What does it do?
A compressor is a dynamics processor - it helps to maintain a steady volume even when the incoming signal is fluctuating wildly. Have you ever wondered how a singer can seem to go from a whisper to a scream without any drastic change in overall volume? That’s compression at work. Put simply, it makes the loud bits quieter and the quiet bits louder.
Its near-twin, the limiter, only does half of that: it keeps the loud bits down.
Famed producer Tony Visconti (T-Rex, David Bowie) once called compression “the sound of rock”, and he wasn’t far wrong. Compression is used all over modern music (some say too much), and is a sort of secret weapon for achieving a professional sound.
How does it work?
Compressors can be difficult to fathom until you put them to use. They vary in complexity, with the simplest vintage-style examples having only input and output knobs (turn the former up and the latter down for more effect).
Others provide a user-definable volume threshold above which the effect will kick in and a ratio that describes the difference between the unprocessed (or ‘dry’) signal and the effected (or ‘wet’) output.
The higher the ratio, the greater the effect. A setting of 1:1 does nothing, while a setting of 12:1 is pretty drastic. There may also be a means by which you can control how fast the compressor works; this might be accomplished with attack and release parameters, for example.
Use it on...
Use compression on everything that requires taming. Vocals are a given, unless your singer has great mic skills. Drums, too, can be molded into shape with a compressor.
Entire mixes are often (over-)compressed to make them sound louder. Go easy, though - too much compression or limiting can suck the life out of your tracks.