$$ \begin{aligned} & \Delta x=\frac{\lambda}{2 \pi} \times\left(\frac{\pi}{3}\right)=\left(\frac{\lambda}{6}\right) \\\\ & \Rightarrow \quad \frac{\lambda}{6}=6 \mathrm{~m} \\\\ & \quad \lambda=36 \mathrm{~m} \\\\ & U=f\lambda=500 \mathrm{~Hz} \times 36 \\\\ & =18000 \mathrm{~m} / \mathrm{s} \\\\ & =18 \mathrm{~km} / \mathrm{s} \end{aligned} $$