Audacity noise gate: what it does and when not to use one

A noise gate is a volume switch that opens when the signal is loud enough and closes when it is not. Above your threshold, audio passes through untouched. Below it, the gate shuts and you get silence.

That is fundamentally different from noise reduction, which analyses the noise and subtracts it from the whole recording including the parts with speech in them. A gate does nothing at all while someone is talking.

So a gate removes noise between words and leaves it untouched underneath them. If the hiss is audible while someone speaks, a gate will not help, and the silence between sentences will make the contrast more noticeable rather than less.

Audacity does not ship with a noise gate enabled. It is a Nyquist plugin available from the Audacity plugin library, and it needs installing before it appears in the menus.

Download the .ny file, then in Audacity go to Tools, Nyquist Plugin Installer, and select it. Restart Audacity, then enable it under Tools, Plugin Manager if it is not already active.

Once installed it appears under Effect, usually in the Noise Removal and Repair group alongside Noise Reduction.

The two-step install is worth knowing about, because a lot of tutorials assume it is already there and people conclude their Audacity is broken.

Gate threshold is the level below which the gate closes. Set it just above your background noise and just below your quietest speech. Look at the waveform: find the loudest point of the room tone and the quietest word, and aim between them.

If those two overlap, which happens with a noisy room or a quiet speaker, no threshold works and a gate is the wrong tool for that recording.

Attack and decay control how fast the gate opens and closes. Fast closing chops the tails off words and sounds abrupt. Slower decay lets sentences trail off naturally, and around 250 milliseconds is a reasonable starting point for speech.

Level reduction is how far the gate turns things down when closed. Do not set it to full silence. Reducing by 12 to 20 dB keeps a little room tone, which sounds far more natural than absolute nothing.

Multi-microphone recordings are the classic case. Three people at a table, each with a microphone, and each microphone picking up the other two. Gating each track so it only opens when that person speaks dramatically cleans up the result.

Recordings with a consistent low-level intrusion also suit it: a computer fan, a fridge, distant traffic, where the noise is well below the speech and only bothers you in the pauses.

And live performance, where a gate stops a microphone picking up everything else on stage. That is where gates came from, and the use they are still best at.

For one person in a room with some background noise, Noise Reduction is the right tool. It reduces the noise everywhere, including under the voice, which is what actually makes a recording sound clean.

For pauses that are too long rather than too noisy, Truncate Silence shortens them without gating anything.

And the real fix for most noise problems is not an effect. Moving closer to the microphone raises your voice relative to the room more effectively than any processing, and costs nothing.

Hilite handles the noise side automatically without a gate, as part of enhancing a recording. We make it, so weigh that as you like. Audacity's own Noise Reduction is free and handles most of these cases well.