Compressors are probably the best known dynamic processors. Compressors use for maintain the audio signal. If the level of an input signal rises above user specificed trechold, the compressor reduces the amount by which the signal's level can continue to rise. If the input signal stays below treshold, it is not affected. 

     The ratio parameter determines the extent to which the of a signal above the treshold is compressed. For example, if the ratio is 3:1, and the input signal is above the treshold, a 3 dB increase in the input level results in a 1 dB increase in the output level. Other compression parameter include input gain and output gain. The input gain control (sometimes called "makeup gain") is used to adjust overall imput level to compensate for a particularly strong or weak incoming signal without clipping the compressor's signal circuitry. Raising the outpiut level also rises low level signals, including noise. Compressors has attack and release parameters.The attack parameter determines how long the input signal rises above the treshold before the compressor starts attenuating.  The release parameter determines how long it takes for the compressor to stop working.