The art of noise
The secret life of the noise gate
Open your gate and find the key to a better sound
In the light of digital recording, MIDI and multitrackers, Bob Dormon re-assesses the role of that old professional recording device, the noise gate - and finds that good gatekeeping is as relevant as it ever was...
The primary function of a noise gate is to eliminate noise when there is no sound to be heard. However, as the tutorial on this month's re:mix demonstrates, the fun doesn't stop there. Noise gates can provide a variety of creative functions.
Before we begin to examine these alternatives, you might be tempted to wonder whether these ubiquitous studio tools are still necessary nowadays. Digital recording, vastly improved analog noise reduction systems and sophisticated automated muting (now appearing on portastudios!) might be expected to have consigned them to the scrapheap. A quick browse through the pages of this month's issue of the mix confirms that this is not so. Manufacturers like Aphex are still confident enough to produce gates such as the 105, reviewed on page 36. So why do we need them in the face of all this complementary technology? Noise isn't the problem it used to be... or is it?
Simply turning a track on or off - which is effectively what happens with any kind of muting system - works well enough, but do you really want to do it for every single line performed by a soloist or vocalist? You'll get there in the end no doubt, but the precision and overall simplicity of getting a noise gate to do the dirty work for you will save a lot of time. And in a commercial studio situation, time is your master.
Many decades ago, the fidelity of recording equipment - and the domestic gear that was used to reproduce vinyl or magnetic tape recordings - left a lot to be desired. As more of these 'classic' recordings are made available on CD, the shortcomings of the recording studios of the era become apparent. At the time, these recordings sounded fine as they matched the capabilities of the technology reproducing them. Now that digital recording provides a dynamic range (ie. the difference between the loudest and the quietest sound audible) that exceeds the limits of these old recordings, the discerning listener can detect the noise that the engineers of the time fought to restrain?
The dynamic range of digital recorders also shows up the imperfections that exist on recordings made today. A simple DAT machine will soon let you know how quiet your desk or tape machine is, and so it is important to control every aspect of the recording process to reduce the engineer's arch-enemy: Noise. Its elimination calls for a judicious combination of muting, gating, plain old-fashioned quality instruments and outboard gear.
The controls that exist on noise gates are pretty much universal with one or two exceptions. Setting up a noise gate is really just a decision of where you draw the line between a usable sound/signal and the inherent noise of the material you are gating. A noise gate is your audio bouncer. You set the dress code and if something is not up to standard, then don't let it in! This is done by adjusting the threshold control, ie. changing the rules to suit your party. The threshold simply sets the boundary that the incoming signals have to cross. If the signal is not loud enough to breach this threshold setting, it won't get through, and if everything's set correctly, you won't hear it either. If the sound is loud enough, the gate will open and will only close again when the signal level drops below that set by the threshold.
On more expensive noise gates, you can also define what frequencies will trigger the gating action. The threshold setting may not be enough to isolate, say, a bass drum when there's a snare beating away in the background. By using a combination of high and low pass filters, you can alter the sensitivity of a noise gate so that it will only respond to a particular range of frequencies.
The Attack, Hold and Release (or Decay) functions on a noise gate are not too dissimilar to those encountered on most synthesizers. The difference is that these functions can be applied to any incoming signal. You are not restricted by the sounds of your synth or memory limits of your sampler.
The attack will control how fast the gate responds to an incoming signal that has exceeded the level set by the threshold. Sometimes, if the attack is set too fast, an annoying click is audible. Making the attack time a little slower usually overcomes this problem without compromising the attack of the gated signal. You can deliberately set the attack to a slow time, so that sounds appear to fade in. The piano example on track 8 of this month's re:mix highlights this effect.
"Noise gates can provide a variety of creative functions"
Once the gate has opened, the length of time it stays open can be adjusted using the Hold and Release functions. The release control is really the opposite of the attack. It sets how quickly the gate closes once the sound level has dropped below the threshold. On percussive sounds you might want to set this to a fairly rapid response. While on more expressive instruments such as acoustic guitar, or material that has reverb or echoes on it, a slower release is required.
You might think that careful use of the release control would make the hold setting redundant. But this is not so in most cases. If, for example, you were gating drums or percussion, you might find that simply using the attack and release controls would prevent you from hearing the complete sound. The rattle of a snare might be truncated or faded out by the release in such a way that some of the presence of the original (ungated) sound is lost. Using the hold control allows you to keep the gate open for a specific amount of time before it begins to close. You can then hear the whole sound, and by adjusting the hold and release times you can set a 'gating envelope' that is determined by time as well as signal level. This is also very useful when you are using external sounds to trigger the gate's action. More on that in a moment.
