Problem 4.162

A point isotropic sound source is located on the perpendicular to the plane of a ring drawn through the centre $O$ of the ring. The distance between the point $O$ and the source is $l=1.00 \mathrm{~m},$ the radius of the ring is $R=0.50 \mathrm{~m} .$ Find the mean energy flow across the area enclosed by the ring if at the point $O$ the intensity of sound is equal to $I_{0}=30 \mu \mathrm{W} / \mathrm{m}^{2}$. The damping of the waves is negligible.