The Cambridge Dictionary defines compression as the act of pressing something into a smaller space or applying pressure from multiple sides until it becomes smaller. If we talk about audio, compression is the process of reducing the volume of a signal.

But in the same way when something is compressed into a smaller space, it’s forced to change its shape, the same happens to the audio signal. That’s why compression is much more than controlling audio peaks. It’s about changing the audio qualities of a sound, affecting its envelope and perceived volume. It’s an art form in itself that changed the way recorded music sounded.

In this article, we will look at why compression is an essential tool in music production and how it’s used to create the sound of records.

Compressor 101: How Audio Compression Works

A compressor is a device that helps even out elements in the mix so they are easier to balance. This way, no elements unintentionally jump out in volume, and we don’t have to manually adjust their volume constantly.

You probably have read this a million times, but a compressor works by setting a threshold level that triggers volume reduction whenever the signal exceeds it, and it stops when the volume falls below the threshold. Simple.

It gets tricky when we talk about how the compressor reacts. That means how fast or slow the compressor reduces the volume, how much it’s reduced, and how fast or slow the signal returns to the original volume when it drops below the threshold. These are what we call attack, ratio and release. Setting these 4 parameters will affect the sound envelope and, therefore, its tone and presence.

On top of all this, we also have makeup gain, a volume knob in the compressor that restores perceived volume to its pre-compression level. This control emphasizes the compressor’s effect, especially when it’s applied aggressively.

But why do compressors sound different from each other? Let’s find out.

Behind the Controls: Types of Audio Compressors

No two compressors are designed equally. In fact, the different technologies and topologies used in the original analogue compressor units imparted a specific color to the audio signal, leading engineers to use this equipment as color boxes as well as levellers.

How the sound level is detected, how gain reduction is applied, and where the side-chain is in the circuit affect the sound of the compressor. Some of the most classic compressor types are VCA, optical, FET, or vari-mu compressors, but they can also be described as feed-forward or feed-back compressors, depending on when the circuit’s controller receives the audio signal. Each of these types of devices compresses and changes the tone of the sound signal differently.

VCA and FET compressors respond faster than optical compressors because of differences in their detection circuits. On the other hand, as we explained in our article TuCo: Behind the Scenes, vari-mu compressors are program-dependent, meaning the ratio varies with the amount of signal that exceeds the threshold.

Modern digital technologies have allowed programmers to emulate all these compressors and bring them into our DAW sessions. Not only that, but in some cases, they even improve their performance and capabilities through modern digital workflows. An example of this is Sonimus’ TuCo and Satson Compressor. These not only emulate specific compressors but also bring them into the digital era with new features.

Compressors: saturation and distortion in mixing

Every time we change the shape of a signal, we are distorting it, and as we mentioned at the beginning, compressing audio will cause a change in its envelope, resulting in the addition of harmonics. As the compression increases, it starts causing a smooth saturation, but a heavy compression will produce a distorted sound, which we all love when we do parallel compression.

On top of that, some compressor units have colorful amplifiers with tubes or transformers on their circuit that add extra harmonics when the signal is processed. This way it is easy to see how you can use compressors for much more than just levelling signals.

Some engineers use compressors to add color, distortion, or just to thicken up the signal passing through them. A good example is Michael Brauer’s multi-bus processing, where the different elements in the mix are grouped and processed by different compressors (VCA, FET, Valve…) to add specific character before summing everything together. For Michael, compression is much more than just controlling volume; it’s about cohesion, tone and character.

Now that you will never look at a compressor the same way again, next time you use it in your mix, try to think beyond the dynamics and look for what’s going to add to the overall tone and presence of the song. And if you want to learn more about compression and how to get the most out of it, subscribe to our newsletter to get our latest articles right in your inbox.