Intensity of Sound

What is a decibel?

A decibel (dB) is the internationally adopted unit for the relative intensity of sound. The sound intensity of 0 decibel is the threshold of human hearing, 10-16 watts/cm2. This corresponds to a pressure of 2 X 10-5 newtons/m2 or 2 billionths of atmospheric pressure. The ear is extremely sensitive! The decibel scale is a logarithmic scale, meaning for every 10 decibels, the intensity is increased by a factor of ten. For example, a change from 30 decibels to 40 decibels means the sound will be ten times more intense. A change from 30 decibels to 50 decibels would mean the new sound would be one hundred times more intense.

The following chart shows a typical sound environment, how many times louder those levels are than the threshold of human hearing, and the relative intensity of that sound compared to the threshold of hearing.

Sound Times More Intencse Relative Intensity (dB)
Loss of hearing 1 x 1015 150
Rocket launch 1 x 1014 140
Jet engine 50 meters away 1 x 1013 130
Threshold of pain 1 x 1012 120
Rock concert 1 x 1011 110
Lawnmower 1 x 1010 100
Factory 1 x 109 90
Motorcycle 1 x 108 80
Automobiles driving by 1 x 107 70
Vacuum cleaner 1,000,000 60
Normal speech 100,000 50
Library 10,000 40
Close whisper 1,000 30
Leaves rustling in the wind 100 20
Breathing/whisper 5 meters away 10 10
Threshold of hearing 0 0


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