What is a noise gate?

Noise gates in plain English

A noise gate is an automatic mute button. While the sound going through it is louder than the threshold, the gate is open and the sound passes straight through. When the level drops under the threshold, the gate closes and turns the sound down, often to silence, until the next loud moment opens it again.

The name comes from the job it was first used for. Tape hiss, amp hum and the air conditioning in a live room are all quiet, and you notice them most in the gaps when nobody is playing. A gate leaves the playing alone and shuts the gaps.

It's a dynamics processor, the same family as a compressor, and it shares some of the same controls. They work in opposite directions, though. A compressor turns down sounds that go over its threshold, while a gate turns down sounds that stay under it.

Why use a noise gate?

Mostly for cleaning up recordings. The microphone on a snare also picks up the hi-hat and the kick, which engineers call bleed or spill, and the tom mics hear the whole kit even though the toms might only play a few fills in a song. High-gain guitar amps hiss and hum whenever the guitarist stops playing. Put a gate on each of those tracks and the gaps go quiet.

On drums a gate shapes the sound as well. Cutting off the ring after each hit makes a kick or snare shorter, so each hit stands out from the space around it, and a loose-sounding kit starts to sound tighter.

Gates have creative uses too, like chopping a pad into a rhythm or stopping a big reverb dead after every snare.

Reading the lab

The big screen shows the whole loop. The pale shape behind is the original and the pink shape in front is what comes out of the gate. In the lane underneath, teal shading marks where the gate is open, and the amber line shows how far it's turning the sound down: at the top of the lane the gate is open, and at the bottom it's closed by the full range. Hover over any point for the exact figure.

The square screen is the transfer curve, with input level along the bottom and output level up the side. Above the threshold the line runs diagonally, so what goes in comes out unchanged. At the threshold it drops straight down by the range, which is the whole job of a gate drawn as one shape. The close-up next to it shows about 400 ms around the loudest hit, so you can watch the gate open, hold and close on a single note.

The panel on the right counts how many times the gate opens and how much of the loop it spends open and closed, measured from the sound you're hearing. A/B switches the gate off with the loudness matched, so the gated loop can't win just by being louder or quieter.

The drum and percussion loops come from Sounds Like You packs. Acoustic guitar is a recording with a pause of about three seconds in the middle, where the level falls to around −50 dB, so it's a good test for a gate. You can also upload a file or record eight seconds from your microphone, and if you have a noisy recording to hand, that's the best test of all.

The controls

Gates in different DAWs use slightly different names for the same things. Logic Pro's Noise Gate calls the range Reduction, and Ableton Live's Gate calls it Floor. The jobs don't change.

Threshold

The threshold is the level the sound has to reach to open the gate, in dBFS. The right setting sits between two numbers: the level of the noise you want gone, and the level of the quietest note you want to keep.

Say the hiss on a guitar track sits around −55 dB and the softest note you care about peaks at −30 dB. Anywhere between them will work, and a threshold near the middle, around −42 dB, leaves room for error on both sides. When those two numbers are close together, or the noise is louder than the quietest notes, no threshold can separate them, and a gate is the wrong tool.

The drum loop in the lab shows the problem. Between hits the level falls to around −40 to −50 dB, but the soft ghost hits peak anywhere from about −45 dB up to −15 dB. Any threshold high enough to clean every gap also eats some of the ghost notes.

Range

Range sets how far the gate turns the sound down when it's closed. At 80 dB a closed gate is as good as silent. At 10 to 20 dB it only tucks the gaps back, which on drums keeps a little of the room and the rest of the kit, so the drums still sound like they were played in one space.

Full muting is clean on a solo instrument. In a busy mix it can draw attention to itself, because a track dropping in and out of total silence is easier to notice than a low hiss. Start with less range than you think you need and add more only if the gaps still bother you with the whole mix playing.

Attack

Attack is how quickly the gate opens once the sound crosses the threshold. On drums you want it fast, under 1 ms, because the transient at the front of each hit is over within a few milliseconds and a slow attack fades it in. Slower attacks of 5 to 20 ms suit sounds that start softly, like a bowed string or a pad. They also hide the click a gate can make when it jumps open in the middle of a low, sustained note.

