a unit used to express the intensity of a sound wave, equal to 20 times the common logarithm of the ratio of the pressure produced by the sound wave to a reference pressure, usually 0.0002 microbar.

a unit of power ratio, the number of units being equal to a constant times the logarithm to the base 10 of the intensities of two sources.

a unit used to compare two voltages or currents, equal to 20 times the common logarithm of the ratio of the voltages or currents measured across equal resistances. Abbreviation: dB, db

a unit for comparing two currents, voltages, or power levels, equal to one tenth of a bel

a similar unit for measuring the intensity of a sound. It is equal to ten times the logarithm to the base ten of the ratio of the intensity of the sound to be measured to the intensity of some reference sound, usually the lowest audible note of the same frequency

Progress in science and industry is constantly demanding new terms and one of the latest of these is the word "decibel," coined by telephone engineers to describe the efficiency of telephone circuits. It is a substitute for the phrase "transmission unit." The actual unit decided upon was first called "bel," after the inventor of the telephone. The bel, however, is larger than is needed in practice, and, therefore, a unit one-tenth as large was adopted by engineers and named the decibel. ["Popular Mechanics," May 1929]

A unit used to express relative difference in power or intensity, usually between two acoustic or electric signals, equal to ten times the common logarithm of the ratio of the two levels.

A unit used to measure the power of a signal, such as an electrical signal or sound, relative to some reference level. An increase of ten decibels in the power of a signal is equivalent to increasing its power by a factor of ten. As a measure of sound intensity, a zero-decibel reference is stipulated to be the lowest level audible to the human ear; the speaking voice of most people ranges from 45 to 75 decibels.