Of all dynamics processors, gates are the easiest to understand and use. Commonly referred to as "noise gates" they are in their simplest form, merely a switch. A switch which gets rid of the noise you hear during quieter parts of a track by muting the sound. What makes them special is the ways they can be used to both clean up and radically alter the sound processed through them.

How Gates Work
Noise is usually masked (hidden) by the signal, as it is normally at a much lower level in comparison - particularly with most modern equipment, which is relatively quiet. However, as the signal fades away in quieter sections of the music, the noise can become noticable and even quite annoying. A gate works by switching off the signal when it falls below a certain level (the threshold), muting both the signal and the noise. Various controls set how fast the gate starts working (Attack), how fast it stops working (Decay) and by how much it reduces the level (attenuation). And then there is the magical 'trigger' input...

A waveform of a distorted electric guitar with hum and hiss showing in the quieter sections	The same waveform after gating to remove the noise

The Controls

Threshold - As you lower the threshold level you can hear more of the signal, and also the noise in the gaps where all should be quiet. As you raise the threshold, you'll find that more and more of the signal is gated out, until you don't hear anything - including the bits you want. The ideal setting is where you hear all the signal you want, and everything you want to get rid of is gated out. Sometimes you will need pretty high threshold setting (when you are micing one part of a drum kit and are trying to gate out the other parts of the kit), while in other applications you will want it set relatively low (when you are micing an acoustic instrument and just need to get rid of soft sounds like player movement and breathing, etc.).

Attenuation - This control sets the level that the signal is reduced by when the gate is closed, -60dB or more is as near as damn to completely off. With lower attenuation settings you'll hear some of the noise coming through between the wanted sections of the signal - it might seem pretty pointless to let a small amount of the noise you are trying to get rid of through, but it can be useful when heavier attenuation is too obvious.

Attack and Decay - With these you "shape" the envelope of the sound as it starts and finishes, the aim is (usually) to change from silence to signal and back again as naturally as possible. If these settings are wrong, you'll either hear a little bit of noise as the sound starts and finishes, or the sound will be noticeably clipped short.

Trigger Input - The trigger input allows you to use a second audio signal to open and close the gate - regardless of what signal is being gated. More on this just now...

Gates as an Effect
There are a few ways to get some obvious effects from gates. Some highly desirable and tasty stuff indeed.

Slow Attack Settings - with a slow attack, a gate will release slowly, changing the attack of the sound. It's like using a volume control to 'swell' into a note. Boss has an effect called "Slow Gear" on some of its multieffects which basically does this.

Gated Reverbs - using the original signal as a trigger input, the sound with added reverb is gated. This makes for a very controlled reverb sound - allowing huge reverbs to be used which are strictly controlled, without adding to much sustain to the sound. Much beloved effect on snare drums.

'Synching' Instruments - The common example is locking the bass guitar with the kick drum. Using the bass drum as a trigger input, the bass guitar is gated so that it only sounds exactly when the kick drum does. An ideal cleanup for a sloppy bassist. Another example is using a rhythmic instrument to supply a rhythm to a sustained sound like distorted guitar or, quite commonly to a synth pad. The sustained sound is given a pulse by using the rhythmic instrument as the trigger input.

Reverse Gates - On some gates, it is possible to reverse the effect of the trigger input, so that the gated signal is only passed when there is no signal at the trigger input. This makes sure that the two sounds are always in counterpoint to each other.

Ducking - Think of when a DJ speaks over music, and the level of the music drops to make space for the voice. This is ducking. This is done by using a reverse gate where the voice is the trigger input and the music is the gated signal. The attenuation is kept lower than usual, as you still want the gated signal to come through - just at a reduced level. Most commonly used by DJ's and in any voice-over application, but it can be used in any situation where you need one sound to make space for another (get creative folks!).