What is compression in music?
When starting out on your production journey compression can be pretty confusing. Knowing when to use it & how it acts on your mix is difficult and it’s often crucial to getting a coherent mix or master.
We know for sure it was something that we found extremely baffling when starting out, so we’re here to help if you feel a little bit lost.
In this article we’re going to cover the basics of compression in music, in an easy to understand format to help demystify its uses and uncover its true power & potential.
What Is Compression In Music?
Compression is the process of reducing the dynamic range of the loudest and quietest parts in your audio signal. Essentially, when you’re applying compression, you are squashing the loudest and quietest parts of your audio signal closer to each other.
By doing this, it reduces the distance in volume between the lowest and highest points of your signal, giving it a somewhat ‘glued’ or ‘squashed’ sound.
From the two signals above, you’ll notice just how harsh compression can be if we over-use it. It can cause a mix to sound boxy or lifeless if we don’t use it correctly. So, that’s why we also need to understand how much to use & when to make sure our mixes sound incredible.
To give an analogy, compression is a bit like cooking: use too much spice and you’ve ruined your meal, but use enough and all the flavours combine together to make an incredibly tantalising, cohesive experience.
So what are the controls on the compressor and what do they do?
WHAT IS COMPRESSION IN MUSIC: Compression Controls
In this section you’ll find a list of all the compression controls that can be used to change your sound.
In order to understand compression and mixing fully, we recommend checking out this mixing a track from start to finish guide.
On a compressor you’ll commonly find:
Threshold is how loud the signal has to get before the compressor begins to act on it. The value which you set this, is the level at where the compressor will begin to do its magic. So, if we were to set it at -22db (decibels) for instance, the compressor would begin to squash the audio whenever the signal goes above -22db.
Ratio is how much compression is applied to the signal after it passes the threshold value in volume. The lower the number (1.5:1, 2:1, 3:1), the less compression that will be applied. The higher the number (5:1, 7:1 etc), the more compression will be applied.
Let’s say we have a 3:1 ratio, for every 3dbs your audio goes over the threshold value, it will only allow 1db to pass.
If that audio was to go 6dbs over the threshold, with the same ratio of 3:1, then only 2dbs would pass. With a 3:1 ratio, the volume is reduced by a factor of 3.
Attack is much like attack in the ADSR of a synthesiser. It’s how quickly the compression is applied to the audio signal.
Release is, you’ve probably guessed it, how quickly the compression stops after the signal falls under the threshold in volume.
The knee of your compressor is how it reacts to the audio signal, when the threshold volume has been hit. It’s often described as how smoothly your compression kicks in after the threshold is hit.
A hard knee will squash the signal straight away, whereas a soft knee will bring the compression in more gently.
Make-Up gain gives you the ability to boost the signal back to it’s original volume with the applied compression. When you compress, the volume will often dip significantly, which means you need to bring it back up to an audible area. Make-Up gain is your friend here.
Input is the volume of your audio as it’s first going into the compressor. It’s basically just turning up the un-compressed signal, so the compressor can ‘hear’ it more.
The output of your compressor allows you to boost the level of your signal from the compressor. It’s basically just turning up the compressed signal.
WHAT IS COMPRESSION IN MUSIC: When To Use Compression
Now we understand what a compressor is and how it reacts with our audio, when is a good time to use it? Like with everything in mixing, this question is subjective to your mix and what your goal is.
Your compression should preserve the transients in your audio signal, not destroy them. You want to have a good level of dynamic range throughout your mix to add depth and character to it.
The loudness of a track often translates to high energy and low energy points. If your mix was the same volume throughout, there would be no difference between the chorus and breakdown in terms of volume and it would make the piece more static.
We’re looking for full, dynamic mixes that sound cohesive and glue together nicely, not brick walls.
When you’re looking to add compression to your mix you need to ask yourself why you’re doing it.
What is the purpose of compressing your sound?
Is it to make it thicker?
