Compressors & Limiters
Compressors reduce 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.
When signal levels get too high when recording, there will be distortion. Compressors and limiters provide protection against sudden transient sounds that could distort your sound or damage your equipment. It is also used for cutting tracks and adjusting the mix. It can smooth volume changes, adjust the dynamic range and balance of a track.
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. And 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.