A unit for some logarithmic quantities. It is dimensionally equal to the number one, and scales by decimal logarithms. The symbol is B.

What that means is that a value of 2 B is ten times bigger than one of 1 B, and 3 B is ten times bigger than 2 B, and so on.

The bel is named after Alexander Graham Bell, who worked on early versions of the microphone and telephone. It has been around for many years, mainly in the form decibel (symbol dB), such that 30 dB is ten times more than 20 dB, 80 dB is ten times more than 70 dB, and so on.

The decibel was used for the measurement of loudness. It has now been adopted for use with the SI for any quantity where a decimal logarithmic scale is appropriate. It is important to specify the reference level, because these are not absolute magnitudes but scalars.

The use of the units bel and decibel was agreed on by the CIPM (International Committee for Weights and Measures) in 1998 to be presented to the 21st CGPM (General Conference) in 1999. I have not been able to locate confirmation that the CGPM accepted them, but normally I think with CIPM acceptance it's a shoo-in. It does not become an SI unit in the strict sense, but is officially approved for use with the SI.

At the same time they were to adopt the new unit neper, a natural logarithm, as an SI unit.