Answer
$$1.71 \times 10^{-7} \mathrm{m}
$$
Work Step by Step
Using Eq. $17-27$ with $I_{1}=9.60 \times 10^{-4} \mathrm{W} / \mathrm{m}^{2},$
$\omega=2 \pi(2000 \mathrm{Hz}), v=343 \mathrm{m} / \mathrm{s},$
and $\rho=$ $1.21 \mathrm{kg} / \mathrm{m}^{3},$ we obtain
$$
s_{m}=\sqrt{\frac{2 I}{\rho v \omega^{2}}}=1.71 \times 10^{-7} \mathrm{m}
$$
