How to Use Expanders and Gates to Reduce Audio Background Noise in DaVinci Resolve

Recording audio at home is always a tricky feat, from microphones picking up sounds you hadn't even realized where there to begin with, or squeezing in a functional workplace. A big culprit in my recordings, even after I've done everything I can, is the computer fan. The good news is that there's great tools in DaVinci Resolve's Fairlight tools to manage those pesky background artefacts!
In this lesson we'll go over the Gate and Expander in the Dynamics Panel. You'll learn how to use to this feature to make your audio sound cleaner.



The Dynamics Panel is a powerful tool that's located within the mixer on the bottom right-hand of the screen. It contains a limiter, compression and a gate/expander, but in this tutorial, all we'll be looking at is the later. So go ahead open start up Fairlight, import a clip, and open up Dynamics.

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What are Gates and Expanders?
The Gate and Expander is pretty much the opposite of the compressor and limiter. They're all used to manage audio levels, but instead of reducing louder aspects of a clip, the gate and expander are used to manage the quieter, unwanted noises, like background hisses and hums. Thankfully understanding how this feature works is pretty straightforward once you get familiar with it's functions.:
Expander
The expander takes your quietest signals in your clip and lowers their volumes in relation to the louder sections. For example: you have a track of dialogue with a quiet fan in the background, the expander will work to quiet the background noise while leaving the dialogue untouched.
Gate
The gate does the same thing only you can set your point and effectively mute anything below threshold (if you wanted to).
Dials, Knobs, and Settings
Let's get comfortable with what each control is and what it does. 

Figure 1. Expander

Under Expander and Gate you'll see dials along to the top labelled Threshold, Range, Ratio and Attack, Hold, Release along the bottom.
Turning on the expander and gate is done by clicking the title within each panel. Once you turn on either feature, a few lines will appear in the graph above; a yellow/green line that's on the diagonal and a teal line on the vertical. These respectively represent the audio signal (yellow/green) and threshold (teal). This means the gate/expander will kick in and reduce the volume of the single below that intersection.
One thing to note. You can't use both gate and expander at the same time. This is because while they're identical in effect, the methods they use are different. 

Threshold is the point in which gate begins to act. This is measured is decibels. The default setting is -35db.

Range allows you to set by how much you decide to cut the volume down by.

Ratio works in a relationship with threshold. By setting your ratio to 2:1 and your threshold at the default -35db, you're really saying that for every 2db below your threshold, you'll be reducing it by half. When using gate, this feature is greyed out.

Along the bottom of the section, you'll see Attack, Hold, and Release. 

Attack is how quickly the gate acts once hitting the threshold/ how quickly the music ducks.

Hold is long the expander/gate stays on for after the signal is back below the threshold/how long the music track the stays suppressed.

Release is the time it takes to go back to the signal being unaffected.

Generally speaking when I use the expander I tend to stick with the default settings. They do a good job, and if I want to have more customization, I think using the gate is a better choice. 
Using a Noise Gate in Fairlight (Fan and CPU)
The first thing you'll notice when you switch from expander to gate is that even with same default settings, the audio is being treated vary differently. Take a look at both figure 1 and 2 and you'll notice the audio signal when using the gate has a more dramatic fall off. This change is caused by the ratio no longer having an impact.

Figure 2. Fairlight Gate

How you decide to use the noise gate will depend on the audio you're dealing with. In this example we're using it limit the the background noise from a fan or computer. On average this kind of noise sits between -30db to -40db. That means the default setting of -35db is a really great starting position. 
I prefer doing my voice-overs in Fairlight, unfortunately my CPU gets pushed when editing and can make a bit more noise than average computer. To adjust for that, try raising your threshold and the range. Start slowly and make gradual adjustments until you're happy with the sound. You can also use gain reduction meter on the left to get a visual of the impact.
if you are editing a short clip or a fragment of a clip, use the looping option (figure 3) to avoid the headache of starting the track over. To do this click the loop icon, select your clip or highlight the section using the Range Selection (R as a shortcut), go backing the gate and hit Option + /.

Figure 3. Range Selection and Looping Audio

Lastly use Attack, hold and release to shape and style the way the gate fades in and out. Unless it's an aesthetic we're actively going for, I don't like noise gates starting and stoping abruptly. To make a smooth and seamless fade, a good starting position is to adjust Hold to about a half second (500ms) and the Release to about 500-700ms. 
That's it!
Remember that less is more and that great audio starts with a great recording. These features will help shape and polish audio that just needs a little help, but with too much of a good thing added, you'll end up with a audio that's muddy.
If you want to learn more about DaVinci Resolve and Fairlight check out these articles to get you going!

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About the Authors
Andre Bluteau is musician and filmmaker from Canada. While primarily working on personal projects, Bluteau has worked produced and edited music videos for Polaris and Juno nominated artists. His work can be heard at Baker and Andre Bluteau as well as on YouTube.