Decibel Calculator.
Translate ratios into decibel (dB) and back.
There are two types of translations:

10*log(P/P0) and 20*log(U/U0).

From Wikipedia, the free encyclopedia:

"A decibel (dB) is one tenth of a bel (B), i.e. 1B = 10dB. The bel is the logarithm of the ratio between two power quantities of 10:1, and between 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.
Two signals that differ by one decibel have a power ratio of 1.258925411794167 and an amplitude ratio of 1.122018454301963."