Decibel

A decibel (or dB) measures ratios of power or intensity. It expresses them as an exponential function. One bel is a power ratio of 10:1, and is divided into ten decibels. An increase of three decibels is approximately a doubling of power. Decibels are often used in measuring telecommunication signals. With electric audio signals, there are several decibel units, relative to several bases. For example, dBm is relative to one milliwatt.

It's been said that the smallest difference humans can hear is 0 dB and is related to the Absolute threshold of hearing, although this is very subjective at best.