The intensity of a sound is a measure of the strength of the sound wave as it is indicative of the energy transported by the wave to a specific unit of area. Recall that
sound intensity is proportional to the square of the amplitude of the sound wave AND
amplitude is a measure of the pressure changes associated with the wave.
The commonly used measure of sound level is the decibel (dB).
Sound level in decibel is calculated using the sound intensity (
I) relative to a reference intensity (
I0).
The reference intensity is usually the lowest (or threshold) intensity of sound that a person with normal hearing can perceive at 1000 Hz.
(
I0 = 10
–12 W m
–2)
The logarithmic relationship and the factor of 10 are used because our ears are not very sensitive to changes in intensity.
For example we perceive a loud rock concert to be twice as loud as a loud conversation whereas the ratio of the sound intensities is actually a factor of 1 million.