Gates are known to be the second most common dynamic range processor in mixing after compressors.
Gates are also called noise-gates, a name that implies their traditional usage as noise eliminators. Noise, however, is not the only thing gates are employed to reduce. The rumble produced by the floor tom when the rest of the drum kit is played is often gated. The hi-hats spill in a snare track and the headphone spill on a vocal track are just two examples. In addition to clearing up spills they are also used for more creative task such as tightening drums, adding punch or applying dynamic movement. Gates affect signals below threshold; these are either attenuated or muted. Signals above the threshold pass through unaffected, unless some attack is applied. A gate only cares whether the signal is above or below the threshold. It is said, that the gate is closed when the signal is below the threshold and open when the signal is above. The threshold settings on a gate might seem straightforward, the threshold is set below the wanted signals and above the unwanted signals.
The level detection on most gate’s is based on peak-sensing, which allows quick gate opening once the signal exceeds the threshold. Level fluctuations are more profound with peak sensing than with RMS. While fluctuating in level, signals may cross the threshold in both directions many times over a short period of time. Which causes rapid opening and closing of the gate, which produces a type of distortion called chattering; a way of reason to overcome chattering is by having two thresholds. One above which the gate opens, another below which the gate closes. Having two threshold controls would be cumbersome since adjustments to one will call for adjustments to the other. Now, instead of providing two controls, gates offer a single threshold and a control called hysteresis. The threshold is the opening threshold, and the hysteresis determines how many dB below it the closing threshold is set. When hysteresis control is given, these figures provide a good starting point.
Attack controls how quickly the gate opens, and release controls how quickly the gate closes. The response times on a gate are normally set in milliseconds. Attack times usually span between 0.010 ms (10 µs) and 100 ms. Release times are often within the 5-300 ms range. Like with compressors, both the attack and release times determine the rate of gain change. For instance, a gate might define that response times are referenced to 10 dB of gain change. The practical outcome of this is that the range affects the overall time it takes a gate to open and close where appropriate, this can be achieve faster attack and release times by dialing a smaller range. A longer attack on a gate means that less of the natural attack is retained, a longer release on a gate means that more of the natural decay is retained. Very short attack and release times can cause audible clicks due to abrupt level changes. A steep level rise produced by fast attack generates high frequencies, the shorter the attack the higher the frequency. The same applies to release, depending on the gated signal, fast response times might also alter the low and mid frequencies, which essentially means that a gate can affect any part of the frequency spectrum a side-effect have to be observed. The attack setting has an extra weight when gating transients, especially those with low-frequency content like kicks. With most of the impact gearing up at the beginning of the cycle, a gate is most likely to reshape the important part of the waveform, either with a short attack that produces sharp level changes or with a longer attack that reshapes the waveform and withholds the kick impact. One way or another, kick’s tonality is very likely to change, with the lows, mids, and highs, all likely to suffer. Very short settings can also cause low-frequency distortion due to the gate operation within the cycles of a low-frequency waveform. Longer attack, release or hold can rectify this issue. In the case of percussive performance, we must consider how the release and hold settings, which affect the length of the sound, lock to the rhythmical feel of the track. In addition, the gate fully close before the next hit arrives, or successive hits will be gated differently. An issue with gates is that short attacks are often wanted, so more of the signal above the threshold passes through, and short release, so the signal below the threshold is attenuated quickly.
Compressors, on the other hand, often work on 20 dB or so, and moreover, gain changes are not as steep due to the gradual development of the input signal. A gate has no such softening mechanism; it’s either open or closed, and there is often quite some gain involved in toggling between the two. Short attack and release settings are not always appropriate though. Longer times are often used when the gated instrument has long natural attack and release. Longer attack times might let us lower the threshold by a small amount. The reason for this is that false triggers will not be long enough to become audible.
[All information in this article was sited and noted from Mixing Audio: concepts, practices, and tools. written by Roey Izhaki]
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