A noise gate reduces or attenuates its output when the input falls below a certain level. A noise gate that shuts off the signal completely tends to have a rather disturbing effect as the sudden change in background noise is rather obvious. Thus, many gates tend to reduce the level by a finite amount rather than shutting completely. This is actually the description of an expander rather than a gate, as the overall effect is to expand the dynamic range (the loud sounds remain the same, but the soft sounds are made softer). This, you may realise, is the exact opposite of a compressor (see the Q&A on compressors).

Like compressors, there is usually control of the attack of the gate, or how many milliseconds it takes to start to reduce the output level. Also, there is control of the hold time, which forces the gate to stay open for a fixed period even if the threshold signal has recovered. This is used to prevent 'chattering' or rapid opening/closing of the gate when the input signal is hovering aroung the threshold point. The release control sets the time it takes for the level to return to its above threshold level.

Whether a unit is a gate or expander, they tend to be used across individual input channels, or occasionally across a group output,. Their job is to effectively mute temporarily unused channels in live or recorded work. In live situations, they are most often used for gating drums.

Many units also have a trigger input, which is used when a second signal is used to open the gate. The commonest use of this technique is to use a kick drum signal to open the snare drum mixer channel, so snare and kick are synchronised for more impact.