A Beginner’s Guide to Compression: Part 1 – The Basics

Compression is possibly the most misunderstood tool in the music production toolbox, but also one of the most useful. Really understanding how compression works is essential to using it effectively, and good use of it is at the heart of the way probably all of your favourite records sound.

Here then, is our back to basics guide for those that could do with a little more knowledge, and those that could do with a reminder.

What is it?

The most basic function of a compressor is to act as an automatic gain rider. You might hear it said that ‘the most transparent compressor is you’.

That’s because at its most practical, a compressor is simply an automated way of carrying out fader rides to even out a level. If you ever do any work in broadcast or film, this is the common use of a compression – using it to make sure nothing goes over a pre-specified level.

The consequence of pushing the loud bits down is that the dynamic range is reduced. If you then amplify the compressed signal, this will have the effect of bringing up low level detail as well as reducing peaks. Ironically, by pushing high levels down you can make something louder and/or achieve that mythical ‘thickness’. This is the one of the more common uses in music production – particularly in mastering.

How does it work?

To understand how a compressor works, let’s take a look at the controls you’ll find on nearly every compressor:

Threshold is where you set the level at which the compressor will act. So if you set a threshold of -20dB, any signal that reaches that level will be affected by the compressor. Anything below that will remain untouched.

This is why it’s important to always tweak the threshold on compression presets – the threshold is entirely dependent on the signal you feed into it. If you’ve recorded something quiet and the preset has a threshold of -5dB, it’s not going to do anything.

Ratio specifies how much the gain is reduced by once it hits the threshold. So, if you’ve got your threshold at -20dB with a ratio of 2:1 and you have a -18dB signal, (2dB over the threshold) the compressor will reduce the level by 1dB (to -19dB). If the signal is 4dB over the threshold, a ratio of 2:1 will reduce it by 2dB.

Attack specifies how quickly the compressor will act once the signal goes over the threshold. Leaving a longer attack time will let more transients (the initial ‘attack’ portion of the signal) through resulting in a more natural sound, a shorter one will allow less and result in a more ‘squashed’ sound.

The release control sets how long it will take for the signal to return back to its original level after the compressor has acted. How you set this really depends on what you’re trying to achieve. If you’re trying to even out a level, longer times (200-300ms) are best, but if you’re just trying to address peaks, shorter release times are more appropriate.

Makeup gain/output
This control sets how loud the final compressed signal is outputted. It is often called Makeup Gain as a reference to how you are making the finished signal louder to compensate for reducing the loud portions of the signal.

Understanding how these controls work will go a long way to effective use of compression. The trick is to know what you’re trying to achieve first, and knowing how to achieve it second.

Next time we’ll look at common uses of compression in music production. If you have any questions, post them in the comments!

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