Attenuate a recording while it sits below a level, and let it through once it rises above one. It is the standard fix for tape hiss, computer fan noise, or room tone between spoken phrases.
Threshold and hysteresis
The threshold is the level that opens the gate. Everything quieter is attenuated by the range setting.
Hysteresis lowers the level at which the gate closes again, so opening at -45 dBFS with 3 dB of hysteresis means closing only once the signal drops below -48 dBFS. A signal hovering at the threshold would otherwise flick the gate open and shut many times a second, which sounds like chattering or a stutter.
Range: mute or attenuate
Range is how far the closed gate attenuates, not a switch between on and off.
At -60 dB the gaps are silent. At -12 dB the noise is pulled well down but still present, which usually sounds more natural on speech, because a recording that alternates between room tone and pure digital silence draws attention to the gate. At 0 dB the gate is inactive and the file passes untouched.
Hold stops the gate falling shut mid-phrase
Hold is a minimum time, after the signal drops below the closing level, before the gate is allowed to start closing. Speech has short gaps inside it, and without hold the gate closes across every one.
120 ms covers ordinary consonant gaps. Raise it if the gate is closing between words rather than between sentences.
Attack and release
Attack is how fast the gate opens. A short 1 to 5 ms keeps the front of each word intact. Longer settings soften the entry, which can suit sustained instruments but clips syllables on speech.
Release is how fast it closes once hold has elapsed. Short releases cut off reverb tails and sound abrupt. 200 to 400 ms is a reasonable starting range for voice.
Example: a voice recording with fan noise
The fan reads around -52 dBFS, the quietest words around -34 dBFS.
Set the threshold to -45 dBFS, hysteresis to 3 dB, hold to 150 ms, attack to 2 ms, release to 300 ms, and range to -25 dB. The fan drops well below audibility in the gaps, the words are untouched, and the transition does not read as a hard mute.
Detection
The gate follows the peak level of the loudest channel, with a short decay so a waveform passing through zero between two peaks is not mistaken for silence. Both channels of a stereo file open and close together, so the image does not shift when only one side crosses the threshold.