A noise gate is a dynamics processor that attenuates an audio signal when its level falls below a threshold and lets it pass unchanged when the level rises above it. In mixing, it removes background noise, hum and microphone bleed during the pauses between notes, words or drum hits, so that each track contributes only its intended sound to the mix.
A gate is the opposite of a compressor in spirit: a compressor reduces loud signals, while a gate reduces quiet ones. Its gentler relative, the downward expander, lowers quiet passages gradually instead of closing almost completely.
How a noise gate opens and closes
A noise gate works by measuring the level of a detector signal and controlling a gain stage according to that level. When the detected level rises above the threshold, the gate opens and passes audio at full gain. When the level falls below it, the gate closes and applies the set range of attenuation.
The transition is shaped by time constants. Attack sets how fast the gate opens, hold keeps it open for a minimum time after the signal drops, and release sets how fast it closes. Without these controls, the gate would chatter open and shut many times per second on a decaying note, producing audible chattering or fluttering.
Many gates add hysteresis, meaning the closing threshold sits a few decibels below the opening threshold. This prevents rapid toggling when a signal hovers near the threshold, which is common on sustained notes and reverberant tails.
The following table summarizes the main controls and their typical settings.
| Parameter | Typical range | What it controls |
|---|---|---|
| Threshold | -60 to -20 dBFS | Level at which the gate opens |
| Range | 10 dB to full mute | How much the closed gate attenuates |
| Attack | 0.01 to 10 ms | Speed of opening, preserves transients |
| Hold | 10 to 200 ms | Minimum open time after the signal drops |
| Release | 20 to 1000 ms | Speed of closing, shapes the decay |
| Hysteresis | 2 to 10 dB | Gap between opening and closing levels |
Sidechain filters and key inputs
A noise gate becomes far more precise when its detector listens to a filtered or external signal rather than the audio it processes. Most gates offer sidechain filters, usually a high-pass and a low-pass filter, that shape only the detection path.
On a snare drum microphone, for example, the kick drum bleed can be loud enough to open the gate. A high-pass filter around 150 to 250 Hz in the sidechain makes the detector ignore the kick, so the gate responds only to snare hits. The audio itself remains unfiltered.
An external key input goes further. The gate on one track opens in response to a different track, which allows creative techniques such as gating a low sine tone with the kick drum to add weight, or tightening a bass part so that it plays only when the kick plays. The same routing concept underlies sidechain compression.
Some gates also offer lookahead, which delays the audio by a few milliseconds so the gate can open before the transient arrives. This avoids clipping the first milliseconds of a fast attack, at the cost of added latency.
Noise gate compared with related processors
The noise gate belongs to the family of dynamics processors, and each member treats levels differently. The table below places the gate among its neighbors in mixing.
| Processor | Acts on | Effect | Typical ratio or depth |
|---|---|---|---|
| Noise gate | Signals below threshold | Mutes or strongly attenuates | Very high, often full mute |
| Downward expander | Signals below threshold | Lowers gradually | About 1:2 to 1:4 |
| Compressor | Signals above threshold | Reduces dynamic range | About 2:1 to 10:1 |
| Limiter | Signals above ceiling | Prevents exceeding a level | 20:1 or higher |
| De-esser | Sibilant band above threshold | Reduces harsh consonants | Frequency dependent |
A gate is also distinct from noise reduction software. A gate cannot remove noise while the instrument plays, because it only acts in the gaps. Broadband hiss that sits under a vocal remains audible whenever the gate is open, whereas spectral noise reduction attempts to separate noise from the wanted signal at all times.
Setting a noise gate in a mix
The most reliable method is to set the threshold just above the noise or bleed and just below the quietest wanted sound, then shape the timing by ear. Starting with full range and a fast release makes errors obvious, after which both can be relaxed.
| Source | Suggested settings | Reason |
|---|---|---|
| Toms | Fast attack, 100 ms hold, 200 ms release | Removes long cymbal and snare bleed |
| Snare drum | Sidechain high-pass, 15 to 25 dB range | Ignores kick, keeps some natural room |
| Kick drum | Fast attack, short release | Tightens low end and limits rumble |
| Guitar amplifier | Moderate attack, 100 ms release | Removes hum and pickup buzz in pauses |
| Spoken voice | Expander rather than hard gate | Keeps breaths natural and avoids cut-off words |
A partial range of 10 to 20 dB often sounds more natural than a full mute, because a little bleed keeps the drum kit coherent. Gating is also best placed before compression, since a compressor raises the quiet noise floor that the gate is trying to remove.
Common noise gate mistakes
Most gating problems come from thresholds and time constants that do not match the material. The symptoms are easy to hear once an engineer knows them.
- Clipped attacks result from a slow attack or a threshold set too high; faster attack or lookahead restores the transient.
- Truncated decays on toms or vocals come from a release that is too short; a longer hold and release let the tail breathe.
- Chattering on sustained notes means the signal hovers near the threshold; adding hysteresis or hold solves it.
- Pumping background noise appears when the gate opens and reveals hiss that vanishes again in the gaps; a smaller range or an expander hides the change.
- Missed soft notes occur when the threshold excludes quiet playing; automation or manual editing is then a better tool than a gate.
The simple explanation
A noise gate is like an automatic door that opens only for loud enough sounds. When a singer sings or a drum is hit, the door opens and the sound passes through. When the performer stops, the door closes, and the hiss, hum or sound from nearby instruments is shut out.
The settings decide how loud a sound must be to open the door, how quickly it opens, and how gently it closes, so the music does not sound chopped.
How it is used in practice
A mix engineer works on a live-recorded rock song with eight drum microphones. The two tom microphones play only a few fills, but they pick up loud cymbals and snare throughout the song, which makes the whole kit sound washy and smeared.
The engineer inserts a gate on each tom with a threshold around -30 dBFS, a fast attack, about 120 ms hold and a 250 ms release, and sets a sidechain high-pass filter so the kick drum does not open the gates. The range is set to 18 dB rather than full mute, keeping a hint of natural room sound.
The result is a tighter, clearer drum sound in which the tom fills still ring naturally, while cymbal bleed no longer muddies the mix between fills.