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, very simple. And if you want it even louder, why, you turn it up again.
You see, you can't actually just keep turning it up. Audio hardware has a limit to how loud a sound goes through it without you jizzing up the wires and it's components. As you push, let's say, a mixer harder and harder, it will begin to distort as it just can't handle. The more you push above this limit, the more your sound suffers. This is called going into the RED!
This is really bad, don't do it kids!
So we now have a kerfuffle, we want to increase the volume because little Pete has a really quiet voice during interviews, but his interviewer has a loud voice. If we turned the volume up for little Pete, the sound would go into the Red when our booming interviewer speaks which makes his voice indistinguishable (just like a Dalek talking on a walkie talkie).
One solution for this is to "ride" the faders, that is, have the volume fader all the way up for little Pete and drop it back down for when our interviewer (just identified as Brian Blessed) speaks. This makes the loud voice quieter in comparison so both can be heard.
Imagine automating that into a gadget...
Now then you haven't even seen what a Compressor looks like, let's look at a few...
This is a hardware compressor
This is a bunch of software compressors
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, a free DAW that I recommend meaning that, along with many other audio processing plugins, you don't need to download it separately. Here it is:
It looks as smooth as sandpaper compared to other software compressors (many have a modelled GUI to match real life compressors) but it is perhaps clearer in it's simplicity.
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.
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 (Brian Blessed's loudest moment) or we can mercilessly compress the whole thing (including little Pete's voice). Now think for a moment, we want little Pete to be louder and Brian Blessed to be relatively quieter. So there is no point crushing the volume of everyone. If we can find a threshold that is louder than little Pete, he will stay the same volume but if the threshold also manages to be below the average volume of Mr Blessed his voice will quieten.
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
Release: How long will the compressor continue compressing
The picture below demonstrates what a compressor does (visually) to audio. On first glance they may
seem different but on closer inspection it is clear they are the same recording but just with the loud bits
reduced and the quiet bits increased.
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.
Your face when you learn to compress