The amount of energy flowing per unit time through a unit area that is perpendicular to the direction in which the sound waves are travelling is known as sound intensity. Sound intensity is given by the equation:
where P is the power through an area A. The SI unit of sound intensity is W/m2. Many sound intensity measurements are made relative to a standard threshold hearing intensity I0.
I0 = 10-12 watts/m2 = 10-16 watts/cm2
The decibel scale is the commonly used approach to measure the sound intensity
I(dB) = 10 log10[I/I0]
The human ear can detect the sound of very low intensity. The smallest detectable sound intensity is 1 x 10-12 W / m2. If the sound moves away uniformly from the source, the intensity decreases further away from the source. The intensity is inversely proportional to the square of the distance from the source.