Your audio journey – Part 42 – Noise gate / Downward expander

In this part I’ll explain how to set and use a noise gate or downward expander to remove annoying background noise.

It’s important to say that the aim is always to record in an environment where you’re not picking up background noise but sometimes that’s not possible.

In that case there’s a downward expander or a noise gate which can trap out the audio.

A downward expander is like a compressor but it works on audio below the threshold. A normal compressor works to reduce audio above the threshold to reduce dynamic range, a downward expander works to compress audio beneath the threshold.

A noise gate (or autogate) in Adobe Audition works the same. All audio under the threshold is acted upon but the minute audio crosses the threshold the gate doesn’t act.

There are certain situations where the noise gate and downward expander will be of little use as both only act while the audio is under the threshold but the minute audio reaches or crosses above the threshold all audio, including whatever the background noise is, is allowed to pass untouched by either process. Therefore if the noise overpowers the audio then neither of those will work.

In order to set a noise gate or an expander the first thing you want to do is get an idea of what you’re dealing with and for that reason you want to open the amplitude statistics window. Pick a selection of the audio this is just the noise, avoid mouth sounds or breaths, and scan that selection. The maximum amplitude is the figure you need.

You can then set a noise gate or downward expander based upon this figure.

It’s useful to pop upon both spectral frequency analysis and the preview window to make sure it’s set correctly but your ears will be the final decider. Listen to the rest of the audio and make sure that your noise gate or downward expander hasn’t cut off any of the other audio you wanted to keep. In the event it has you may well be better off using a noise gate where you can adjust the attack, release and hold settings as well as the threshold.

The aim is to get audio recorded without background noise so in terms of using either of these use them as the first step in “repairing” your audio before going on to process it with EQ, compression etc.