dB: What is a decibel?
(From UNSW website by Joe Wolfe ): http://www.animations.physics.unsw.edu.au/jw/dB.htm
· The decibel ( dB) is used to measure sound level, but it is also widely used in electronics, signals and communication. The dB is a logarithmic (log) unit used to describe a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things.
· Suppose that we have 2 microphones and we want to measure the difference between their powers in decibel.
· 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.
· As is shown on the graph, which plots 10 log (P2/P1) against P2/P1. To continue the example, 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 1,000,000 = 60 dB.
· Decibel can describe very big ratios using numbers of modest size.
· There is sound pressure and sound power. Power of Sound is in wave and is equal to squared pressure .
· 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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