# decibel

##### Non-SI unit accepted for use with SI
Name Symbol Quantity Value in SI units
decibel dB logarithmic ratio

### Definition

The decibel, symbol dB, is a non-SI unit accepted for use with the SI.

One decibel is equal to one tenth of a bel, symbol B. i.e. 1 dB = 0.1 B, where one bel is equal to the decadic logarithm of a ratio between two power quantities of 10:1, or the decadic logarithm of a ratio between two root-power quantities of √10:1

The decibel is used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm.

The decibel is a relative unit of measurement equal to one tenth of a bel. It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 10110, or root-power ratio of 10120.

The decibel expresses a change in value (e.g. +1 dB or −1 dB) or an absolute value.

When expressing a power ratio, it is defined as ten times the logarithm in base 10. That is, a change in power by a factor of 10 corresponds to a 10 dB change in level.

When expressing root-power quantities, a change in amplitude by a factor of 10 corresponds to a 20 dB change in level.

The decibel scales differ by a factor of two, so that the related power and root-power levels change by the same value in linear systems, where power is proportional to the square of amplitude.