A measurement of sound intensity utilizing a relative scale based in the ratio of one sound intensity to another, one usually being a standard intensity. The scale of decibel level is a logarithmic one; the decibel is defined as 20 times the common logarithm of the ratio of the pressure produced by the sound in question to that of a standard reference pressure (which is usually considered to be 0.0002 microbar, one microbar being equal to one millionth of an atmosphere). Thus a sound 20 dB higher than another is 100 times greater in intensity. A sound 30 dB higher than another is 1,000 times greater in intensity. A decibel is also defined as one-tenth of a Bel unit (a unit named for acoustics pioneer and telephone-inventor Alexander Graham Bell).

A decibel is also used to measure voltage levels.

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