What is audio gating?
Audio gating is the control of when and to what degree audio passes through a channel. It’s based on factors including the signal level strength. In some cases, it’s referred to as a noise gate.
Imagine standing in front of a fence gate and whispering, “I want to come in.” Nothing happens. Then you yell, “I WANT TO COME IN” and this time the gate opens. The higher volume level is what opened the gate.
Gating is more than controlling if a signal passes through the channel. Other properties can be controlled:
The speed of the opening and closing.
The time the gate stays open.
How soon the gate can be fully opened again.
Using these properties, a number of audio gating techniques can be used.
Why use audio gating?
There are three main reasons for gating.
1. Reduction of unwanted external sounds
The stage is full of sound, from the instruments and singers to the wash of floor monitors and slapback. An open microphone should pick up the intended source and very little else.
For example, look at a vocal microphone. The microphone should capture the singer’s voice but it should not reproduce the stage sounds into the house mix. Use of a gate would stop those lower signal level stage noises from being broadcast when the singer wasn’t singing.
2. Reduction of natural channel noise
Gating can be used for clearing up a signal. Consider an electric guitar signal which sends hum along the line. This is sound coming from the intended instrument but it’s not a useful sound. In this case, noise gating could be used to keep out the hum when the musician wasn’t playing. Knowing how to use a noise gate will definitely improve the house mix by reducing the extra buzz from electric guitars and amplifiers.
3. Use as an effect
Audio gating as an effect, now that’s where things get interesting. Imagine the sound of a snare being hit and the sound naturally decaying in volume. Now imagine the signal was cut off after the initial hit was heard. This type of effect could be used on multiple instruments but you have to ask the question, “would it benefit the sound?”
Another way to use gating as an effect is by adding punch. For example, imagine the sound of a kick drum being struck. It takes fractions of a second for the volume to build. By altering the speed in which the gate opens and the signal strength required to open it, you can give it a punchier sound by only opening the gate once the initial sound level is much higher.
Gates are controlled through setting five parameters; threshold, attack, release, range, and hold. There is one exception; some multi-function rack units such as “compressor/gates” only allow for control of the threshold and maybe one other parameter where all other parameters are fixed.
This shows the speed in which the sound is processed once it reaches the threshold, then the gate is opened, it’s held open after dropping below the threshold, and then the gate is closed during the release time.
Threshold, is, “the key to the gate.” It’s the decibel level in which the gate opens or closes. The higher the threshold, the louder the sound (stronger the signal) must be to open the gate. The lower the threshold, the more sound that will pass through when the gate is open.
The trick is finding the right spot to open the gate which accounts for player or vocal volume nuances. To put it in practical terms, musicians don’t always play or sing at the same level they did during practice. For this reason, it’s helpful to find that ideal threshold and then back it off a few decibels to account for these changes.
Attack & Release
Audio gating, which seems like an on-or-off control, creates a better sound when the speed of the opening and closing varies based on the need. The sound of a drum being struck and the sound of a singer are both sounds that have a natural decay. To close those channels the moment their signal level drops below too high of a point would be like suddenly muting their channel.
The attack time controls the speed of the gate opening. The time is in microseconds, under one second. A gate that opens too quickly on a slower signal attack can produce a click sound. The simple solution is to extend the attack time until the click sound disappears. Consider very fast attack times for percussive instruments and slower times (10 ms or more) for everything else.
The release (decay) time is key to a natural sounding audio decay. It’s measured in microseconds to seconds. Setting this time has much to do with how the gate is being used – noise elimination or effect. For noise elimination from a line, such as with an electric guitar, you want a natural decay which ends with the line noise being cut out. If using it as an effect then it would be set much faster.
Maximum range applied.
The range is the amount of decibel reduction to the signal once it’s gated. Fully engaged, think of it as an iron gate – nothing is getting through. An iron gate isn’t necessarily a good thing. The range can be set to a lesser decibel cut, such as -15dB. This means with the gate closed, some of the sounds can be heard, though to a lesser extent.
A gate set to the maximum range creates a dead sounding instrument. In the case of a snare drum and a few incorrect settings, the natural decay of the snare wouldn’t be heard. By setting a small range, the decay would come through though the degree and time it lasted could still be controlled. You might think of it as a controlled fade.
A benefit of using a smaller range is it decreases the processor’s time to open the gate. For example, a -90 dB change to the threshold takes longer than a -15 dB change because the attack time indicates the rate at which the gain changes. It would be like driving zero-to-60 MPH versus 45-to-60 MPH.
The hold time is the minimum time the gate is held open. The hold time can be controlled though often internally set to a minimum of 20-30 milliseconds to prevent chatter. Chatter is the constant opening and closing of a gate due to a high-speed fluctuating signal level.
Another type of chatter-control is called hysteresis. In this case, the threshold automatically increases for subsequent gate openings and decreases a few decibels for closing the gate. The idea is a sound of fast varying levels would not produce chattering.
Warning: Incorrect gating wreaks havoc on your mix!
There are side effects and problems associated with using the automated process of gating.
Although this was discussed within the “hold” section above, it’s possible that poor gating settings still allow chatter to occur. Put on headphones, hit the PFL button for the channel, and listen to the results of the gating.
2. Loss of desired sound.
Musicians can play or singer with volume dynamics. Softer here, louder there, this is part of song arrangement. However, a gate might close when that happens. Back off the threshold if this is happening.
3. Loss of natural sound
By gating, there is the possibility of shortening the decay rate of a sound. Imagine placing a tight gate on a piano so the notes end abruptly instead of naturally ring out. Gating doesn’t need to be used on everything. But when it is used, listen to the results in headphones to check it and alter settings appropriately.
There is intentional distortion, like that used with an electric guitar or used as an effect on an input. There is also bad distortion and one way that can occur is as a result of gating.
Imagine a gate setting on a snare where the gate is opened and then a slow release is applied to capture the full decay. However, if the snare is played again during the release portion, the gate won’t open until the end of the release time and therefore would not open until after the snare is initially struck. You’d only hear a partial snare sound after the snare was hit. Repeat this over and over and it produces a distorted sound.
CLOSED = the gate is on, very little sound will pass to the listener
OPEN = No gate is used
Imagine that you have a vocal track and you hear some hiss. You set a gate on the track so that the hiss cannot be heard. When the singer starts to sing, their sound goes over the threshold and the Gate opens, letting sound be heard. If the volume is close to the threshold, the gate may close and open rapidly creating the chattering effect.
Hysteresis will allow you to keep the Gate open longer before it re-engages or closes. If you set hysteresis to -10 dB, and the threshold was at -20dB, the gate would only close again at -30dB. Then the cycle repeats. So... gate turns on the first time when sound is below -20dB, when it opens it will stay open until it goes below -30dB. Essentially you have added another threshold.