(dB), unit for expressing the ratio between two amounts of electric or acoustic power or for measuring the relative loudness of sounds. One decibel (0.1 bel) equals 10 times the common logarithm of the power ratio—i.e., doubling the intensity of a sound means an increase of a little more than three dB. In ordinary usage, specification of the intensity of a sound implies a comparison of the intensity of the sound with that of a sound just perceptible to the human ear. For example, a 90-dB, or 9-bel, sound is nine powers of 10 (i.e., 10⁹, or 1,000,000,000) times more intense than a barely detectable sound. Decibels are also used to express the ratio of the magnitudes of two electric voltages or currents (or analogous acoustic quantities); in this usage one dB equals 20 times the common logarithm of the ratio.

The term bel is derived from the name of Alexander Graham Bell (Bell, Alexander Graham), inventor of the telephone.