How does a noise gate / expander work?
Like a compressor, a noise gate is an automated volume control. But with a different effect: when the sound is very soft, the noise gate pushes it all the way down to zero.
Why? Well, say you have a little bit of hiss in your vocal recording, you’ll probably only hear it when the vocalist is quiet. When she talks, her singing or speech masks the noise.
In that case, a noise gate is useful: At all times when it’s almost quiet anyway, the volume goes to zero and you don’t hear that hiss anymore.
A noise gate has the same controls you will find on a compressor.
With the threshold you determine at which volume the noise gate should ‘close’ (volume to zero). This threshold works exactly the other way around compared to a compressor: if the noise is below the threshold value, it will be adjusted.
With the attack you determine how fast the noise gate opens when the sound is below the threshold value and with the release you determine how fast it closes when the sound is above the threshold value again.
An expander is actually the same as a noise gate, but on that you can also set the ratio. That means the sound is not set to zero when it gets below the threshold, but you can determine that yourself with the ratio. At a ratio of 20:1, a sound that is 2 dB below the threshold value will be attenuated by 40 dB.
Many noise gates and expanders also have a ‘range‘ function: With this function you determine the maximum attenuation.
For example, if you set that range to -10dB, the sound below the threshold will be attenuated by a maximum of 10dB. Whatever ratio you use.
Practical example: This can sometimes work well on a voice recording with too loud breaths. You might want to hear it to keep it natural, but not so loud! Put an expander on it with a range of e.g. 7 dB and set it to work at the exact volume of the breaths. Net effect is that all breaths are turned down 7 dB. This is not a fool-proof system by the way: sometimes whole words are also turned down by 7 dB. Keep using your ears!