Definition and examples
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 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.