Noise Gates

The noise gate has two functions:

To assist in managing noise.
As a creative device.

In its principal role of managing noise, the noise gate can be applied to a single unmixed signal that has periods of silence, during which noise is apparent. This is important enough to state again, that the noise gate can be used thus:

On a single, unmixed signal.
The signal has periods of silence (i.e. when the instrument isn't playing), during which noise is apparent.
It cannot be used on a mix of signals (at least this is very unlikely).
There is no benefit in using it unless there are sections of silence.

The classical example of where the noise gate would be appropriate is the electric guitar, where the amplifier is likely to be noisy. When the guitarist plays, the sound of the instrument drowns out the noise so there is no problem. When the guitarist isn't playing however, the noise of the amplifier becomes apparent, and irritating. In this situation, what the noise gate does is detect when the signal level is high, when it assumes that the guitar is playing, and opens fully to allow the signal through unimpeded. When the gate detects that the signal level is low, it assumes that the guitarist is not playing and closes completely, blocking off the noise.

Another classic use of the noise gate is on a drum kit, where several might be used. The conventional method of miking up a drum kit demands a mic on each drum, a mic on the hihat and two overhead mics. The problem with this is that the mics are all so close together that each mic picks up every instrument of the kit to an extent, as well as its own instrument. This inevitably blurs the sound. To make it more focussed, all the mics except the overheads are gated so that each channel is only open when the drum is actually sounding. In this situation it is often subjectively better to set the gate so that it attenuates when closed, by perhaps 10 dB, rather than cut off completely. In live sound, this technique is often extended to virtually every microphone for the entire band.

Since the noise gate can only process one unmixed signal, the place to connect it is in the channel insert point of the mixing console. As with equalizers and compressors, gates are not used via the aux send and return loop. If the console you are working with does not have a patchbay, you will have to make up a special adapter (Y) lead if your console has the usual single stereo jack send/return insert point.

One feature of the noise gate that contrasts with other kinds of outboard units is that there are no presets, and that twiddling the knobs will not produce a satisfactory result by chance (as it sometimes might with an effects unit).

Controls

It helps to have a suitable sound source playing at this point, one with sections that should be silent, but in which noise is irritatingly apparent. We'll go through the controls, one by one. It's good to have a rough starting point, and the example above of the classic Drawmer DS201 is a good one. With another model of gate, simply set similar values for levels and timings.

The Threshold control sets the level at which the gate will decide whether to open or close. When the signal is above the threshold, the gate will be open. When it is below the threshold, the gate will be closed. With the control set at minimum, the signal will always be above the threshold, even when only noise is present. With the control set to maximum, then the signal will never rise above the threshold and the gate will be closed. Somewhere in between there will be an optimum point where the wanted signal gets through, and the unwanted noise is blocked. It is important to make sure that when the musician plays, the first note gets through, right from the initial attack. Also, when the musician stops, the decay of the last note is maintained adequately before the gate closes. There are controls other than the threshold control that influence this, but the threshold must be set as precisely as possible before continuing.
The Range control sets the degree of attenuation when the gate is closed. Noise gates are commonly set to maximum attenuation unless there is a good reason to do otherwise. On a single signal, the gating effect will be obvious, but of course it should not be so in the context of the entire mix. If the opening and closing of the gate is still noticeable, then the range control should be set to achieve the best compromise.
The Hold control sets a time period during which the gate will remain fully open, even though the signal has just dropped below the threshold. If the hold time is set to zero, when the signal crosses the threshold there will be a period of uncertainty when the gate doesn't know for sure whether it is supposed to be open or closed and it will change state rapidly a number of times, causing what is sometimes known as 'jitter' (not to be confused with digital jitter). The hold control is usually set to the minimum value that causes jitter to cease. (Some gates alternatively have a Hysteresis control. This sets a separate threshold for signals that are rising in level than for signals that are falling in level. Having a variable hysteresis control is actually better than having a hold control.
The Attack and Decay (also known as Release) controls are used to shape the envelope of the sound as it comes in and goes away with the object of changing smoothly from silence to signal, then signal back to silence, without cutting off any of the wanted sound, nor letting any noise get through.
Stereo Link is a function only used when applying a twin channel gate to a stereo signal. An example would be an instrument played through a noisy old analog chorus unit (because you just like the sound!). When this is on, both channels are forced to open and close at the same time. If this is left off for a stereo signal, no matter how carefully you set the threshold controls the channels will change state at slightly different times. You won't believe how dreadful it sounds until you try it.

Filters

Basic gating is easy enough when the sound source is well differentiated between signal and noise. But there are times when the noise is so high in level that setting the threshold to a point that fits neatly between the signal and the noise is impossible. A good example would be the drum kit, particularly the snare drum and hihat. The snare and hihat are so close together that the snare mic will pick up as much level from the hihat as it does from its own instrument. It will be impossible to set the threshold for correct gating.

Expander

An expander is a more sophisticated form of gate. Whereas a gate is either on or off, an expander increases the dynamic range when the signal is below the threshold.

When the signal is above the threshold level, there is no change.
When the signal is below the threshold, the quieter the signal gets, the expander will make it even more so.

The expander has a ratio control like a compressor. 1:1 means no expansion; 20:1 means that the expander is working almost like a gate.

The expander isn't as useful a device as one might think. The problem is noise modulation - as the signal level goes up and down, below the threshold, the noise level will also go up and down. This is an unpleasant effect known as 'breathing' or 'pumping'. Breathing and pumping can be evident in the compressor, but since it is high level signals that are being processed, they are better able to mask the noise.

