The compression effect amplifies the input signal in such a way that louder signals are amplified less, and softer signals are amplified more. It is essentially a variable gain amplifier, whose gain is inversely dependant on the volume of the input signal. Compression is used by most radio stations to reduce the dynamic range of the audio tracks, and to protect radios from transients such as feedback. It is also used in studio recordings, to give the recording a constant volume, especially to vocals. Using a compressor for a guitar recording makes finger-picked passages sound smoother, and makes clean lead passages sound smoother.
Compression tends to increase background noise, especially during periods of silence.
Thus, a noise gate is usually used in conjunction with the compressor. An expander performs the opposite effect of the compressor. This effect is used to increase the dynamic range of a signal. This is useful for eliminating background noises, such as hiss or hum, during periods of silence in a recording or performance. At other times, the recording or performance usually drowns out the background noise.