A compressor/limiter reduces the difference between the loudest and quietest passages of a signal. It works on a threshold system where signals exceeding the threshold are processed and those falling below it pass through unchanged. When a signal exceeds the threshold the compressor automatically reduces the gain.

How much gain reduction is applied depends on the 'compression ratio' which on most compressors is variable: the higher the ratio, the stronger the compression. Very high ratios cause the compressor to act as a limiter where the input signal is prevented from ever
exceeding the threshold.

Compressors are the most commonly used processor and are particularly popular for maintaining constant vocal levels Live and in the Studio. This is because singers tend to vary their levels the most out of all instruments. Compressors help to achieve the sought after
tight, "punchy" sound.