Gates and expanders can be very useful tools to help cut out ambient noise in a recording, or as a musical effect. They work very similarly to compressors, so here’s our guide to how gates and expanders work, and how to use them effectively.
Expanders are basically the opposite to compressors, so instead of reducing the dynamic range they increase it. Ultimately this means they are useful to reduce or cut out (‘gate’) quieter sounds in a recording. Often this is noise or ambience, but can also be used to reduce the sustain of a drum, or to shorten reverb. As with our guide to compression, we will follow the two examples of vocals and snare drum.
This is the level that incoming audio needs to reach for the expander to stop having an effect, i.e. everything below the threshold is expanded and made quieter. To reduce background noise on vocals, this needs to be quite low so that the singer’s voice is always above the threshold. If you want a very short, crisp and punch snare drum then an expander with a higher threshold will reduce the drum’s sustain. make sure the initial drum hit goes above the threshold, but drops below almost immediately.
The ratio describes how much expander will increase the dynamic range when it is effect. So for a ratio of 2:1, as the incoming audio level decreases by 1dB the outgoing audio level decreases by 2dB. For vocals we want this ratio to be very high cut out a lot of noise. On the snare drum, we don’t want to eliminate the drum sustain altogether or we would lose the tone completely, so 2:1 or 3:1 is probably adequate
This determines how far below the threshold the expander continues to work. So, if your threshold is at -30dB and the range is -10dB then the plugin will expand any audio between -30dB and -40dB. Any audio below this range (below -40dB in this case) will have the same dynamic range as the original audio but will just be quieter. For the vocalist, we want the rang eto be as big as possible. The snare drum only needs a little reduction of the sustain, so the range should only be 5 – 10dB and then tweak the threshold to get it working on the sustain properly.
This sets how quickly the expander stops having an effect after the level crosses above the threshold. For snare drum, this needs to be very short – probably less than a millisecond, so as not to clip the sound of the initial drum hit. Vocals need a slightly longer attack to sound natural, but an attack much longer than a few milliseconds can start to clip the beginning of words.
This sets a time delay before the expander starts to work, after the level goes above the threshold. This can be useful on snare drums to control exactly when in the sustain you want to start reducing the level. Depending on your attack time, this is likely to be in the 5 – 10 millisecond range. For vocals, the hold can be useful to make sure that the end of words are not cut off by the expander. To set the hold time for vocals, you simply need to experiments and find a setting that doesn’t clip words, and does cut out enough background noise.
This sets how quickly the expander starts to reduce the gain after the audio level drops below the threshold. Snare drums will need a fairly fast release, around 2 – 3 milliseconds to catch enough of the sustain but not the initial hit. The vocals will need a slightly longer release of about 10 to 15 milliseconds, or the effect will sound obvious and distracting.
Gates are simply an extreme case of expanders, they have a very high ratio and range to completely cut out anything below the threshold, just like we have set up with the vocal example.