Hold

Hold keeps the gate open for a set time after the sound drops back under the threshold, before it starts to close. It's there to stop chatter, the fast flutter you get when a note fades slowly through the threshold, dipping under it and back over several times on the way down. Every dip shuts the gate for a moment, and the end of each note gets a rattly, stuttering edge.

For short drum hits, 20 to 50 ms of hold is plenty. Guitars and vocals ring on for longer and usually need 100 ms or more.

Release

Release is how long the gate takes to close once the hold time runs out. A release under 20 ms cuts the end of each note off abruptly. From 100 to 300 ms it fades the tail out, which sounds more natural, though the gate then spends more time partly open and lets more noise through.

On a busy part, hold and release together have to fit inside the gap between notes, or the gate never gets the chance to close. At 120 BPM sixteenth notes are 125 ms apart. A 40 ms hold and a 60 ms release add up to 100 ms, which leaves only 25 ms for each hit to fall back under the threshold before the next one arrives. Push the release to 250 ms on a part like that and the gate never fully shuts, which is what step six of the lab shows on the drum loop.

Hysteresis, lookahead and the sidechain filter

Fuller gates add a few more controls. The lab leaves them out so you can hear the main five on their own.

Hysteresis gives the gate two thresholds, one to open and a lower one to close, usually a few decibels apart. Once a note has opened the gate it stays open until the level falls well below the opening point, so a note wobbling around the threshold can't flick it on and off. Logic's Noise Gate has a Hysteresis control; in Ableton's Gate the same thing is called Return.

Lookahead delays the audio by a few milliseconds so the gate can see a hit coming and already be open when it arrives. You keep the whole transient even with a gentle attack, at the cost of a small delay that your DAW normally compensates for.

The sidechain filter, sometimes called key filtering, puts an EQ on what the gate listens to without changing what you hear. On a snare mic, a high-pass filter on the sidechain stops kick bleed from opening the gate, since most of the kick's energy is low. On a kick mic, a low-pass filter does the same job for hi-hat bleed. Hardware gates such as the Drawmer DS201 put these filters on the front panel for exactly this reason.

How to set a noise gate, step by step

This order works on almost any track, and it keeps you from setting the threshold by guesswork.

1. Start with the gate wide openThreshold all the way down, range at maximum, attack under 1 ms, hold around 30 ms and release around 100 ms.
2. Measure the noise and the notesSolo the track and read the input meter in a gap, then on the quietest note you want to keep. Those two levels are the edges of your threshold.
3. Raise the thresholdBring it up until the gaps go quiet, and stop before the quietest notes start to lose their beginnings. If they do, back it off by a few decibels.
4. Set hold and release on the ends of notesLengthen the hold until any flutter stops, then set the release so tails fade out instead of stopping dead.
5. Bring the range backWith the rest of the mix playing, reduce the range until the gaps are only as quiet as they need to be.
6. Compare with the gate offUnsolo and A/B. Noise that was obvious on solo can disappear under the full track, and then a lighter gate, or none at all, is the better choice.

Noise gate settings for common sounds

The threshold always comes from your recording, so it isn't in the table. Treat these values as starting points and adjust them with the track playing.

High-gain guitar gets the fastest, deepest settings because that's where the gate is part of the sound. Tight metal rhythm parts depend on the amp noise vanishing the instant the guitarist mutes the strings, so the gaps between chugs are silent.

Using a noise gate on drums

Multitrack drum recordings are where you'll reach for a gate most. Toms are the clearest case: the tom mics sit close to the snare and cymbals and hear both, and gating them takes a lot of cymbal wash and snare out of the mix during the long stretches when the toms aren't playing. Editing the tom tracks by hand, deleting everything between fills, does the same job and is often more reliable, because an edit can't miss a soft hit or open on a loud snare.

