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What a noise gate does

A noise gate is a door for sound. While the recording is louder than the threshold the door is open and everything passes. When the level drops under it, in the pause after a sentence or the gap between two notes, the door closes and the sound is turned down by the range. Set the threshold a few dB over the noise, not a few dB under the voice, and the voice never notices it is there.

This gate measures the quietest tenth of your recording, the hiss or fan in the pauses, and suggests a threshold 10 dB over it. It checks the level every 10 ms on whichever channel is loudest, and opens and closes every channel together so a stereo file keeps its image. Because it reads the whole file first, it opens just before each sound starts, so first consonants and the attack of a note are kept. The gain moves on a smooth curve over the attack and release times, so opening and closing never click.

A gate does nothing while it is open. Hiss under the words stays exactly as loud as it was, and now it starts and stops with the voice. For quiet noise, 30 dB or more under the speech, nobody notices. For louder noise, a gate on its own can sound worse than no gate, and noise removal is the right tool.

Audacity has the same effect under Effect > Noise removal and repair > Noise gate, with the same controls: gate threshold, attack, hold, decay (the release here) and level reduction (the range). Its Analyze Noise Level option reads the floor for you to type in; this gate reads it and starts the threshold from it, then lets you compare before and after.

Gate, noise reduction or cutting the pauses?

• Use the gate when the noise is only a problem in the pauses: faint hiss, a quiet fan or amp noise you stop hearing once someone talks or plays.
• Use background noise removal when you can hear the noise under the words. A gate is open while someone talks, so that noise stays exactly as loud.
• Use the silence trimmer when the pauses are too long, not too noisy. It removes time; the gate keeps the timing and only turns the gaps down.

Noise gate vs noise reduction plays one reading under a fan through two gate settings and through noise removal, with the level of the pauses and of the noise under the words measured for each.

When a gate is the wrong tool

• Noise you hear under the words. A gate is open while someone talks, so it passes the fan with the voice. Use background noise removal, which works under the words too.
• Noise nearly as loud as the voice. With less than about 15 dB between them, the gate cannot tell a soft word from the fan and either cuts words or flutters in the gaps.
• Another voice, a television or a cough. Anything over the threshold opens the gate, so these stay in. Cut them in the audio editor.
• Long fades and reverb tails. A gate shortens the natural decay of a note or a room. Use a longer release, or Gentle range, so the tail fades instead of stopping.