A decibel (dB) is one tenth of a bel (B),
i.e. 1 B = 10 dB.
The bel is the logarithm of the ratio of two power quantities of 10:1, and for two field quantities in the ratio 
A field quantity is a quantity such as voltage, current, sound pressure, electric field strength, velocity and charge density, the square of which in linear systems is proportional to power.
A power quantity is a power or a quantity directly proportional to power, e.g. energy density, acoustic intensity and luminous intensity.
The calculation of the ratio in decibels varies depending on whether the quantity being measured is a power quantity or a field quantity.
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