Sound intensity is the quality that allows us to characterize whether a sound is strong or weak and depends on the energy the sound wave transfers.
Sound intensity (I) is physically defined as the sound power received per unit area of a surface, ie:
But as power can be defined by the ratio of energy per unit of time:
Then we can also express the intensity by:
The most commonly used units for intensity are W / m² and J / s.m².
It's called minimal physical intensity, or hearing threshold, the lowest value of sound intensity still audible:
It's called maximum physical intensity, or pain threshold, the highest value of loudness tolerable to the ear:
As an observer moves away from a sound source, the sound intensity or sound level (β) decreases logarithmically, being represented by the equation:
The unit used for the sound level is Bel (B), but as this unit is large compared to most everyday sound level values, its usual multiple is the decibel (dB), so that 1B = 10dB.