Sound measurement

The decibel (dB) is a logarithmic part of measurement that expresses the magnitude of a physical quantity (usually power) qualified to a specified or implied reference level. Its logarithmic natural history allows very large or very small ratios to be represented by a convenient number, in a related manner to scientific notation. Since it expresses a ratio of two quantities, it is a dimensionless unit. The decibel is useful for a extensive variety of measurements in acoustics, physics, electronics and other disciplines because it linearizes a physical value -- e.g. light intensity or level of noise -- in which exponential changes of magnitude are perceived by humans as being more or less linearly related.

The decibel is not a SI unit. In April 2003, the International Committee for Weights and Measures (CIPM) measured a recommendation for its inclusion in the SI system and decided not to adopt that recommendation. Subsequent the SI convention, the d is lowercase, as it represents the SI prefix deci-, and the B is capitalized, as it is an abbreviation of a name-derived unit, the bel.