decibel (dB) at English => English (The Britannica Concise) Of Explained:
Unit for measuring relative intensities of sounds or relative amounts of acoustic or electric power. One decibel (0.1 bel) is defined as 10 times the common logarithm of the power ratio and is roughly equal to the faintest audible sound. Because the scale is logarithmic, doubling the intensity means an increase of a little more than 3 dB. A 90-dB, or 9-bel, sound is 109, or 1 billion times more intense than a barely detectable sound of 1 dB. Decibels are also used to express the ratio of the magnitudes of two electric voltages or currents; in this usage 1 dB equals 20 times the common logarithm of the ratio. The bel is named for A. G. Bell.