decibel

[des-uh-bel, -buhl]
noun Physics.
1.
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.
2.
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.
3.
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

Origin:
1925–30; deci- + bel

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World English Dictionary
decibel (ˈdɛsɪˌbɛl)
 
n
1.  a unit for comparing two currents, voltages, or power levels, equal to one tenth of a bel
2.  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

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Etymonline
Word Origin & History

decibel
1928, coined from L. decibus "tenth," from decem "ten" (see ten) + bel unit for measuring sound, after Alexander Graham Bell.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

decibel de·ci·bel (děs'ə-bəl, -běl')
n.
Abbr. dB
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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
decibel   (děs'ə-bəl)  Pronunciation Key 
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.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
decibel [(des-uh-buhl, des-uh-bel)]

A unit of measurement of the volume of sounds.

The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Once merely a whisper, its decibels intensify with the advancing years.
Hip-hop music crackled through the sound system, a few decibels too loud.
As would be expected from such a combination, the band plays a robust blend of
  rock at high decibels.
Together, these two performers promise an evening of righteous indignation
  delivered at high decibels.
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