Gating and expansion work similarly to compression. While compressors automatically turn the volume down when the input audio rises above the threshold, gates and expanders automatically turn the volume down when the input audio falls below the threshold.
The simplest example of this is a basic noise gate – it mutes the audio when the instrument isn’t playing. This works when the threshold is set just a little higher than the background noise. When the instrument isn’t playing, the background noise is below the threshold so the gate ‘closes’ – it mutes the audio (turns it all the way down). When the instrument is playing, however, the audio level rises above the threshold and the gate ‘opens’ – letting the audio through.
Gates often have fewer controls than compressors. Some gates have many controls, but almost all have the following:
- Threshold – This sets the level below which the audio is muted. When the input audio is quieter than this level, the sound will be muted. When the input audio is louder than this level, the sound will pass through.
- Attack time – This sets the time for the gate to change from closed to open. Usually this should be as fast as possible, but sometimes this can result in a sharp click or unnatural sound when the gate opens. Increasing the attack time results in a softer, smoother sound.
- Release time – This sets the time for the gate to change from open to closed. Setting this correctly is important for instruments that have a natural decay (such as acoustic guitars or drums). Often the decay can still be heard ‘under’ the background noise, and closing the gate too fast can unnaturally cut off the end of the instrument’s decay. In these cases, the background noise is preventing the threshold from being any lower. Increasing the release time will give the instrument’s decay more time to fully die out before the gate is closed.
Expanders are gentler versions of gates. Instead of muting the audio, they simply reduce the volume. This often sounds more natural and gentle than a gate because the background noise doesn’t come in and out as dramatically. Expanders usually have an extra control that gates don’t – ratio. This sets the degree by which the volume is reduced when the input audio falls below the threshold. Expanders can be more useful for mixes that need to retain a natural ambience – especially acoustic and folk music.
Like compression, don’t assume that since you’ve got a gate or expander that you must use it! Unlike compression, I’d recommend not even trying it for most tracks. Only try it if you have a track that has noticeable background noise that is distracting in between the instrument playing. This can be more noticeable if the track is being compressed, because a compressor can often turn up the background noise in between the instrument playing. If possible, apply the gate before the compressor.
The necessity for gates and expanders is greatly reduced these days because most recording and mixing equipment produces very little background noise and it’s usually easy to record in a quiet enough location.
One exception to this is high-gain guitar amps. The high gain and distortion greatly increases the level of the background noise – sometimes this background noise is almost as loud as the guitar itself. In these cases a gate or expander can be very useful in cleaning up the audio track.