What’s That? Compression Pt.1

Compression is one of the most valuable tools in the audio engineer’s arsenal, making the foundations of a great sounding mix alongside EQ. However, compression can be quite a hard concept to grasp - especially when the market is saturated with compressors in so many different forms. So hopefully this “What’s That?” series will be of good help to anyone new to this!

The Fundamentals - What Actually is Compression?

Just to be clear - we are talking about dynamic range compression here, not audio codecs!

Dynamic range compression is a process of reducing the peaks of loud audio (or boosting quiet troughs) in order to achieve a more consistent overall volume, therefore compressing the dynamic range of an audio source. We do this to make instruments “sit” nicely in the mix, so that they won’t be lost when the bassist is playing softer, or be overbearing when the guitarist decides to aggressively strum accents in a chord progression.

Of course, another effective way to do this is to go over every single note and slightly adjust the volume to stay consistent. However, that is very time consuming. Compression is also able to change the very fine details of a sound too, for instance how much attack or sustain a snare drum will have. That’s a more complicated use that I will go over later!

How Do You Use One of These?

A stock compressor that comes free with Presonus Studio One

Many vintage compressor emulations have different quirks as to how they operate, but pretty much all (downwards) compressors rely on the same principles as the ones shown here.

Threshold: The volume at which the compressor starts compressing. On the compressor shown above, any audio above -24dB will start to be compressed. Anything quieter than -24dB just remains the same.

Ratio: Determines how much compression is applied to the signal above the threshold. 1:1 ratio won’t do anything. A 4:1 ratio like shown above means that for every 4dB of audio over the threshold, the compressor will only actually output 1dB more.

Attack: How fast the compressor reacts to audio above the threshold. A slow attack is like the compressor is slightly drunk … so if the audio is quick enough, it can go past -24dB without getting caught. Especially useful for transient material like a snare where we want to try and preserve the initial hit.

Release: How fast the compressor lets go of the audio when it is back down below the threshold. A faster release is more aggressive and generally brings out the tail/ambience out of a sound out more, where as a slower release generally helps the audio sit more smoothly back into the mix.

Knee: Mainly a feature on modern compressors, the knee feature affects the gradient as to when the compression acts at the threshold. The sharp knee shown above means that the compression will act strictly only above -24dB. A soft knee means that the compression would be eased into the signal around -24dB.

Hopefully this covers the basics of compression, if you would like to know anything further then please don’t hesitate to comment down below!

In part 2 I’ll delve into vintage compressor models & their moden fancy counterparts.

Thanks for having a read :)