Decibel
The decibel is a logarithmic unit used to relate two quantities, abbreviated as "dB". It was named in honour of Alexander Graham Bell. A decibel is one tenth of a bel. The decibel is not an SI unit, although the CIPM has recommended its inclusion in the SI system.
It is defined as 10 log(I1/I0) where I0 and I1 are the two quantities to be related.
Uses
Acoustics
The decibel is often used in acoustics to quantify sound pressure relative to a reference pressure level of 20 micropascals or 0.02 mPa.
The reason for this is that the ear is capable of hearing a very large range of sounds. The ratio of the sound pressure that causes permanent damage from short exposure to the limit that (undamaged) ears can hear is more than a million. To deal with such a range, logarithmic units are useful: the log of a million is 6, so this ratio represents a difference of 120 dB. Psychologists also say that our sense of hearing is roughly logarithmic. In other words, they think that you have to increase the sound intensity by the same factor to have the same increase in loudness.
Electronics
In radio electronics, the decibel is used to describe the ratio between two measurements of electrical power. It can also be combined with a suffix to create an absolute unit of electrical power. For example, it can be combined with "m" for "milliwatt" to produce the "dBm". 0dBm is one milliwatt, and 1dBm is one decibel greater than 0dBm, or about 1.26mW.