Mix a pop song - 4 - Using Noise Gates

A "Noise Gate" - as many of you will know already - is a device that only lets sound through if it is louder than a set amount.

So in some ways it is a bit like a dodgy connection lead! - except that:

• You can precisely calibrate when it cuts in
• It doesn't crackle when it cuts out - the sound fades gently away at a predetermined rate

Gates are used to cut out unwanted background sound when an instrument isn't playing. Naturally when the instrument does play then you will hear the background sound switch in as well as the instrument. This doesn't usually matter, because the playing of the instrument normally masks the background noise. You could use a gate - for example - to cut out all that amplifier hum and hiss in-between parts of a guitar solo - it would certainly be objectionable to have it going on throughout the whole song when the guitar isn't playing.

Of course there is normally some sound quality loss when going through an analog noise-gate, but this is not usually significant and it shouldn't concern you. Also, if you are using a compressor on something already, then any built-in noise gate on that compressor will use the same gain-control circuitry anyway and so there is no further loss in signal quality - the gate effectively comes "for free".

If you are using a mastering compressor, then its built-in noise gate is useful as it can act as a quick-and-easy way of trimming off all that background noise before the first downbeat of a song. It certainly saves a lot of time by eliminating the need to edit by hand in a sound editor later!

However, in many individual instrument cases within a multitrack recording, you can do a better job by hand instead of using a noise gate. Either by erasing the parts of tracks where the instruments are not meant to be playing (be careful though!), or simply by automating the mix to mute channels when people aren't supposed to be playing.

Where noise gates come into their own is when gating a signal that comes and goes rapidly - and drums are a prime example of this. It would obviously take an impossible amount of time to try and accurately "erase" the gap between every signal snare drum beat, and this is especially the kind of situation where the automatic nature of a noise gate is at its best.

Specifically, there are five uses of noise gates that I can think of (at this moment) that relate directly to drums:

1. Gating sampled drums to shorten their length
2. Creating "gated" reverb
3. Helping to eliminate "spill"
4. Helping to eliminate "resonance"
5. Creating a "clinically clean" drum sound

Lets look at each of these in detail.

In each case when gating drums, the "Attack" should normally be set to its fastest, unless you are using a very, very fast gate which might introduce an audible "click" when the gates cuts in.

Gating sampled drums to shorten their length

There's little (or no) benefit in gating sampled sounds from a drum machine simply to remove noise, as the noise is not normally significant (an obvious exception to this is if you have taken samples from another recording which is noisy). However, if you are mixing a pre-recorded tape, you can use a noise gate to shorten the length of (for example) the snare drum. This is something that you would otherwise not be able to do without access to the original sampler or drum machine. The "hold" and - particularly - the "release" controls are the ones that affect this.

Creating "gated" reverb

These days, most digital reverbs have a "gated" reverb preset, so the need to gate reverb by hand has almost completely disappeared. Previously, you would have to feed the stereo reverb return through a pair of "linked" noise gates, and feed the "side chain" of the gate - the signal path that triggers it to open - from the direct snare sound. You would need to use a noise gate with a precise "hold" control - which keeps the gate open after it has triggered - in order to specify how long the "gated" sound lasts. An abrupt "release" setting after a fairly long "hold" setting usually gives the most dramatic effect. Normally, exactly one beat or half-beat sounds about right. "Drawmer" noise gates became famous as the best tool to achieve this in the analog world.

Compressing ambient microphones (if present on the multitrack recording) can lead to a spectacularly loud sounding drum kit, and gating such a stereo signal in the same way as gating reverb can heighten the dramatic effect even more.

Helping to eliminate "spill"

With a real drum kit, most microphones will pick up a significant amount of sound from the other drums. This can severely restrict your ability to EQ one drum without it affecting the rest of the drum balance. By gating each drum separately, this gives you more flexibility. But there are a couple of problems.

Firstly, when you EQ the drums, this can easily make the spill from other drums so loud that the gate "misfires" and opens up when it shouldn't. If you raise the "threshold" control to compensate, you risk not triggering on important drum beats. The classic problem case for this, is a loud hi-hat causing a snare drum gate which has had a lot of high EQ added (for a brighter snare sound) to open on a hi-hat beat instead of a snare beat. This sounds particularly loud and objectionable, and really sounds like a mistake when it happens. One solution to this is to gate the snare before the equaliser. Some gates also have separate, simple EQ controls on the trigger signal so you can roll off (for example) the high-frequency troublesome hi-hat spill which is causing the gate to misfire. However the problem with doing this, is that it can make the gate open a little bit too late, losing some of the impact of the snare drum. Sometimes the only way to solve this, is to duplicate the snare drum track, move the copied track back a few milliseconds, and use that as the basis for the signal feeding the gates "sidechain". This will make the gate open up before the snare beat has even happened which is pretty cool. Some software controlled gates (even hardware-based digital ones) have a delay available for the direct signal path in order to achieve much the same result. Naturally in this case, it means that you will have to move the master track back a few seconds in order to compensate, but on digital multitracks this is relatively easy, and it avoids the inconvenience of having to prepare a separate snare track just to trigger the gate. If you do this, remember to jot this down in the notes that accompany the multitrack, as people using the multitrack later will simply think that the snare is out of time with everything else.

