The noise gate has two functions:
- To assist in managing noise.
- As a creative device.
In its principal role of managing noise, the noise gate can be applied to a
single unmixed signal that has periods of silence, during which noise is apparent.
This is important enough to state again, that the noise gate can be used thus:
- On a single, unmixed signal.
- The signal has periods of silence (i.e. when the instrument isn't playing),
during which noise is apparent.
- It cannot be used on a mix of signals (at least this is very unlikely).
- There is no benefit in using it unless there are sections of silence.
The classical example of where the noise gate would be appropriate is the electric
guitar, where the amplifier is likely to be noisy.
When the guitarist plays, the sound of the instrument drowns out the noise
so there is no problem.
When the guitarist isn't playing however, the noise of the amplifier becomes
apparent, and irritating. In this situation, what the noise gate does is detect
when the signal level is high, when it assumes that the guitar is playing, and
opens fully to allow the signal through unimpeded.
When the gate detects that the signal level is low, it assumes that the guitarist
is not playing and closes completely, blocking off the noise.
Another classic use of the noise gate is on a drum kit, where several might
be used. The conventional method of miking up a drum kit demands a mic on each
drum, a mic on the hihat and two overhead mics.
The problem with this is that the mics are all so close together that each
mic picks up every instrument of the kit to an extent, as well as its own instrument.
This inevitably blurs the sound.
To make it more focussed, all the mics except the overheads are gated so that
each channel is only open when the drum is actually sounding. In this situation
it is often subjectively better to set the gate so that it attenuates when closed,
by perhaps 10 dB, rather than cut off completely.
In live sound, this technique is often extended to virtually every microphone
for the entire band.