What Is Audio Compression?
Audio compression is the process of reducing your audio’s dynamic range. By reducing the dynamic range, you even out the loudest & quietest parts of an audio signal, giving a ‘glued’/’squashed’ sound.
Take a vocal track, for instance. Vocalists often have a huge dynamic range, & can go from extremely quiet parts to bellowing loud. To make the take sound more natural, and consistent, we can even out these changes using a compressor so they are more similar in volume.
From the two signals above, you’ll notice just how harsh audio 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.
Audio Compression Controls
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 gain reduction will occur (where the compressor starts squashing the sound).
So, if we were to set it at -22db (decibels), for instance, gain reduction would occur & squash the audio signals whenever they go 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 lower the gain reduction. 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 and Release Controls
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 level has been reached. 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 after the compressor has reduced the gain. 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 level of your audio fed into the compressor. It’s basically just turning up the un-compressed signal, so the audio compressor can ‘hear’ it more & apply compression more accurately.
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.
When To Use Audio Compression
Now we understand what an audio compressor is, 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 (unless you want a destroyed sound). 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.
The Free Compressor Fact Sheet
How To Use A Compressor
After understanding how compressors work, 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 in your audio. You’ll want the output level to be the same volume as the input, so you can bypass and activate the compressor to see how the audio has changed.
Think of it as an A/B split test.
Set The Ratio
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.
Set The Threshold
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.
Attack and Release Times
Attack and release are controls for how fast compression reacts & how fast it stops reacting.
Set the attack time. Your attack is how fast your compression acts. So, if you want your compression to slam in, have a quick attack time. If you want your compression to come in slower, preserving peaks of drum transients for instance, have a slower attack time.
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 knee. 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 compressor has made in your sound & determine whether it’s needed or not.
source: Ask Audio
Compression is pretty subjective to your mix. You need to listen to your sound, & what the difference is with a compressor on & off to determine if you *actually* need it or not.
Buuuutt it’s good to have some examples to work from. So below we’ve left some compression examples you can try out in your music production sessions.
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) 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 weak area for a lot of music production beginners, so only use it if you know what you’re doing.
If you don’t, listen to & work out how it changes your music. Don’t be afraid to twist all the knobs & push the threshold & ratio high. If you do this, you’ll be able to hear how it’s affecting your music more easily.
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. We can then blend the two to get a wicked punchy sound.
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, so we’ll need to send it to a bus, otherwise it will sound horrible.
Blend the bus signal in with the original signal.
You can also check out our how to mix drums tutorial for a more in-depth guide on Kick Drum compression & use of soft clippers etc.
Compression is great to use on bass to give a thicker & tighter sound to your low end. Check out our how to compress bass guide for a full guide on how to get your bass sounding thick & full!
Sidechain compression can be used to make a pumping effect in your tracks. If you’ve ever listened to House Music, you’ll know this is an extremely crucial element to getting things to ‘bounce’.
Sidechaining is usually added to melodic elements, like the bass, chords or lead lines. And, the trigger for the sidechain is almost always the kick drum or snare drum.
To sidechain everything to your kick, you would select the kick audio as your sidechain input.
You would then increase the compression ratio a lot (inf:1 if you can). After this you lower the threshold.
As you’re playing the kick & side chained chords – you will hear it duck more as you increase the compression ratio & turn down the threshold.
Set your attack and release: attack at around 5ms & a quick release of 20-50ms.
If you hear a click, it’s because your attack is too fast. Play around with the attack time until this disappears.
Audio compression is the process of reducing the dynamic range of the loudest and quietest peaks in your audio signal. When applying this compression it ‘squashes’ you audio closer together, causing gain reduction. This gives a more consistent volume overall & even out recordings.
Remember to only use compression when you know what the purpose is, & when you can hear how it’s affecting the audio.
Compression is great for evening out vocal takes, getting thick, solid bass & is incredible for making drums pop through a mix.
If you want to learn about soft clipping (great for punchy drums), check our what is soft clipping article.