stuttering effects and more.
Noise gates usually have five main parameters: threshold, ratio (reduction), attack, hold and release.
- Threshold – this sets the level at which the gate opens to let the sound through.
- Ratio – the balance between the original sound and the gated sound. For example, instead of using the gate to totally mute the background ambience on a vocal track, you could allow some of the ambience to still be heard if so desired.
- Attack – sets the time it takes for the gate go from a closed state to an open state.
- Hold – allows the gate to be held in an open state after the signal level has fallen below the threshold.
- Release- use this to set how long it takes for the gate to go from fully open to fully closed. A fast release quickly cuts off the sound whereas slower release is more like a fade out. Beware not to set the release to fast as it can induce a clicking sound.
A more detailed look at noise gates can be found here: http://en.wikipedia.org/wiki/Noise_gate