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What is an Audio Compressor?


Learning to use an audio compressor in today's home studio is one of the most important steps you can take to improve your music productions. It was one of the very first things I learned about (alongside reverb).


A vocal take, a drum performance, a guitar recording - any musical performance has a natural range in volume, from the quietest part to the loudest part. This is known as the dynamic range.


But when the time comes to mix your track, these volume ranges can become a bit of a problem. Imagine these large volume changes spread over 20 or 30 different recorded tracks as well - your whole mixing effort would be a constant fight to get the volumes of all your tracks to sit together as a whole.




What is Audio Compression?


So what is audio compression and what can it do to help? It's a common question I hear all the time.


Basically, compression reduces the dynamic range of your recording by bringing down the level of the loudest parts, meaning the loud and quiet parts are now closer together in volume and the natural volume variations are less obvious.


The audio compressor unit can then boost the overall level of this compressed signal. So the end result is that the quieter parts sound like they've been boosted in volume to be closer to the louder parts.


The dynamic volume changes of a recording are now under more control, and a knock-on effect is that the overall level of the compressed recording can be increased inside your mix. The recording will also sit inside your whole mix much more easily.




Compression Controls


The compression device itself has many different controls that can have an effect on the sound you're processing. I'll run through the main controls that are commonly found.


Input Gain

This controls the level of the signal going into the audio compressor.


Threshold

Compression brings down the overall level of the loudest parts of your recording. But how does the compressor know which part of the signal is 'loud' and which part of the signal to compress? By setting the threshold.


The threshold sets the level at which the compressor kicks-in and starts changing the dynamics of the recording. So for example, if you set your threshold at -20 dB, everything below this level will not be affected by the compressor. But everything louder than this level (-20 dB) will be compressed.


Ratio

How much will the signal be compressed once it's gone over this threshold level? This is controlled with the ratio. The higher the ratio, the more compression there is.


The easiest way to show you how the ratio works is by showing you some numbers.


  • If the ratio is 1:1, there is no compression at all.
  • If the ratio is set at 2:1, for every 2 dB of sound that goes over the threshold, you get 1 dB of output above the threshold. So if the signal goes over the threshold by 10 dB, the compressor reduces this signal so it's now 5 dB over the threshold.
  • If the ratio goes up to 8:1, for every 8 dB of sound over the threshold you would get 1dB of output above the threshold. So if the signal goes over the threshold by 16 dB, the compressor reduces this so only 2 dB goes over the threshold.



Graph showing the link between the threshold and the ratio settings on an audio compressor

 



Attack

This is the time that the compressor takes to act on the input, once the sound level has gone over the threshold level. It's usually measured in milliseconds (ms).


Release

This is the time that the compressor takes to let the signal return to normal once it has fallen below the threshold level. Again, usually measured in ms.


Output Gain (Make-up Gain)

If the audio signal has been compressed, the overall level of the signal will be reduced. Increasing the output gain raises the level coming out of the compressor, so the volume can be more easily matched to the levels of the rest of your tracks in your mix.


Knee

Soft-knee compression is gentler on the sound as it goes through the audio compressor - the change from uncompressed to compressed sound is smoother. Hard-knee compression is a more immediate and obvious effect.




Final Thoughts


The compressor is an essential device for your studio. But before you use one, it's important to understand the names of the various controls, how they work, and how they interact with each other.


Once you have this understanding, using an audio compressor in your studio becomes much easier and can lead to both technical and creative applications of this very important piece of equipment.



 

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