Gates and expanders are far too often overlooked in terms of a simple solution to several problems.
Gating is the best way of cleaning up any mix, cutting out any unwanted resonances, tails, or unwanted artefacts, whereas expanders are the opposite and can create blasts of excitement or help bring out individual parts of the mix for brief periods of time.
Let’s look at some of the most common uses.
Expanders
Expanding is not a trick settled on early in my career, I actually didn't see the point of it for a long time - that was until I saw a clever trick used by Adam Getgood where he would sidechain the snare track to the room mics, and place a gate/expander on that track.
An expander is simply an inverted gate, so; for example instead of reducing by 60:1 you would adjust the opposite to say 1:10 to expand the volume rather than reduce.
In this example from Adam, instead of using the snare to trigger the gate to open, and the room mics being muted until the snare drum hit, it triggered the expander to push the level of the room mics up every time the snare hit creating this massive snare sound for a split second with the room mics completely untouched for the rest of the time (because the last thing you want to do is gate your room mics, right?)
This sounds complicated but try out these settings the next time you mix, and send your snare to the sidechain input of your room mics - remember to set the gate/expander plugin you choose to use the sidechain input as well!
Gating
The best way to clean up your mix before any sort of processing is gating.
Either doing this on the way in, or the first plugin you place in the chain should ideally be a gate, especially if there are unwanted artefacts common to the instrument or source you’re recording - for example: vocals, guitars, close mic’d drums, etc.
But this is not to say you should gate everything; Overhead mics and room mics for drums would be a silly thing to gate unless for a very specific sound or purpose within a track.
Toms are a great example of the best uses of gates for me. The most important parameter here would be the release. Taming toms can be difficult when starting out in the mixing world, but learning how to tame the length of the tom hit was paramount to then being able to EQ for me.
If it’s too boomy, or the ring of the instrument (toms included) is too long, then set the release to a lesser number; I usually start at 500ms and work my way down. If the sound is being cut off too quickly, raise the release to gently gate in a much more transparent nature.
Vocals are the easiest thing to start practicing gating on and experimenting with parameters. The main thing you’re looking to eliminate from these sources is mic stand rumble or rattle, unwanted breathes or artefacts like a car driving past, or someone forgetting to set their phone to silent during recording.
Try these simple techniques going forward, if you’re not already and hear how much clearer your mixes will be without even attempting any EQ or compression.