Intensity

Intensity is

• the power per unit area carried by the wave,
• measured in watts per square meter (W/m$^2$).

Sound intensity is

• a measure of the sound power that contacts an area (e.g. eardrum, microphone).

Sound is audible to humans when its intensity is above

$$I_0 = 10^{-12}$$   $$\mbox{ W/m$^2$}$$$$,$$

with 1 W/m$^2$ being the threshold of feeling.

Intensity is related to pressure squared:

$$I = p^2/(\rho c),$$

where

• $p$ is the pressure,
• $\rho$ is the density of air (kg/m$^3$), and
• $c$ is the speed of sound (m/s).