Mixing : Volume controls part 3 - Noise gates

Operating similarly to a compressor / limiter, noise gates are often packaged together in one box with them, know the differences

Like compressors / Limiters, a noise gate turns the volume down. The difference is that a compressor / limiter turns the volume down above the threshold, while a noise gate drops the volume when the volume falls bellow the threshold. However, since the volume is being turned down on a sound that is already low in volume, normally a noise gate will turn off the sound completely.

Noise gates have three main functions: to get rid of noise, to get rid of bleed, and to shorten the duration of a sound.

Noise Eradication

The first function of a noise gate is to get rid of noise, hiss or anything annoying that is low in volume. Noise gates only get rid of background noises when a sound is not playing. Noise gates don't get rid of noises when a sound is not playing. Noise gates don't get rid of noises while the main signal is present; however you normally can't hear the noise when the sound is playing.

One function of a noise gate is to get rid of amp noise when a guitar is not playing. Say you have a guitar amp set on 11 with lots of distortion. When not playing, the amp makes a huge "cushhhhhh" sound (when the guitar is playing, you don't hear the amp noise, it gets cut off. Whenever the guitar player is not playing, you now hear silence.

It is important not to chop off any of the guitar sound. All it takes is for the musician to play a soft note, and the noise gate will chop the sound right off. Noise gates can also be used to get rid of noise from tape hiss, cheap effects units, dogs, kids, and crickets. 

Bleed eradication

Another common use of a noise gate is to remove the bleed from other instruments in the room. When a mic is on an instrument, the sound of that instrument will be loudest in the microphone. Therefore, it is easy to set the threshold of a noise gate between the sound and the bleed, so that the bleed gets turned off.

The obvious advantage of isolating a sound like this is that you have more individual control over volume, equalization, panning and effects. Once a sound is isolated with a noise gate, any changes you make with a sound manipulator will only change the one sound you are working on. Gates can be especially effective on drums to isolate each drum. This is especially important on a snare if you are going to put a lot of reverb on it. Without the gate you end up with reverb on the hit as well. Another advantage of isolation is that it helps to eliminate phase cancellation (we'll discuss this more later).

But most importantly, by removing the bleed, you will then hear the sound in only one microphone. This has the effect of putting the instrument in one precise spot between the speakers, instead of being spread in stereo. For example, consider the miking of a hi-hat cymbal. Besides being picked up by the hi-hat mic, the hi-hat is also being picked up by the snare drum mic. If the hi-hat mic is panned to one side and the snare mic (with the hi-hat bleed) is panned to the center, the hi-hat appears to be spread in stereo between the speakers. It is no longer clear and distinct at a single spot in the mix. Putting a noise gate on the snare mix turns off the hi-hat when the snare is not playing. The isolated image of the hi-hat when the snare is not playing. The isolated image of the hi-hat in it's own mic, you will now appear to be the crystal clear and precisely defined wherever the hi-hat mic is placed in the mix.

Shortening the Duration

You can also use the noise gate to shorten the duration of a sound. The noise gate will cut off both the attack and release of a sound because these are commonly the softest parts of the sound. This can be quite an unusual effect.

A noise gate can also be put on reverb to chop off the release resulting in the well-known effect referred to as gated reverb.

Visually, when volume is shown as front to back and the volume is lss than the threshold setting, the sound will disappear. If the low volume sound is noise, bleed or the attack and release of a sound, it gets cut off.

You can read more about the most important things you need to know about mixing, and how to setup the compressors properly depending on the task you want to realize in Dave Gibson's book "The Art of Mixing 2nd Edition" A visual guide to recording engineer and production.