Of all of the tools in your musical toolbox, compression seems to have an unshakeable mystique.
This might be because it is so versatile, with both super-subtle and extreme applications being common in music production.
There is also the fact that compression is quite a transparent effect. It’s not like stomping on a distortion pedal where the result is immediately obvious!
In fact, well-used compression can be completely unnoticeable, though if it wasn’t there you’d definitely miss it.
So far, so mysterious! How can you unlock the magic box that is compression?
The first step is to understand what compression actually does, and then to get to know how it can be applied to create the results that you want, whether you’re using a studio hardware compressor, a plugin, or a stompbox.
That sounds like a big task, but we’re here to help with a simple guide to what compression means.
You’ll quickly discover that compression isn’t as mystical as it appears, and then you’ll be able to use it in your music with no worries.
The first step in understanding compression is to understand sound.
Every sound is expressed as a wave that has louder and quieter parts. Think about when you play a note on a guitar or piano.
It starts out loud, then dies away over time. Alternatively, sing a song.
You’ll notice that your voice isn’t the same volume the whole way through. As you wind up into a big chorus, you’ll get louder and more passionate.
That’s how good performances should be, full of dynamics and feel.
However, this can present a problem from a recording and mixing point of view.
Sounds that suddenly jump out at the listener aren’t generally what you’re looking for, but the sonic characteristics of those louder sounds are important for bringing your performance to life.
Squashing The Peaks
At its very simplest, compression reduces the volume of the loudest parts of a sound. Say you’ve got a nice guitar part, and in some bits, you dig in slightly harder.
Adding some compression will make those parts where you play harder less different in volume from the parts where you play softly.
When you apply a compressor, you are setting a point at which the volume of your sound is reduced.
You also set how much it is reduced by, how fast this happens, and how long the reduction lasts.
This is why it’s important to think about the character of the sound that you’re working with. Next up, we’ll talk about how you go about squashing those peaks to suit your sound.
Threshold And Ratio
Two of the most important controls on any compressor, be it hardware or software, are the threshold and ratio controls.
These set when the compression effect kicks in, and how much of it there is.
The threshold sets the level that your compressor starts working at. Say you have a piano part that normally sits at around -12db, but occasionally reaches -10db.
If you want to bring that in a little while preserving the dynamics, you can set your compressor at around -11db. That way, any signal that rises above 11db will be affected by the compressor.
The ratio control tells your compressor how much to reduce the volume once the signal goes over the threshold you have set.
It is expressed as a ratio. A compressor set at 2:1 lets 1db through for every 2db the signal exceeds the threshold. An 8:1 ratio means that the compressor lets 1db through for every 8db a signal exceeds the threshold, a much greater reduction.
The higher the first number in the ratio, the more extreme the compression will be.
Attack And Release
The two other important functions of your compressor are attack and release.
These control how quickly the compressor kicks in and how long the effect lasts. These two tools are powerful and make a massive difference in how your compressor works, giving you the ability to fine-tune your compression.
Attack governs how quickly the compressor acts when it detects a signal above the threshold you’ve set.
A super-short attack will jump right on that peak and instantly squash it, whereas a longer attack will let the peak happen but suppress the volume of the signal afterward.
Too short an attack can sound unnatural, so even with sounds like drums you probably don’t want an instantaneous attack.
The best way to dial attack in is to set it to a relatively conservative length, and then make it shorter or longer until your ears tell you it’s musically right. Longer attacks generally sound more musical than short ones, but there is a place for both.
Release sets how long the signal’s volume stays suppressed after the compressor kicks in. Setting your release is really a matter of using your ears because what sounds right is so dependent on the specific sound that you’re working with.
Make sure you adjust your attack and release settings in the context of the whole track rather than with it solo.
That way you’ll be able to see if you’re creating an unnatural feel much easier.
The final key part of a compressor is the gain control. Naturally, when you’re compressing a sound you are reducing its volume. You’re going to need to add a little of what’s called make-up gain to bring the track back up to the appropriate level for your mix.
Rather than watching the meters, try to bring your track back into place by ear. Adding gain back in isn’t just a matter of noting your peak reduction and raising it to compensate!
A Note On Sidechains
One powerful effect you can achieve with compression is ducking. Say you have a kick drum and a bass part that occupy a similar frequency range. You can make your kick drum punchier if you can get that bass out of the way!
Setting up a compressor on your bass track but feeding it the kick drum signal via the side chain means that the bass part gets compressed when the kick drum plays.
By playing with the compressor controls you can make this effect subtle or very prominent, whatever suits your sound.
Hopefully, you now have a better understanding of what compression is and how it works. It’s a super-powerful tool that is essential in getting your mixes to sit together properly and for shaping individual vocal and instrument sounds.
The best thing you can do is to load up a compressor and experiment with it to see what you can make it do!