Decibel

The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power) relative to a specified or implied reference level. Its logarithmic nature allows very large or very small ratios to be represented by a convenient number, in a similar manner to scientific notation. Being essentially a ratio, it is a dimensionless unit. Decibels are useful for a wide variety of measurements in acoustics, physics, electronics and other disciplines.

The decibel is not an SI unit, although the International Committee for Weights and Measures (CIPM) has recommended its inclusion in the SI system. Following 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 (see below). The full name decibel follows the usual English capitalization rules for a common noun. The decibel symbol is often qualified with a suffix, which indicates which reference quantity has been assumed. For example, "dBm" indicates that the reference quantity is one milliwatt.

A decibel is one tenth of a bel (B). Devised by engineers of the Bell Telephone Laboratory to quantify the reduction in audio level over a 1 mile (approximately 1.6 km) length of standard telephone cable, the bel was originally called the transmission unit or TU, but was renamed in 1923 or 1924 in honor of the Bell System's founder and telecommunications pioneer Alexander Graham Bell. In many situations, however, the bel proved inconveniently large, so the decibel has become more common.

An increase of 3 dB corresponds to an approximate doubling of power. (In exact terms, the factor is 103/10, or 1.9953, about 0.25% different from exactly 2.) Since in many electrical applications power is proportional to the square of voltage, an increase of 3 dB implies an increase in voltage by a factor of approximately √2, or about 1.414. Similarly, an increase of 6 dB corresponds to approximately four times the power and twice the voltage, and so on. (In exact terms the power factor is 106/10, or about 3.9811, a relative error of about 0.5%.) See the formulas below for further details.