The decibel (dB) is a logarithmic unit used to express the ratio of two values of a physical quantity, often power or intensity. One of these values is often a standard reference value, in which case the decibel is used to express the level of the other value relative to this reference. The number of decibels is ten times the logarithm to base 10 of the ratio of two power quantities, or of the ratio of the squares of two field amplitude quantities.

A change in power by a factor of 10 corresponds to a 10 dB change in level. At the half power point an audio circuit or an antenna exhibits an attenuation of approximately 3 dB. A change in voltage by a factor of 10 results in a change in power by a factor of 100, which corresponds to a 20 dB change in level. A change in voltage ratio by a factor of 2 (equivalently factor of 4 in power change) approximately corresponds to a 6 dB change in level.