About the Decibel

The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level. A decibel is one tenth of a bel (B).

(The bel, named after Alexander Graham Bell, was originally devised by telephone engineers to quantify the reduction in audio level over 1 mile of standard telephone cable. In many situations, however, the bel proved inconveniently large, so the decibel has become more common.)

The decibel is useful for a wide variety of measurements. It is able to conveniently represent very large or small numbers, has a logarithmic scaling that roughly corresponds to the human perception of, for example, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.

If the decibel symbol is qualified with a suffix, it indicates that a reference quantity has been used. For example, "dBm" indicates decibels relative to one milliwatt, while "dBW" is decibels relative to 1 watt. Aerial gains can be specified relative to a theoretical isotropic radiator (dBi), or relative to a half wave dipole (dBd).

Handy Calculators

Power Ratios: [dB=10*log(Pout/Pin)]

Voltage Ratios: [dB=20*log(Vout/Vin)]

Watts < > dBW

Voltage < > dBm: