What is Sound intensity?

The sound intensity $I$ is the acoustic power crossing one square metre facing the wave: $I = P/S$, in $\mathrm{W}/\mathrm{m}^{2}$. The quietest audible sound is about $I_0 = 1.0 \times 10^{-12}\,\mathrm{W}/\mathrm{m}^{2}$; pain begins near $1\,\mathrm{W}/\mathrm{m}^{2}$.