Compression is the reduction of the dynamic range of a signal. Compression may be intentional or one of the effects of a system that is driven to overload. If intentional it is done to ensure suitable placement of sound levels between the lowest and highest distortion-free levels of an electroacoustical system. Compression may be thought of as a form of non-linear amplification where peak levels above a certain threshold are amplified according to input-to-output ratios such as 2:1, 3:1, 5:1, 10:1 or 20:1 which reduce the peak levels accordingly. Compressors may be combined with dynamic range expanders which suppress low level signals below a threshold according to a 1:2 or 1:20 ratio, the latter being called a gate because low level signals such as background noise are effectively eliminated.
Compression is also the portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure.