An easy to understand guide to compression & compressor plugins

Dynamic range compression is a fundamental part of mixing music. And yet, it’s one of the most commonly misunderstood parts of the mixing process. So in this lesson, we’ll break down what audio compression is, and how to use a compression plugin in a way that’s easy to understand.

Be sure to download your free compression cheat sheet here!

What is compression in sound?

Compression is used to reduce an audio signal’s dynamic range. Or in other words, to reduce the difference between loud and quiet. Most commonly, compressors do this by turning the loud parts of a performance down so that they are closer in level to the quieter parts.

But why would you want to do that? Well, though there are numerous instances where compression can come in really useful in a mix, I most often use compression for three things…

What is audio compression used for?

1. Balancing levels with a compressor

The first thing that I use compression for is to balance out the level of a signal so that its level is more consistent and less varied. Think of a vocal track where the singer performs some parts of a song loudly, and other parts quietly. Without compression, during the louder parts of the performance, the vocals will get too loud for the mix. You could turn the level of the vocals down so that these loud parts sit at the right level, but then the quieter parts of the performance will play too quietly.

This can be the case with any instrument with varying levels. Think of a snare track that has a mixture of accents and ghost notes, for example. So applying compression to the signal will turn the louder parts down, making the level more balanced.

2. Achieving punch with a compressor

Another thing I like to do with compression is to use it to add punch, energy, or excitement to an instrument. You do this by altering the relationship between a signal’s attack and decay. To give a track more punch, you use the compressor to turn down the decay portion. This makes the attack more prominent and thus makes instruments like drums sound more punchy and gives guitars and basses more energy. Find out how to do this here.

3. Fattening tracks with a compressor

Thirdly, something that I often use compression for is to make signals sound fatter or fuller. Once again, you do this by reshaping the relationship between the attack and decay. This time, you use the compressor to turn down a signal’s attack so that it is closer in level to the decay. This makes instruments sound fatter and fuller. Find out how to do this here.

How to use a compression plugin

So, how do you use a compressor? Well, the great thing about compression is that the vast majority of compression plugins are controlled using the same six parameters. So no matter which compression plugin you’re using, their controls are largely the same. Let’s look at what each of the parameters on your compression plugins do…

Threshold

The threshold setting on your compressor lets you control which parts of a signal are compressed, and which are not. Any parts of a signal that are louder than the threshold will be compressed. And any parts of the signal that are quieter than the threshold will remain uncompressed. So it’s your way of telling the compressor which parts of the signal are too loud and need to be turned down, and which parts should be left alone.

Depending on the design of your compressor, you’ll interact with the threshold in one of two ways. Often, your compressor will have a threshold parameter which allows you to dial in the threshold setting. But some compressors utilize the threshold differently. Rather than giving you control over the threshold, some compressors have a fixed threshold, and rather than a threshold parameter, they provide you with an ‘input’ parameter. You then use the input parameter to control how much of the signal crosses the threshold.

Click here for a video lesson on your compressor’s threshold:

Ratio

The ratio setting on your compressor lets you control how much the compressor reduces the level of the signal. So any part of the signal that crosses the compressor’s threshold will be turned down by the ratio amount.

The ratio will be set out like this: 1:1, 2:1, 3:1, etc.

A ratio of 1:1 means that there is no compression.

2:1 means that the part of the signal overshooting the compressor’s threshold will be turned down to half its original level. So if the signal overshoots the threshold by 4dB, then the overshoot will be reduced to just 2dB.

4:1 means that any part of the signal overshooting the compressor’s threshold will be turned down to one-fourth of its original level. So if the signal overshoots the threshold by 4dB, then the signal will come out of the compressor just 1dB louder than the threshold.

As such, the ratio allows you to control how much the part of the signal that is being compressed is turned down.

Click here for a video lesson on your compressor’s ratio:

Attack

The attack setting on a compressor controls how quickly the signal is turned down when it overshoots the threshold. As soon as the signal overshoots the threshold, the attack phase begins and the compressor starts to turn the signal down. The faster the attack setting, the faster the signal is turned down. The slower the attack setting, the more gradually the signal is turned down.

Attack time is generally defined in milliseconds, usually ranging from around 0.010ms to 250ms. Faster attack times are great when balancing signals or when you want to make tracks sound fatter. Slower attack times are great when you want to give a track more punch.

When setting attack times, it pays to keep in mind that if the attack is too fast, then it may cause the signal to lose its impact. On the other hand, if the attack is too slow, then the signal might fall back below the threshold before the compressor has been able to turn the signal down a noticeable amount. So listen closely when setting your compressor’s attack time.

Click here for a video lesson on your compressor’s attack:

Release

Knee

Next is the knee setting. Not all compressors give you control over the knee. But if they do, then it’s a great way to fine-tune the way your compressor applies compression to audio signals.

We generally refer to a compressor’s knee setting as being either a ‘hard knee’ or a ‘soft knee’.

With a hard knee setting, any part of the signal that crosses the threshold is compressed by the full ratio amount. A soft knee, however, is different. With a soft knee, the compressor starts to apply compression as the signal approaches the threshold by applying a low ratio amount of compression. And the full ratio amount is not reached until a point past the threshold.

Some compressors give you a button to toggle between hard knee and soft knee. Others let you dial in the knee setting using a decibel value, allowing you to dial in the decibel range over which the compressor builds from 1:1 to the full ratio amount. The threshold usually sits at the centre of this transition zone.

Hard knee settings work well for instruments like drums which have fast peaks. Soft knee settings work well for more fluid sounds like vocals, which often benefit from a more gradual transition between compressed and uncompressed.

Click here for a video lesson on your compressor’s knee:

Makeup Gain

The final of the six most common parameters on a compressor is the makeup gain setting. This might sometimes be labelled as ‘output’.

This is used to control the level of the signal coming out of the compressor. Usually, the signal coming out of the compressor won’t be as loud as it was when it went in. That’s because the compressor has turned parts of the signal down. So the makeup gain setting allows you to control the level of the compressed signal that is coming out of the compressor.

If your compressor doesn’t offer this control, then it probably sets the level of the output automatically to match the input.

Using compression in your mixes

So, whether you want to use compression to give signals more balance, to add punch, or to make tracks fatter, you can achieve all 3 using just the threshold, ratio, attack, release, knee, and makeup gain parameters on your compression plugin.

For a full guide on giving your tracks balance, fatness, and punch, be sure to download your free compression cheat sheet here.

Other compression settings

If you’re looking at your compression plugin, and you have some other options available, or, if you’re already confident with applying compression using the six main compression parameters, then check out how to utilize some of the other options that your compressor might offer, such as the hold parameter, the look ahead parameter, peak/RMS options, or sidechain options.

Finally, what do you use compression for in your mixes? Do you use it to balance signals? Do you use it for fatness or punch? Or do you use it for something entirely different? Be sure to leave your thoughts, comments and questions in the comment section below.