A compressor is a signal processor affecting the dynamics of an audio signal. Input amplitudes above a user-defined threshold are reduced to a degree determined by a user-defined ratio. Attack and release parameters determine, respectively. the rates at which the effect is applied and subsequently removed. Compression is generally used to render a smoother sound.
Limiting is a special case of compression, employing high thresholds, high compression ratios and high rates of attack and release. A wide-bandwidth limiter is generally used to reduce the amplitude of transient peaks, thus facilitating a louder output with nominal audible adulteration of the sound. For the purpose of limiting, a compressor will often be keyed with a filtered input signal or another signal entirely. For instance, a vocal track will commonly be processed with high-frequency limiting, to reduce sibilance without appreciably affecting the overall output level. In this case, the track output is split; one lead goes directly to the compressor input, while the other is processed through an equalizer to emphasize the desired high frequencies, and sent to the compressor key input.
Expansion is basically compression in reverse. Amplitudes above the threshold are increased, while those below the threshold are reduced. Expansion can be used to ameliorate extraneous sounds occuring in the quieter portions of a recorded track.
The effect of a noise gate is similar to that of an expander, except in that audible dynamics are not altered. In a noise gate, the output is simply switched off below a certain threshold.