Helping to eliminate "resonance"

Related to the problem of "spill" is the problem of "resonance". A real drum kit has drum skins that resonate in sympathy with each other. For example, every time that the drummer uses the bass drum, tom-toms will often emit a faint "boo" sound. Similarly, when the bass drum is used, the snare drum often has an irritating sympathetic "rattle", which sounds in the final mix like a rattly bass drum. Gating the drums can help eliminate these types of unwanted noises. If gating makes the kit sound too clean, it is possible on many gates, to let a little of the signal through at all times, so that you are not plunged into total silence when the gates kick in. This is sometimes called "soft gating."

The techniques for gating resonance out of drums are similar to gating "spill". The main difference is that when equalising the trigger signal (the "sidechain"), you sometimes have to do the opposite of what you do for spill. For example, to remove resonance on toms, you often have to use a trigger signal that has had the top AND the bottom rolled off - otherwise the "boo" sound of the resonance as well as hi-hat spill might cause the gate to misfire.

By using a combination of high-frequency and low-frequency rolloff (which are often provided on the gates themselves), it is usually possible to "narrow" the side chain signal to something that makes the gate trigger at the appropriate time, without cutting off the front of the beat.

Cautions on gating a "noisy" drum kit

Gating is not the "holy grail" of a getting a clean drum sound. A clean-sounding drum kit is the best solution. When recording, it is far better to try and persuade the drummer to investigate and eliminate any resonance in their kit rather than thinking that it can all be gated "in the mix". Sometimes, all this takes is careful tuning of the drum kit to avoid resonance and rattles. Other times, "gaffer" tape has to be stuck on the skins of the drums in carefully-chosen places to dampen the resonant frequencies and make the kit sound "tighter". In other cases, you may be regrettably short of time and have to (unfortunately) fix the sound later.

When resonance and spill are severe, gating can ironically draw attention to them (in the form of "breathing", which is the name given to the audible effect of gates and compressors that can obviously be heard working). When resonance and spill are very severe, it can be beyond the ability of gates to fix the problem, and you may have to be more imaginative in looking for a solution.

The most common problem caused by severe spill affecting gating, is when toms that sounded too dull need to be brightened on mixing (just to make the sound acceptable). If there is a lot of spill from the cymbals onto the drum tracks, you can find that if you gate the toms, a rather unpleasant effect occurs when a drum fill follows a loud cymbal crash. What happens is that the cymbals appear to "pulsate" in a thrashy way every time a tom is hit. This is the sound of the Cymbal "spill" breaking through, and is very unpleasant. Perhaps you might decide to take an alternative approach to fixing the problem, such as actually making the toms sound even duller than they did before, by rolling off the top end, and then using something like an Aphex Aural Exciter to re-synthesize the missing top end. That will give you a new top end, without the same level of cymbal breakthrough.

Often when mixing, I prefer not to gate the toms at all, and if gating would otherwise be required - I simply automate the mutes on the tom channels so that they are only switched on when a genuine tom fill occurs. If this makes the drum kit sound too artificial, I use soft fader movements instead to simply lower the level of the toms when they are not being played. I prefer to do this rather than using gates, because otherwise you can find that the whole stereo image of the drum kit changes and moves when the fills are played. Another solution would be to gate the toms as a pair rather than individually. Sometimes this works, but by no means always.

Creating a "clinically clean" drum sound

Finally, even if spill and resonance are not a significant problem in themselves, you can use gates to eliminate even the merest hint of them, and gate every single drum down to its bare essential sound. This will give you a "clinically clean" drum sound, but bear in mind that it will also strip a lot of "life" out of the sound, and you'll probably need a good deal of very high-quality short reverb to put some life back in. The result will probably still sound artificial.

Summary of gating drums

As you can see, gates can be a lot of hard work. If all you are trying to do is to clean up the drum sound a bit, consider simply muting the toms in-between fills using mixer automation (mute and un-mute on (e.g.) snare beats to cover the change in sound). That is frequently all that is required to tidy up the drum sound. Pan-global gating is not usually required and can sound contrived and unnatural.

If you can get away without gating or muting any drum tracks at all, then so much the better.

In any case - as I mentioned at the beginning of this section, make these decisions about the drum sound at this early stage of the mix and not later. If you gate drums towards the end of a mix, the whole atmosphere of the mix can change in a disconcerting way.