Audio Compression Intro and Basics – How to use a compressor

Compression is an essential tool in audio processes. Simply put, a compressor crushes the dynamic range of sound so the quiet bits are louder and the loud bits are quieter.

This sounds quite complicated, so lets look deeper. In sound, e.g. editing an interview, if you want something louder, you turn the volume up. And if you want it even louder, why, you turn it up again.

But you can’t actually just keep turning it up. Audio hardware has a limit to how loud a sound goes through it without you overdriving the signal.

READ MORE: Logic Pro’s Compressor – Save your money and try this tool first!

As you push, let’s say, a mixer harder and harder, it will begin to distort as it just can’t handle the voltage. The more you push above this limit, the more your sound suffers. This is called going into the red.

So we now have a problem, we want to increase the volume but the loud bits can’t get any louder.

One solution for this is to “ride” the faders, that is, have the volume fader all the way up for quiet parts and drop it back down for the loudest. Imagine automating with an effect…

For my example today, I am going to use a simple yet powerful compressor called ReaComp.

It is a software compressor you can add to most Digital Audio Workstations (DAWs) as a VST file.

This one comes native to Reaper, one of the DAWs that I recommend meaning that, along with many other audio processing plugins, you don’t need to download it separately. Here it is:

On careful inspection, most compressors (hardware and software) have the same controls. They will have a threshold, a ratio, an attack and a release.

Other controls include knee-size, pre-comp and more. For the sake of simplicity let’s just look in  very basic detail. 

Threshold

Our “sentencing” control. Everything louder than it will be compressed (reduced in volume).

So we can set the threshold to only the very loudest of bits or we can mercilessly compress the whole thing.

Now think for a moment, we want to boost the quiet parts and make the peak volumes relatively quieter. There is no point crushing the volume of everyone.

If we can find a threshold that is louder than the soft sections, they will stay the same volume. And if the threshold also manages to be below the average volume of the peaks, they will quieten. 

Ratio

This is our “mercy” control. Everything louder than the threshold will be compressed but by how much?

This is what the ratio sets. the higher the ratio, the more merciless the compression.

For example a ratio of 10:1 means that if a sound is 10 dB louder than the threshold volume, after compression, it will only be 1 dB louder.

The higher the ratio, the more obvious the compression. If the ratio is set to infinity. then the audio will never exceed the threshold, this is called limiting.

Attack

How long after the compressor detects the volume exceeding the threshold will it actually start compressing.

Release

How long the compressor will continue compressing.

A quick warning to those using compression. Use it with reason. Don’t just throw a compressor onto any bit of audio if it sounds fine. If you do use a compressor, especially for music, please don’t compress the shit out of it, Loudness Wars prove it isn’t great for music.