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A Guide to Noise Gates in PA Systems
A noise gate is to an expander much as a limiter is to a compressor. Essentially it is an expander that mutes the signal when it falls below the threshold, rather than simply reducing the gain. A downward expander with a ratio higher than 8:1 is effectively acting as a noise gate.
Usually a noise gate is a 19″ 1U box with knobs or buttons (and more often than not a couple of LED meters) on the front. Some compressors have a noise gate function included in the channel facilities.
A noise gate mutes the signal when it falls below a threshold (the threshold is generally user-determined).
It senses the input level, and closes the channel when the input level falls below the threshold. It can have controls for:
The level below which the signal is muted.
How quickly the gate opens once the signal reaches the threshold. On most sounds (particularly percussive sounds), the attack should be very fast to avoid cutting off the beginning of the sound.
Certain important parts of the sound (decaying resonance & natural reverb tails) may fall below the threshold. If the gate is closed too soon, the cut off may be audible (& unnatural).
If a noise gate is used on a small rack-mounted tom, low-frequency sound from the kick drum may exceed any usable threshold. A filter enables the user to tune out (or sometimes tune in) a frequency range, so that the gate only opens when sounds in a particular frequency range exceed the threshold.
Most multi-channel noise gates allow coupling of each pair of channels for stereo use. If gating is applied to left and right channels independently, drop-out of one or other side can result whenever the signal on one side is above and on the other side below the threshold.
If all else fails, read the manual!
Noise gates are usually connected on channel inserts.
Expanders and noise gates can have a disastrous effect if set up badly, and are at best problematic in live performances: spill from other instruments or from monitors can make it impossible to set effective thresholds. With care they can deal with very situation-specific problems (for example, buzzing from a bass guitar can be silenced between songs by careful use of a noise gate with a filter). However, they are most commonly used on drums, either to reduce spill between drums, or to reduce the ringing of undamped toms. In this application, they are usually used on channel inserts.
To use a noise gate on a drum:
Although the cut-off will still be audible, this will be much less noticeable when the whole kit is being used and the whole band is playing. A little reverb on the drum will help make the cut-off less noticeable during solos.
Other than on drums, leave well alone.
If the drums are well tuned and damped (or if ringing toms are an intentional part of the drummer's sound), you probably don't need one. Otherwise, one channel for the kick drum and one for each tom can be useful. Most compressors include a basic noise gate on each channel, which is useful if you need to gate and compress the same signal (kick drum is sometimes a good candidate for this).
One that has at least all the controls described above. Four channels is usually enough (which might mean you need more than one unit).