Decibel << DEHS uh behl >> is a unit used to measure levels of sound, electric power, and related quantities. Its symbol is dB. A decibel equals one-tenth of a bel, a unit named for Scottish-born inventor and educator Alexander Graham Bell.
The softest sound that most people can hear has a level of about 0 dB. Normal conversation takes place at a level of 60 dB. Extremely loud music might measure 120 dB-and listening to such music for a long time could cause hearing loss. A level of 140 dB, perhaps from a jet aircraft taking off nearby, can produce pain.
Decibels represent differences in sound intensity—that is, in sound power per unit of area. Each increase of 10 decibels represents a tenfold increase in power. Thus, for example, a 50-dB sound source delivers 10 times as much power to an area on the surface of the eardrum as a 40-dB source delivers, a 60-dB source delivers 100 times the power of a 40-dB source, and so on. Mathematically, the differences work out such that a doubling of power produces an increase of 3.01 dB.
Normal sound waves carry little power, as the following definition and example will show. Sound intensity can be defined as the rate of transmission of sound energy through a certain curved area of space. This area is a section of the surface of an imaginary sphere surrounding the sound source. Suppose you were listening to a radio that was about 10 feet (3 meters) in front of you. You might consider the radio to be in the center of an imaginary sphere that had a radius of about 10 feet. Your ears would be on the surface of the sphere. Suppose the radio were playing loudly, delivering a sound of 90 dB to your ears. The sound intensity would be only 1/1000 of a watt per square meter of surface area of the sphere.
Electrical engineers sometimes measure power and voltage in decibels. In power measurements, a doubling of power produces a gain of 3.01 dB—as in sound measurements. However, the decibel scale for voltage measurements has a different mathematical basis: A doubling of voltage produces a gain of 6.02 dB.
See also Sound (Intensity and loudness).
