A compressor reduces a signal's dynamic range. It does this by reducing the gain when the signal level is high, making louder passages softer and the dynamic range smaller. It is used in audio recording, production work, noise reduction, and live performance applications. It can smooth volume changes and adjust the dynamic range and balance of a track.

When signal levels get too high during recording or performances, the signal will clip. This can cause distortion or even damage to subsequent devices in your effects chain. A limiter feature on a compressor prevents the signal from exceeding a certain level. This keeps especially loud passages and transient signals from clipping and causing distortion or damage to your equipment. It is also used for cutting tracks and adjusting the mix.

Compression can also increase an instrument's sustain. It amplifies the incoming signal to maintain a constant level, so after "twanging" a string, a little compression will preserve the string's sound. While adding sustain to your arsenal, compression also reduces your dynamics, making it difficult to accent notes and phrases. Be cool about compressing for the sake of sustain.

If you need to have a signal's level controlled by a different signal, it is called 'ducking' (it 'ducks' a signal out of the way), or cross limiting. Here's an example: While music is playing, using the microphone will cause the level of the music to drop so that it's easier to hear the singer. When mixing in the studio, a ducker can also be used to make certain instruments pop out of the mix.

A 'de-esser' is a limiter that monitors only a specific frequency range. It only reduces the level of frequencies in a selected range. This allows you to reduce unwanted sounds. When using a compressor with other effects, many players put it first in the chain. First, it gives you a good signal to work with. When the compressor is on and the output level is increased, the noise will be amplified along with the instrument's sound. Other effects can introduce more noise, so if the compressor is placed after those effects, it will end up amplifying their noises, too.