What Is Compression In Music Production? & When To Use Audio Compression Explained

It’s hard to admit we don’t understand something, especially something that everyone else seems to understand. If you’ve ever played live or tried to record and mix your own music, you’ve probably come across an effect known as compression.

Compression is literally everywhere. Compression and EQ are two of the most important plug-ins or effects to understand if you want to mix your own music. They are the building block for your whole mix.

That said, just because they are basic does not mean they are easy to master. It takes years to truly wield EQ and compression with mastery. Don’t let that intimidate you though, you can definitely start now!

The thing is, there is no point in slapping a compressor on a track if you don’t know what it’s doing. You may in fact be doing more harm than good. However, if you understand the basics of using compression, you’ll be leagues ahead of many home studio owners.

The nice thing about compression is that literally every DAW comes with a built in compressor, and they are usually pretty good. Of course, it’s always possible to upgrade depending on what you’re looking for, but you DAW compressor will be just fine when you’re starting out.

By the end of this guide, I want you to be comfortable with compression. You'll leaarn what the different settings and terms mean, what a compressor does, and what to look for on different compressors.

What Is Compression?

Have you ever noticed how guitar and vocal tracks fluctuate naturally in volume? That’s just how people play/sing. Most of the time, you want these tracks to be upfront and in your face, so to do that, you’ll use a compressor.

The essential thing to understand about compression is what it’s doing to the audio file. Many people tend to think that compression is about making things louder. In fact, it’s the opposite.

Compression makes the peaks of your track quieter.

A compressor is basically an automated volume fader that turns down the peaks of your track – making it more even and easier to turn up in the mix.

Compression squashes the waveform. That way, the track as a whole become more even. Then, you can bring up the overall volume of the track and it will sit nicely in your mix. You can use compression to make things punchier, to shape a sound, to obtain a nice balance and so much more.

It’s hard to fully understand how powerful this tool is without using it over and over again, but you can use compression for a lot of things. You can create a punchier track, your can use it have a track sit on top of the mix, you can even use it on the entire mix to glue it all together.

What Are All Those Knobs For?

In order to properly use a compressor, you’ll need to gain an understanding of the terms and functions available on a standard compression effect or plug-in. Most plug-ins feature the same settings, though some are simpler than others.

Threshold

Possibly the most important knob on any compressor is the threshold. The threshold is what allows the compressor to compress the track and control how much compression is applied.

If the threshold is at zero, you can do whatever you want to the other knobs and nothing will happen. No compression will take place. Threshold is essentially just the volume at which the compressor turns down the signal.

So, if you turn the threshold down just a little bit, it will only affect the very loudest points in the track. This is referred to as a very soft or light compression.

If you turn the threshold down further, the signal will get even more squashed. It will begin to affect the loud peaks as well as the peaks that are quieter. You can squash a signal to the point where it no longer has any dynamics.

You can usually tell how much compression is being applied by looking at a Gain Reduction meter.

Sometimes there’s an In and Out meter, and the Out meter will show how much the volume is being reduced by the compressor.

Generally, for a nice natural sounding compression, you should be looking for around -3db to -5db of gain reduction. However, this is totally subjective. Certain instruments and techniques may involve over-compressing it.

Ratio

This knob was always a little bit mysterious to me. I had read a few tutorials and found through trial and error that a 2:1 ratio tended to sound the best, but I still didn’t really know what it was doing.

However, it’s actually not very complicated. The Ratio knob works in conjunction with the Threshold. The Ratio determines how hard the compressor will kick in when the volume crosses the threshold.

So, a 1:1 ratio means the signal doesn’t get turned down at all when the volume crosses the threshold. A 2:1 ratio means the compressor will turn the signal down by half when it crosses the threshold.

The higher the ratio, the harder the compressor will work. You can experiment with the ratio and the threshold to get as much or as little compression as you need.

For example, you can use the threshold to set when the compressor kicks in. Then, you can use the ratio to fine tune how much you want to turn down the signal when the compressor kicks in.

Attack And Release

So what we’ve learned so far is that the threshold and ratio determine how much compression occurs and to which part of the track. The attack and ratio determines when the compression kicks in and kick out.

Attack and release are great for subtly shaping the tone of a track. It can thicken up a kick, make a snare sound longer, and create a bass tone that is consistent over the course of a song.

The faster you set the attack, the faster the compression squashes the audio when it crosses the threshold. The faster the release, the faster it lets go of the audio.

