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decibel (dB)
A logarithmic unit (see logarithm) used to express power ratio, equal to one-tenth of a bel, widely used in the fields of acoustics and telecommunications. A power source of intensity P has a power level of 10log10P/P0 decibels, where P0 is the intensity of a reference source. The decibel is commonly employed in describing noise levels relative to the threshold of hearing. Doubling the noise level adds 30 to the decibel rating. The faintest audible sound (corresponding to an excess air pressure of 2 × 10-5 pascal) is given an arbitrary value of 0 dB; a whisper has an intensity of about 30 dB, normal speech 50–60 dB, a shout 90 dB, and a jet aircraft 120 dB (about the same as the human pain threshold).
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