Webster's English Dictionary

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decibel

deci.bel \'des-*-.bel, -b*l\ n [ISV deci- + bel] 1a: a unit for expressing 
   the ratio of two amounts of electric or acoustic  signal power equal to 10 
   times the common logarithm of this ratio 1b: a unit for expressing the 
   ratio of the magnitudes of two electric volta ges or currents or analogous 
   acoustic quantities equal to 20 times the common logarithm of the voltage 
   or current ratio 2: a unit for measuring the relative loudness of sounds 
   equal approximately  to the smallest degree of difference of loudness 
   ordinarily detectable by the human ear whose range includes about 130 
   decibels on a scale beginning with 1 for the faintest audible sound