Snares are harder. The quiet ghost notes between the backbeats are part of the groove, and they often sit at the same level as the hi-hat bleed, so a gate set to remove the hats takes the ghost notes too. That's what you heard in the third step of the lab. A modest range of 10 to 15 dB, with a high-pass filter on the sidechain, usually gets you most of the cleanup without losing the feel.

On a sampled kit or a drum loop there's no bleed to remove, so a gate becomes a way of shortening sounds. A short hold and release turns a ringing snare or a long 808 into a short, tight hit. If you want the tail shorter without the hard edge a gate can leave, a transient shaper turning the sustain down does it more gently.

The noise a gate can't remove

A gate only works in the gaps. While a note plays, the gate is open, so any hiss or hum underneath comes through at full level. Most of the time that's fine, since a loud note covers up quiet noise (an effect called masking), which is why the noise was easiest to hear in the gaps to begin with.

For noise you can hear under the playing, like mains hum at 50 Hz in the UK and Europe or 60 Hz in North America, or hiss on a quiet vocal, you need a noise reduction tool such as iZotope RX. Those work frequency by frequency, so they can take the noise out from under the notes, which a gate has no way of doing.

A gate also can't tell good quiet sounds from bad ones. A breath before a vocal line or the squeak of a guitarist's fingers sliding along the strings is quiet, and the gate shuts it out along with the noise. Sometimes that's what you want, and sometimes those small sounds are what make a recording sound like a person played it.

Noise gate vs expander

An expander is the gentle version of the same idea. It turns the sound down gradually as it falls under the threshold, by a ratio: at 1:2, a sound 5 dB under the threshold comes out 10 dB under. Quiet sounds get a bit quieter and very quiet ones get much quieter, with no single moment where the sound switches off.

That gradual slope makes an expander harder to hear working on vocals and dialogue, where a gate's on and off can be audible at the start of every phrase. A gate is better when you want the gaps properly silent, as on toms or high-gain guitar. Some gate plug-ins have an expander mode as well, and the compressor vs expander vs gate comparison puts all three side by side.

Creative uses of a noise gate

Gated reverb

Around 1980, while recording Peter Gabriel's third album, engineer Hugh Padgham and drummer Phil Collins hit on a drum sound that ran through the rest of the decade: a big, bright room sound cut off abruptly by a gate after each hit. The best-known example is the drum fill in Collins's 1981 single “In the Air Tonight”. To get the effect in a DAW, put a reverb on a send, put a gate after the reverb, and feed the gate's sidechain from the dry snare. The reverb then plays at full size while each snare hit lasts and stops dead the moment it ends.

Rhythmic gating

A gate keyed from a rhythm makes a sustained sound play in that rhythm. Put a gate on a pad, feed its sidechain a hi-hat pattern, and the pad only sounds when the hats play. Plug-ins that do this from a built-in step pattern are usually called trance gates. It's the same routing as sidechain compression, with a gate in place of the compressor.

Common noise gate mistakes

Cutting off quiet notes

If a part sounds stiff after gating, the threshold is probably catching ghost notes or the soft starts of quiet phrases. Lower it a few decibels, or use a smaller range so those notes are turned down instead of removed.

Chatter on the ends of notes

A fluttering, buzzy end to every note means the level is wobbling across the threshold as it fades. More hold usually fixes it, and hysteresis helps as well if your gate has it.

Setting it on solo

Noise that sounds awful on a soloed track is often impossible to hear once the whole mix is playing. Set the gate in context, and if you can't hear a difference with it switched off, take it out.

Clicks

A gate that opens or closes almost instantly on a low sound can click, because the level jumps in the middle of a cycle of the wave, and on a 50 Hz bass note one cycle lasts 20 ms. Lengthen the attack or release a little, or use a smaller range.

Putting it after the compressor

A compressor with makeup gain lifts the quiet noise and pulls the loud notes down, so the gap between them shrinks and the threshold gets harder to place. Put the gate first in the chain.

Training your ears to hear a gate

The quiz under the lab hides the controls and matches the levels, then asks whether the gate is on and whether its release is short or long. Listen to the gaps for the first question and to the end of each hit for the second. Eight rounds take a few minutes, and your best score is saved in your browser.

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