Ratio

In a compressor, limiter, or expander, ratio is the amount of output level change that results from a given input level change. For example, in a compressor with a 4:1 ratio, an input level increase of 4 dB will result in an output increase of only 1 dB. An expander might have a ratio of 1:4; for a 1 dB input change, a 4 dB output change will result. Limiters typically have extremely high ratios, some claiming infinity:1, which essentially means that for virtually any input level increase, there will be only a very minor output level increase.