Typically, the Range control is rarely altered for most conventional gating needs. This feature is used to determined how much of the original signal will still be heard when the gate has closed. In general the range is set to maximum attenuation. Effectively, this means nothing will be heard. However, you can vary this so that some of the original signal can be heard, more quietly. This can be useful when the background noise is so intrusive that gating it out entirely sounds too weird to be usable. Trying to gate out noise completely in these situations can draw the listener's attention to the problem. So leaving it in, but at a reduced level, is a more aesthetic solution. Altering the range control allows you to decide what is acceptable.
Some of you will be aware that noise gates are closely related to 'expanders'. Basically, an expander will make the loud bits louder and the quiet bits quieter. You can emulate this effect by adjusting the gate's range and threshold, so that loud signals will be more pronounced and the quieter material reduced in level, by an amount predetermined by the range control.
So far, we've examined how to adjust a noise gate to act on just one signal. You can, however, use an external signal to dictate the opening and closing of the noise gate by employing the 'external key trigger'; a facility that exists on virtually all noise gates. What happens here, is that the noise gate now listens to the incoming signal that appears on its key trigger input and then applies the dynamic gating action invoked by this signal onto another that is fed, as usual, into its gate input. Put simply, you can open and close the gate with one signal, while hearing it work on another. The most common use for this set-up is to have a hi-hat keying the gate on a vocal or synth. The result is a punctuated signal that beats in time to the hi-hat. Check out the CD, and you'll recognise this old trick.
Ducking is the opposite of gating. When a signal exceeds the threshold, the noise gate (or should I say noise duck?) actually closes. When there is no loud signal, everything below the threshold is let through. What's the use of that, you may ask? How about being able to remove individual sounds, rather than isolating them. You might have a beat loop which is perfect apart from the snare, and putting another one on top only makes it sound too loud or not quite right. Removing the offending snare would appear to be the answer.
To do this, it might be easier to adjust the gate as a gate (not a duck) first, so that you can focus on the offending item. Using the frequency controls (if available) certainly helps. Then, flick the gate over to ducking, and hey presto - no more snare... er, just a nasty little blip. Well, I never said it was perfect, but there are ways around this. With any luck, your replacement snare will mask this short blip. Without going to a lot of trouble, there's not much you can do to avoid this with only a simple noise gate. After all, they can't see into the future... or can they?
When is a gate not a gate? When it's an autopanner. The final example on the CD shows how you can use a pair of gates to create this effect. It's a lot easier if the gates can be linked as a stereo pair. Thus the action of the one gate is identical to that of the other, but with a difference. The noise gate must be set so that one side is ducking and the other side gating. You create the autopan effect by having one side open while the other closes. An equal and opposite action. When a signal is present, one gate will open and the ducking side will close. Once the signal drops below the Threshold, then the ducking side opens and the gating side closes. It's like doing a see-saw action with a left and right fader.
"Manufacturers like Aphex are still confident enough to produce gates such as the 105"
A point to remember is that the output of the pair of gates must be panned left and right, and if it's a mono signal going in, then you'll have to parallel it (ie. have two of the same signal) so that the left and right gates can be fed independently. What is tricky, is getting the gates to respond at the speed that you want. Working on the Attack and Release times holds the key (if you'll pardon the pun). A slow attack means that the ducking side will take a while to close while the gating side gradually opens, and is a good starting point. Altering the threshold will also determine when the panning action will go the other way. Don't forget that as the signal dies away then the gating side will close, and the sound image will shift over to the ducking side again. This does mean that the autopan effect is tied to the dynamics of the incoming sound. By using the external key trigger inputs, you can have more control over the autopanning operation by simply sending in a signal at regular intervals to start things off.
If noise gates are thin on the ground where you are working, plan their use in advance. For instance, you may want to gate a vocalist to reduce headphone spill, various gasps and foot noises. You may also want to gate the snare and bass drum of a kit, and the lead guitarist's abominably noisy stack. If you've only got two gates, then you'll have to prioritize.
It's always best to gate vocals on the mix. This way you can be sure of getting it right without losing any of the original performance. The last thing you want to do is ruin the best take by being a goosestepping, noisegating fascist. If you must do that, use the drums! Their dynamics are generally loud enough for the gate to cope without too much of a risk to the performance. And if you've got filtering options, then you can be specific about what goes down.
As for the noise-polluting guitarist, you'd be better off gating him at mixdown, if there aren't enough gates to go round while recording. Guitarists frequently prefer to control their sound from their own roster of effects. That being the case, you'll have to bear in mind the various repeats that may be played as part of the sound/playing technique. However, if there is a gate at hand it's worth a try, especially if you can provide the effects for the player.
Generally speaking, the more black and white things are about sound or no sound, the more able you are to gate with comfort. If the dynamics vary a great deal, it's best to grab it while you can and worry about the gating afterwards. Just remember to earmark a gate for this task during the mix. So gate with it, dude!
On The Re:Mix CD:
36 Noise-gating tutorial
This disk has been archived in full and disk images and further downloads are available at Archive.org - Re:Mix #2.
Feature by Bob Dormon
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