Backstage Technical Services | Sound Guide

Sound Guide - What a noise gate does

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Introduction

If you actually listen to an instrument like a drum you'll find that its significantly different to what you hear on your CD at home. You might feel that the kick drum hasn't enough 'kick' or that the snare drum hasn't enough 'snap' and the reason is that during mixing, the signal will have been fed through a noise gate before being used. Essentially this piece of equipment only lets through loud bits of sound and cuts off quiet parts.

Imagine that you have a door with a spring on it. Hit the door softly and the door won't open but hit it hard and you'll be able to open it, letting you through. A very similar thing happens inside a gate where a signal above a certain strength (the threshold) will be able to pass through.

Effects of a noise gate
The effects of threshold, attack and release on the output signal of a gate.

There are three main controls that all gates possess:

Threshold: The point at which a gate will let the sound pass. Any signal under this strength will not be let through and so if the threshold is set too high, it is possible for there to be complete silence.
Attack: The time between the signal reaching the threshold srength and the gate actually opening to let sound through. Don't confuse this with delay, the signal isn't paused, the gate just waits before acting.
Release: The time between the signal falling below the threshold and the gate actually closing to prevent sound passing through. A higher release time usually produces a softer sound since you can hear the drum vibrating freely.

Traffic light display

Many gates use a series of three LEDs to indicate what it is currently doing to the signal:

LED	Status of the gate
Red	Input signal is below the threshold point. The gate is reducing the output level and so nothing will be coming out from the speakers
Yellow	Input signal falling below the threshold level or rising above it. The gate is using its hold or release circuits.
Green	Input signal is above the threshold level. There is now sound coming from the speakers.

Uses of a gate

There are two main uses of a noise gate:

Change the sound of an instrument such as a drum to give it more impact. By only letting the loudest part of a drum strike through into the mix, the drum will sound quicker and stronger.
Reduce the noise. People talking, interference and other quiet and unwanted sounds can be removed from the mix by using a gate. By setting the threshold so that the gate only lets intended sounds through, there should be silence at all other times. Remember that it is best to remove noise at the source rather than use a gate and that a gate can't remove interference down-stream of it.

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