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Let's get to the heart of the matter: Compressors control sound level. Period. Sound consists of frequency and amplitude, and while equalizers mess with the frequency content, compressors take care of the rest. If you're getting into recording you ought to have at least one compressor, even if you think right now that effects processors and other toys seem like more fun. Without control over the dynamic range of your music, those cool sounds might never be heard!
What's going on in there?
Compressors can modify the levels of your tracks in many ways, and there is an astounding variety of them out there on the market. The primary job of any audio compressor, though, is to make all sounds equal in volume. With all the flashing lights and wiggling meters and knobs, that is the fundamental function. Other than a few oddball designs, they do this by reducing the level of the loud sounds and then boosting the overall volume to compensate; essentially, they make quiet sounds louder and loud sounds quieter. The compressing process goes something like this:
The compressor makes thousands of such adjustments every minute. How, when, and to what extent it makes them depends on how you set the compressor's controls.
How your ears hear
The most familiar compressors are our ears and auditory processing pathways. Part of the reason it's so incredibly compelling to sit in front of a band blasting 130 db off a stage is that your ears are behaving like a pair of compressors: when a painful sound level hits your cochlea, it instantaneously reduces your sensitivity to accommodate the blast of volume. Then your sensitivity quickly returns (within reason) to hear quieter sounds coming after it.
Compressors allow us to have external control rather than leaving the hard work up to the listener's ears. They can manage the dynamic range of sound at much lower volume levels, and this is one of the principle ways that they enhance your music. There are endless applications and tricks for compression, particularly in the multitrack realm, and they are even more important with the advent of computerized hard-disk recording (more on this later).
The first compressors
The earliest use of the compressor was for radio broadcast. Transmitters were delicate things, and overloading the transmitter could cause not just distortion but also physical damage, so the stations needed something to keep levels in check. As time went on, broadcasters realized that listeners were drawn to the loudest sounding stations, so they worked up some tougher compressors, called limiters, that could really be hit with a lot of signal without ever going into overload. The name says it all: these devices limit the maximum signal output to a set level (the model names are brilliant: Volumax, Audimax, Levil Devil....). Limiters not only gave the station a signal-strength advantage, but music and voices sounded louder and more exciting. One obvious current demonstration of this idea is in that Blur hit, "Song #2"- you know, the one that goes "WOO HOO,", where the chorus sounds so terribly exciting. But listen again: those are the same chords and same instruments playing them as in the verses. So, what happened? Well, the whole mix at that point has been slammed into a limiter and it sounds crazy.