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Saturday, March 22, 2008
what is decibel db
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For other uses, see Decibel (disambiguation).
The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a physical quantity (usually power or intensity) relative to a specified or implied reference level. Since it expresses a ratio of two (same unit) quantities, it is a dimensionless unit. A decibel is one tenth of a bel (B).
The decibel is useful for a wide variety of measurements in science and engineering (e.g. acoustics and electronics) and other disciplines. It confers a number of advantages, such as the ability to conveniently represent very large or small numbers, a logarithmic scaling that roughly corresponds to the human perception of, for example, sound and light, and the ability to carry out multiplication of ratios by simple addition and subtraction.
The decibel is not an SI unit. However, following the SI convention, the d is lowercase, as it represents the SI prefix deci-, and the B is capitalized, as it is an abbreviation of a name-derived unit (the bel). The full name decibel follows the usual English capitalization rules for a common noun.
The decibel symbol is often qualified with a suffix, which indicates which reference quantity has been assumed. For example, "dBm" indicates that the reference quantity is one milliwatt. The practice of attaching a suffix in this way, though not permitted by SI,[1] is widely followed.
The definitions of the decibel and bel use base-10 logarithms. For a similar unit using natural logarithms to base e, see neper.
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What is a decibel?And what are the different types of decibel measurement: dB, dBA, dBC, dBV, dBm and dBi? How are they related to loudness, to phons and to sones? This page describes and compares them all and gives sound file examples. A related page allows you to measure your hearing response and to compare with standard hearing curves.
Definition and examples
Sound files to show the size of a decibel
Standard reference levels ("absolute" sound level)
Logarithmic response, psychophysical measures, sones and phons
Recording level and decibels (dBV and dBm)
Intensity, radiation and dB
dBi and anisotropic radiation
Example problems using dB for amplifier gain, speaker power, hearing sensitivity etc.
Occupational health and safety and the law
Related pages
What is a logarithm? A brief introduction.
Definition and examplesThe decibel (dB) is used to measure sound level, but it is also widely used in electronics, signals and communication. The dB is a logarithmic unit used to describe a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things. Later on we relate dB to the phon and the sone (units related to loudness). But first, to get a taste for logarithmic units, let's look at some numbers. (If you have forgotten, go to What is a logarithm?)
For instance, suppose we have two loudspeakers, the first playing a sound with power P1, and another playing a louder version of the same sound with power P2, but everything else (how far away, frequency) kept the same.
The difference in decibels between the two is defined to be
10 log (P2/P1) dB where the log is to base 10.If the second produces twice as much power than the first, the difference in dB is
10 log (P2/P1) = 10 log 2 = 3 dB.If the second had 10 times the power of the first, the difference in dB would be
10 log (P2/P1)= 10 log 10 = 10 dB.If the second had a million times the power of the first, the difference in dB would be
10 log (P2/P1) = 10 log 1000000 = 60 dB.
This example shows one feature of decibel scales that is useful in discussing sound: they can describe very big ratios using numbers of modest size. But note that the decibel describes a ratio: so far we have not said what power either of the speakers radiates, only the ratio of powers. (Note also the factor 10 in the definition, which puts the 'deci' in decibel).
Plot of 10 log (P2/P1)
Sound pressure, sound level and dB. Sound is usually measured with microphones and they respond (approximately) proportionally to the sound pressure, p. Now the power in a sound wave, all else equal, goes as the square of the pressure. (Similarly, electrical power in a resistor goes as the square of the voltage.) The log of the square of x is just 2 log x, so this introduces a factor of 2 when we convert to decibels for pressures. The difference in sound pressure level between two sounds with p1 and p2 is therefore:
20 log (p2/p1) dB = 10 log (p22/p12) dB = 10 log (P2/P1) dB where again the log is to base 10.
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http://en.wikipedia.org/wiki/Decibel
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