An Introduction To Compression
 - By Mo Volans

In this series of features I’ll be focusing on one of the corner stones of audio production technique; dynamics. Although most of us use a dynamics processor, in the form of a compressor or limiter, in every session it’s always good to go back to basics and get to grips with what’s going on under the hood.

In this first installment I’ll be looking at compression; namely how it works, when to use it and it and the parameters you can expect to find on a typical compressor. The beauty of using something as straight forward as a compressor is that once you master one model you should be able to use them all.

The plug-ins I’ll be using in every instalment are all from the Plugin Alliance website and are available as fully functional 14 day trials.

What Is Compression?

At its most basic compression can be thought of as an automatic level control. To help visualize how this works imagine a vocal performance that is well recorded but contains loud and quiet sections. Although the quality of the audio may not be in question to get the best out of the recording we need to address the level issue.

One way of dealing with this is to ‘ride’ the fader to ensure that the vocal is at an optimum level and relates well to other mix elements at any point in the project. This can also be achieved using automation. Amongst other things, a compressor that is set up correctly will essentially perform this function for you.

When used creatively compressors can be used for many things besides level adjustment but originally this was their primary purpose. By reducing dynamic range using compression, recordings can be made more uniform ultimately making them easier to mix and often more pleasing to the listener.

A Compressor’s Controls

Of course not all compressors are identical, you’re going to see different parameters, metering and design from one model to another; however once you have mastered one model it’s very likely you’ll be able to quickly adapt your skill set to another.

One thing is for sure the vast majority of compressors are going to feature at least some of the same parameters. The main controls you’ll see across the board are Threshold, Ratio, Attack and Release. On top of these you are likely to see some kind of input and / or output gain. Some basic metering showing, at the very least, gain reduction is probably going to make an appearance on most compressors.

Starting at the top; Threshold tells the compressor at what point it should start applying gain reduction. As you lower this control you allow the compressor to ‘catch’ the peaks of the transient events in your audio. Setting the threshold correctly will mean the compressor is only triggered when the louder sections of your recording are played back.

The next parameter - Ratio- shares a direct relationship with the threshold control; The ratio essentially decides how much gain reduction is applied once the threshold is reached. A typical ratio setting of 6:1 would result in an input signal that is 6db over the current threshold setting outputting at 1db over the threshold.

An example of more mild compression would be a compressor set to a 2:1 ratio. In this case when the incoming signal rises by 10db over the threshold the compressor will do its job ensuring that the output will only rise by 5db, effectively halving the level.

Attack and Release determine the speed at which the compressor applies gain reduction and in turn recovers to it’s original state. If present these controls are especially useful when working with percussive sounds. For example the snap of a snare drum can be tamed or the room sound in a tom recording can be allowed to pass through the compression process unadulterated.

Some compressors may lack one or both of these controls. In this case its highly likely that the attack and release settings are what’s known as ‘program dependant’. This basically means that some clever electronics are setting the attack and release times for you depending on what you feed the compressor with. This fire and forget approach can work very well with vocals for example but if you need find control over these settings you may want to look for a model with attack and release present.

Creative Compression And Beyond…

This is a basic outline of what the average compressor is capable of and the controls you can expect to fine but of course this is just the tip of the transient - so to speak! Compressors can be pushed well beyond their intended purpose and will produce all sorts of cool effects at the outer edge of their capabilities.

In future installments of this series we’ll be looking at things like parallel compression, how to ‘slam’ your compressor, limiting vs compression and buss compression and multi-band processing. Stay tuned as we dig deeper and progress onto the more advanced aspects of compression.

While you're waiting for the next one you can grab 14 day demos of the compressors featured here from the Plugin Alliance website!