decibel
(symbol: dB) n. a logarithmic unit used to express the ratio of acoustic or electric power (intensity). An increase of 1 bel is a 10-fold increase in intensity; a decibel is one tenth of a bel and is the more commonly used unit, partly because a 1 dB change in intensity is just detectable (approximately and under laboratory conditions). The sound intensity (the numerator of the intensity ratio) or sound level is usually specified in decibels sound-pressure level (dB SPL). The reference intensity (the denominator of the intensity ratio) for dB SPL is 10–12W/m2 and corresponds to a sound pressure of 20 μPa (micropascal). A sound presented at the reference intensity has a level of 0 dB SPL and is close to the absolute threshold for a 1 kHz tone. Often SPL is omitted but is implied from the context: A “60 dB sound” usually means 60 dB SPL. The decibel, because it is a logarithmic
measure, has caused much confusion. For example, decibels do not add: Two unrelated 60 dB SPL sounds when sounded simultaneously produce a 63 dB SPL sound, not a 120 dB SPL sound.