Compression ratio is a term used in internal combustion engines and audio compressors. In internal combustion engines, it is the ratio between the volume of the cylinder and combustion chamber at their maximum and minimum values. It is a fundamental specification for such engines and is measured in two ways: the static compression ratio, which is calculated based on the relative volumes of the combustion chamber and the cylinder when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke. The dynamic compression ratio is a more advanced calculation that also takes into account gases entering and exiting the cylinder during the compression phase. A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.
In audio compressors, the compression ratio determines how much gain reduction the compressor applies when the signal passes a threshold level. For example, a ratio of 4:1 means that for every 4 dB the signal rises above the threshold, the compressor will increase the output by 1 dB. Whereas a ratio of 10:1 or higher will make the compressor effectively act as a limiter. However, if the ratio is 1:1, no compression will occur. The ratio knob sets the amount of applied compression, and the higher the ratio, the more extreme the compression.
