Audio Gating Basics

Learn step-by-step how to set gates and expanders to reduce noise, drum bleed, and shape transients. Learn when to use a gate, when to use an expander and most importantly, learn how to do it in a way that sounds natural in your mix.

Audio Gating Basics

 

Noise gates are a member of the dynamics processor family, designed to reduce the gain of any signal below a certain point. They make quiet passages even quieter and can help to isolate drum hits or other instruments in recorded signals that are showered in noise bleed.

Gates are typically used for:

  • Removing bleed and shaping transients on drum mics
  • Taming noisy guitar amp recordings
  • Removing ambient noise from vocal recordings

Gates work similarly to compressors in that their processing is dependent on a level-based threshold set by the mixer. While compressors attenuate signals above the threshold, noise gates attenuate signals below it. Any signal louder than the user-defined threshold passes right through, but any signal quieter than the threshold is sharply reduced downward.

Know the difference: Gates vs. expanders

Gates operate at extremely high ratios, typically 100:1 or higher, meaning that once a signal falls below the threshold, its gain is reduced to 1/100th of what it was before. In a practical sense, when the gate is closed, the signal is muted to the noise floor.

Most noise gates also include an expander option for a less extreme form of gain reduction. When using an expander, signals that are below the threshold are reduced at a much less dramatic ratio: often around 2:1 or 4:1. At a 2:1 ratio, a signal below the threshold will have half of its original gain.

In essence, the only difference between gates and expanders is that gates operate at extremely high ratios, inducing a much sharper gain reduction below the threshold.

Understand the controls: What, why, how

At their core, gates are simple: they’re either open or closed. But setting how they are triggered and the timing of how they open and close is where greater attention to detail is needed:

  • Threshold: Controls the dB level above which signals pass through the gate. For instance, if the threshold is set at -20 dB, only signals louder than that will pass through (say, -19.99 dB and above). Some gates, like the ones in the C1 Compressor, eMo D5 Dynamics and Scheps Omni Channel plugins, have separate thresholds for when the gate opens and when it closes, which helps make a smoother exit as a sound decays.
  • Floor: Sets how far down a gated signal goes when the gate is closed. If you have the noise floor set as low as it goes, the signal will be completely muted when the gate is closed. If you set the floor to be in the audible range, say 12 dB below where the threshold is, then as a signal falls below, it will be reduced by 12 dB.
  • Attack: Controls how quickly the gate is opened once the signal exceeds the threshold.
  • Hold: Controls how long the gate stays open after the signal exceeds the threshold, regardless of when the signal drops back down. This guarantees the gate to stay open for a set amount of time.
  • Release: Controls how quickly the gate closes after the signal falls below the threshold, or after the hold time runs up. The attack and release settings shape the way a gated signal sounds, and greatly affect how it cuts through the mix.
  • Sidechain: Some noise gates include internal sidechain options, which allows the gate’s trigger to see only a limited portion of the frequency spectrum. Use this to fine-tune the performance of the gate so it reacts more closely to your desired signal, instead of being misfired by surrounding noise bleed.
Setting a gate's sidechain can be extremely helpful in getting a clean gated sound. For example, on a kick drum mic, you can set an internal sidechain with a high pass filter starting at 200 Hz and a low pass from about 4 kHz. This will allow the gate to focus on the transient response of the kick, while ignoring the boomy sub-frequencies and interference from rimshots on the nearby snare.

Most gates also allow sidechaining from an external source, so that you can trigger your gate with a signal other than the one it’s gating. A common application for this is the classic gated reverb sound: when the snare is sent to a sidechain, triggering the gate to open on a heavily compressed reverb with each hit. (Think Phil Collins' "In the Air Tonight" or Prince's "I Would Die 4 U.")

In this video excerpt, engineer/mixer Ethan Mates (Red Hot Chili Peppers, Linkin Park) and FOH mixer Ken "Pooch" Van Druten (Jay Z, Linkin Park) take a look at gating in a live multitrack recording of a Linkin Park show. In the video, see how Ethan sets his kick mic’s gate to be triggered externally by a coinciding kick sample, using the eMo D5:


 

In this video clip, see how mixer Michael White explains gate settings, then gates his snare to reduce bleed and gain control of the snare's attack and release. At the beginning, notice the difference in the gain response curve as he briefly flips into expander mode:

 

6 steps to setting a gate or expander in your mix

Now that you know each control, it's time to get started:

  1. Start with the threshold turned all the way up (0 dB), then slowly reduce it until you begin to hear the signal pass through the gate.

    If you’re gating a transient-heavy signal like a drum mic, set the threshold so that even the quietest drum hit opens the gate, but erroneous drum bleed doesn’t. This may not be completely possible with every recording, so get as close as you can. For example, it can be particularly difficult to let the ghost notes of a snare drum through without letting the bleed from the hi hat also trigger the gate. Proper mic placement is crucial.

    For now, just focus on the triggering of the gate, not on the release.
  2. Set the gate's internal (or external) sidechain to finely tune the performance of the gate. The goal here is to have the gate’s trigger focus on the desired signal, and ignore the unwanted bleed. Monitor the gate’s sidechain, and use the available filters to isolate the main instrument as much as you can. Then, switch back out of the sidechain monitor mode, and make sure it’s triggering how you want it to.
  3. Next, it’s time to adjust the time settings. Start with a quick attack time, and increase it until you hear the instrument's initial punch become fairly softened. Then, pull it back to find your desired amount of punch and snap. Listen within the context of the whole mix so that you can find the best amount of transient punch that works in the track. If you want it to sound further back in the mix, go with a smoother, longer attack time. When working in expander mode this may be a little less dramatic, but nonetheless important in transient shaping.
  4. If you need to space out the time between the gate’s opening and its release, increase the hold time a bit, or try decreasing the separate closing threshold. Find what works best for your sound, as different instruments behave very differently!
  5. Next, increase the release time to make the gate close more gently, or decrease to make it close more quickly. Sans extreme creative effects, it’s usually desirable to have a decay that is smooth and gentle: one which matches the pace and decay of the instrument, and is quick enough to close before unwanted noise becomes obvious.

    The best release settings will heavily depend on the natural decay of the instrument, the tempo of the music, and the bleed, room tone, reverb, or whatever it is that you're trying to attenuate. You generally want to cradle the sound so that it enters and exits the mix in a natural way. If possible, try to time the release of the gate to the tempo of the song, so it closes within an 1/8 or 1/4 note of opening.
  6. If the closed gate's muting effect is a little too clean or too dramatic, try increasing the floor setting or switching to expander mode; sometimes a touch of bleed or room tone can work well to make a mix sound believable and alive!
Take your time and experiment with small changes to find the perfect settings for gating your signal. How you set your gate or expander will vary with each application. Either way, it's an important part of tidying up recordings and, if done well, will help immensely in getting the mix to sound clean and finished.

 

Want to get deeper into shaping transients? Dive into these 6 transient shaping tips for drums.

Do you have any gating tips that we missed? Let us know in the comments below.

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