An Etymological Dictionary of Astronomy and Astrophysics
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A dimensionless unit to measure sound. One bel designates a ratio 10:1 between two quantities, P₁ and P₀, which have the dimension of a power: n [B] = log (P₁/P₀), in Bel units, with → natural logarithm. If one sound is 2 bels louder than another, this means the louder sound is 100 times more intense than the fainter one. It is also common to use this definition for quantities that are proportional to a power, such as energy, work, intensity, or voltage. The bel was too large for everyday use, so the → decibel (dB), equal to 0.1 bel, is more commonly used.

This unit was put forward by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922), who also invented techniques for teaching speech to the deaf.