The solution to this problem is found in the filter controls. Just like a compressor, a noise gate has a 'side chain' signal that is used to control the behavior of the processed signal. In a gate, this is commonly called the trigger, or key signal. In normal use with the filters set to their end stop positions - so that they have no effect - the key signal is identical to the input signal. Indeed it is just tapped off from it. To stop the hihat from opening the gate on the snare mic, simply filter out the high frequencies from the key. The snare is sufficiently rich in low frequencies that it will still open the gate, but the lesser LF content of the hihat is not enough to do the same. If there was a mic on the hihat suffering from the same problem, then the low and mid frequencies could be filtered out of the key, and the rich high frequency energy of the hihat would still open the gate.

One important point to note is that the filters have no effect on the frequency balance of the output signal. They only affect the key, which once it has done its job goes nowhere. There is a function, here called 'key listen' that switches the key signal to the output. This can be used for setting up the gate, then switched back to 'gate' for normal operation.

External Key

Suppose you have done all of the above, but the gate still isn't opening and closing reliably on the snare drum, to continue this example. What next? The answer is to use an external key signal to open the gate. Here's the scenario:

Feed the signal from the regular mic through the gate as normal.
Tape a contact mic to the shell of the snare drum.
Feed it via a preamp to the external key input of the gate.
Switch the gate to external key ('EXT').
Set the threshold etc. for reliable triggering.

Now, the gate is triggered by a signal that picks up virtually no external sound - only the sound of the drum. What's more, the signal from the contact mic comes maybe half a millisecond earlier than the signal from the regular mic, which allows the gate to open a little in advance so that the transient of the drum is accurately captured. It has to be said that this is probably an over-elaborate technique for most circumstances. But it works very reliably and is worth knowing about for the occasional difficult situation.

Compressor and Gate

The gate is often thought of as a companion to a compressor. Compression always has the effect of increasing the noise level. This is because the peaks of the signal are brought down in level, bringing them closer to the noise floor. Then make-up gain is applied to bring the overall signal level back up again, raising the noise floor at the same time. Even if there were such a thing as a perfect compressor, this would still happen.

The obvious answer is to use a noise gate to remove the noise, providing the signal meets the criteria for gating as outlined above. There are two schools of thought among engineers as to how the two should be connected:

Some think that it is better to put the gate after the compressor, since the compressor will generate some noise itself, in addition to the noise emphasized by the compression process.
Other engineers will say that since the signal has a greater dynamic range before compression, it is easier to set up the gate for reliable triggering if it comes before the compressor.

Gate Effects

The noise gate is also capable of a variety of envelope shaping effects, and is a highly creative tool as well as a problem solver. The classic trick is to put a more-or-less continuous signal through the gate, such as heavily distorted chords from an electric guitar, and then use the external key to chop it up into a rhythm. Like this:

Connect the guitar, through a distortion unit, to the gate in the normal way.
Connect a drum machine, or other rhythmic source synchronized to the track, to the external key input.
Switch the gate to external key.
Set the threshold so that the gate triggers on a signal from the external key.
Adjust the attack, hold and release controls to achieve the desired envelope.

Powerful though MIDI sequencing may be, you can't get the same sound in any other way. This is well worth trying.

Another useful gate effect is to compress the sound of an individual drum, then gate it. This works particularly well on drum samples which have a little bit of reverb on them. The compressor can shape the envelope of the sound by emphasizing the attack (by setting a slow attack time on the compressor, allowing the initial transient to get through unaltered), or by allowing the reverb to increase in level as the drum dies away. The noise gate can then further process the envelope using the attack, hold and release controls.

Gated Reverb

First popularized in the 1980s, gated reverb has become something of a cliche. But as a technique, it is still well worth knowing about. It goes like this:

Connect the snare drum mic (say) to the mixing console in the normal way.
Through an auxiliary send, send some of the signal to a reverb unit.
Bring the output of the reverb back to a channel with an insert point. (If your console's auxiliary returns have insert points, then they will work fine).
Connect the noise gate to the insert point send and return of the reverb channel.
Connect the insert send of the snare channel to the gate's external key input. (You could alternatively derive this signal from another auxiliary send). Set the gate to external key (EXT).
Set the hold and release controls so that the reverb extends beyond the end of the dry snare drum sound, but then dies away suddenly (long hold/short release).

You now have Phil Collins-style gated reverb! You could use a distant mic as the reverb source, as an alternative to the reverb unit.

Short-Answer Questions

Please answer these questions in your own words, in 100 words or fewer per question. Copy each question before your answer. Do not always attempt the shortest answer possible, but give an answer that would explain the topic to an intelligent reader, unless of course the question obviously asks for a one- or two-word answer.

Is a noise gate usually effective on a mixed stereo signal?
Is a noise gate effective on a signal where the instrument plays all the time?
Is a noise gate effective on a single noisy signal where there gaps in the playing?
Why is it often considered beneficial to gate some or all of the mics on a drum kit (except the overheads)?
How many gates are often used in live sound: none, a few or many?
How is the noise gate connected to the mixing console?
What is the function of the Threshold control?
What is the function of the Range control?
Comment on the Attack, Hold and Release (Decay) controls.
What would happen if a stereo signal was gated, but the stereo link function was not selected?
Why are side-chain filters beneficial?
Comment on the use of an external key to improve the reliability of gating.
What is an expander?
Why are compressors and gates often used together?
Comment on envelope shaping using a noise gate.
Comment on gated reverb.

Notes for Presentation

Preface each answer with the question as given.

The work must be word processed and spelling checked. Technical terms and proper names must have correct spelling. Use standard formal English. Make every attempt to use grammar that would be respected by a well-educated person.