What is compression? Of all the words thrown around in a control room, it is the one most likely to spook a client, yet the idea behind it is simple: controlling the gap between the loudest and quietest parts of a performance. Here are the key concepts, threshold, ratio, attack and release, in plain English. ‘Compression’ is definitely a word that can spook newbies in there studio. Here’s a bit of an explanation.
What Is Compression?
Compression is the controlled reduction in dynamic range of an audio signal. Dynamic range is the difference in volume between loud and soft. Compressors will often have controllable parameters such as attack, release, input gain, output gain, ratio and threshold.
The threshold is the designated volume at which the compressor will begin to operate on the signal. Once this threshold has been crossed the compressor will reduce the volume to a predetermined fraction of the input signal. This is called the ratio. The ‘attack’ is how quickly the compressor will reduce the signal to the targeted ratio and the ‘release’ is how quickly the compressor will cease reducing once the signal has fallen back below the threshold.
Threshold, Ratio and the Singer Example
Imagine that you were listening to a recorded singer and you had your hand on the volume knob (aka fader). Sometimes the singer would sing softly and every now and then they would sing loudly. When they sing loud you decide to pull down the volume. When they return to a more acceptable volume you bring the volume back up. This is essentially what a compressor does to an audio signal except that a compressor is automated. When the fader is pulled down it is referred to as ‘reduction’. How quickly you pull down the fader is called the ‘attack’ and how quickly you bring that fader back up to the original volume is called ‘release’. The volume at which you decide to start turning down the volume is the ‘threshold’ and the amount that you choose to reduce by is referred to as the ‘ratio’.
Two more controls finish the picture. Attack sets how quickly the compressor clamps down once the signal crosses the threshold; a slow attack lets the initial transient through, which is why drums keep their punch. Release sets how quickly it lets go afterwards, and a release that is too fast makes the level pump audibly. Most of the art of compression is in balancing these two against the tempo and feel of the song.
Knowing what is compression and what each control does makes you a far better collaborator in the mix. If you would like to hear it applied properly, see our mixing and mastering services or contact us, and read more about dynamic range compression on Wikipedia.