Attack and release are fun to fiddle and experiment with once you’ve figured out what the threshold and ratio do. The changes in the audio can be fairly obvious if you’re making big changes in attack and release.

To start hearing what these knobs do, set both in the middle, and set a fairly hard hitting compression with the threshold and ratio. Then move the attack and release around, and you’ll start to hear what sounds good on different tracks or instruments.

I often end up using very slow attack times on things like drums and the master fader. It gives a sort of gluing quality to the whole mix.

Makeup Gain

Sometimes just called “gain”, the makeup gain will turn up your whole track after you’ve compressed it. The reason you want to do this is that everything you’ve done so far is turning down the track. So you’ll need to use the gain to turn it back up to where it needs to be.

Generally speaking, the goal is to get the track back to the volume it was at before you started compressing it. This way you can use the volume fader to put the track back where it needs to be.

The other reason you’ll want to return the track to its original volume is to make sure that your compression moves were effective. Our ears are easily fooled by “louder is better”. Turning the gain up to the original track volume will let you bypass the plug-in to see what’s actually going on.

The Five Most Important Knobs

Threshold, ratio, attack, release, and gain will be your main tools when wielding a compressor. Some compressors will only have these knobs to fiddle with, however, some will have a few more. Until you’ve learned what the five most important ones do, don’t worry about the others.

That said, if you’re curious about what the ‘knee’ does, read on.

So, There Are Other Knobs…

As I said, there are a few other features on a compressor that are less important, more subtle, but worth knowing. I found it frustrating being completely in the dark about what exactly these other functions do, so I’m here to tell you.

The Mysterious “Knee”

Ah. The “knee”. What does this knob do? I swear to you, I’ve fiddled with the knee knob over and over again but couldn’t really hear the difference it was making. But once I found out what it was doing, I started to hear the difference it was making.

So, in the Pro Tools stock DAW compressor, you’ll find a handy little diagram. In it, the orange line represents the threshold. The diagonal line heading towards the orange line represents the audio before compression, and the line after represents the audio after compression.

The higher the ratio, the flatter the post-compression line would become.

As you might expect, given its name, the knee in this diagram is where the white line crosses the the orange vertical line. The white line bends at this point (like a knee).

The knee determines how curved the line is. If the knee is set to 0, then there is no transition from uncompressed to compressed. If you wanted the compression to slowly kick in, compress it lightly when it first crosses the threshold, and compresses it heavier as it gets louder.

As with most of these functions, it’s something you’ll need to fiddle around with. Typically, a low knee on things like drums and rhythm guitar works better. It creates a more upfront sound, and it doesn’t really matter if the compression is somewhat obvious.

On vocals, however, a more drastic knee can make the compression smoother and more natural sounding.

It’s worth noting that a knee is not necessarily going to make or break the track. Sometimes compressors don’t even give you this option. The knee can be a great tool to fine-tune compression on important tracks.

Key Input & Sidechaining

If you’re like pretty much everyone who has ever tried to mix their own song, you’ve probably struggled with getting your kick and bass to play nicely together. They share a lot of the same frequencies and can easily step on each other – especially on different sound systems.

By using the key input and sidechain, you can have your kick drum trigger a compressor on the bass. That means that every time the kick drum hits, it will literally turn down the bass slightly. It’s a very valuable tool that can get your kick and bass working together quickly.

The key input is where you plug in the track you want to trigger the compressor. Put the compressor on the bass, and then set the key input to be the kick drum.

Then, you can set the compressor as you would any normal compressor, except that it is only triggered when the kick drum goes over a set threshold.

It’s not as hard as it sounds! The key is to use it subtly. In most cases, you don’t want the bass becoming audibly louder and softer in the mix. The only time this is used is in electronic music for effect.

The other benefit of this feature is that it can save you time on the editing side. If the drummer and the bassist aren’t sitting in the pocket, you can use this feature to gently create a pocket where the bass and kick are working together.

Gentle sidechaining will take some getting used to. I have destroyed a few mixes with aggressive sidechaining simply because I didn’t know what I was doing. Thankfully, the results are fairly obvious, and you can bypass the function and fiddle around until you're happy with the results.

Final Thoughts

Over time, your instincts will tell you what sounds right. Everybody talks about book smarts, but making a great mix is as much a process of intuition as it is a process of science. You'll see what I mean as you experiment with your mixes.