Add more punch?
Even out the changes in volume?
Or even, to completely destroy the transients and blend them in to add grit?
If you don’t know what the compression is adding or doing to your mix, then don’t add it! Take it off and only use it when you need to. Over compression is a big problem in a lot of people’s mixes and it happens because they don’t know what purpose their compressor serves.
Always trust your ears and listen for what changes compression is making.
WHAT IS COMPRESSION IN MUSIC: How To Use A Compressor
After understanding what it is you need compression to add to your mix, you need to know how to properly set one up.
So here’s your step by step guide to setting up a compressor:
Insert Your Compressor
Insert the compressor on the channel or bus that you want to apply compression to. Here you can change the input level going into your compressor, if the signal is too low.
Turn Off Any Auto-Gain
Make sure that any make-up or auto-gain is set to manual. If you don’t, your audio will be louder and it will become more difficult to hear any difference the compressor has made. You’ll want it to be a similar volume, so you can bypass and activate the compressor to see how the audio has changed. Think of it as an AB split test.
Set the ratio. This is how much your compressor will duck the volume of your audio signal. Start with a 2:1 ratio and adjust after setting the threshold to hear the differences it makes. Do you need to heavily squash your signal, or just add light gluing to make it sound more cohesive? Use higher ratios for squashing and lower ratios for gluing.
Start listening to your audio signal and begin to adjust the threshold, until you see the compressor working. You’ll notice this if you see a needle or bar moving up and down – this is the compressor reducing the gain.
Start adjusting your attack. Your attack is how fast your compression acts, so if you want your compression to slam in, have a quick attack. If you want your compression to come in slower, preserving peaks of drum transients for instance, have a slower attack.
A quick attack will deal with sudden peaks, a slow attack will apply compression after that sudden peak has come through.
Start changing your release time. It’s important to get your release right because it can change your sound massively. A short release can make your compression sound unnatural and digital, whereas a slow release will make it sound smoother. Short releases are often good for drums.
Set the knee dependent on the sound you are compressing. This is how hard or soft the compression will be. If you want it to slam in and sound mechanical, then go for a hard knee – it’s good for drums, bass etc. If you want your compression to be a bit more subtle, go for soft. Soft knees are good for vocals, master busses etc.
It’s time to add that gain in manually. You want to make sure that the gain you add is only what you removed by compressing it. This allows you to check the difference the compression has made in your sound.
WHAT IS COMPRESSION IN MUSIC: Examples Of Use
When using compression it’s entirely subjective to what you are trying to achieve. Make sure you are listening to your audio before to understand whether it really needs compression, or you’re just adding it for the sake of it/someone told you to.
Nevertheless, we know it’s always good to have some examples of compression use, so you can learn.
Using compression on vocals is almost always necessary. When vocalists perform, they have a huge dynamic range which means their volume can go from high to low quickly.
In this example of vocal compression, we want to even out the transients and reduce the dynamic range. A good place to start with vocals is a slow attack, slow release, 2:1 ratio and adjusting the threshold to the level where you want your compression to act. Doing this, will even out our vocal take and make it sound more cohesive.
In certain situations, you’ll need to use more than one compressor. If there are quick peaks in volume, you want to nip these spikes in the bud before evening out the entire take.
To do this, you can use a short attack (10ms), medium release (50ms) and a medium to heavy ratio (6:1) compressor to stop these from cutting through in your mix.
Remember to play around with the settings and listen to what different settings do to your sound. Over compression is a big problem in a lot of mixes, so only use it if you know what you’re doing.
Compression on drums can add a lot of punch and well needed grit, if done right. By using a technique called parallel compression, we have our drums as both a dry signal and wet signal, sending them to a bus and then applying compression.
In this case we would use a heavy ratio (8:1), fast attack (10-30ms) and a hard knee. Applying these settings to our compressor, we get an extremely squashed sound on the bus. The bus is then blended in to add underlying grit to our drum sound.