$\begin{aligned} & \text { Sol. } \left.\quad \Delta \phi=\frac{2 \pi}{\lambda} \Delta x \right\rvert\, \\ & \frac{\pi}{3}=\frac{2 \pi}{\lambda}(6 m) \\ & \Rightarrow \quad \lambda=36 m \\ & \quad V=f \lambda=(500 \mathrm{~Hz})(36 m)=18000 \mathrm{~m} / \mathrm{s}=18 \mathrm{~km} / \mathrm{s}\end{aligned}$