When the ball is dropped, it generates a circular pulse that travels outwards towards the walls of the vessel. Upon reaching the boundary (radius \(r\)), the wave reflects back towards the center. As the reflected circular wave converges back to the single center point, the energy is concentrated into a smaller and smaller perimeter. This focusing effect causes the amplitude to increase drastically, theoretically becoming infinite (maximum) at the center.

The wave pulse travels from the center to the wall and back to the center.
Total Distance \(D = r \text{ (out)} + r \text{ (in)} = 2r\).
Time taken = \(\tau\).
Wave speed \(v\) is given by: \[ v = \frac{\text{Distance}}{\text{Time}} = \frac{2r}{\tau} \]

Answer: \( v = \frac{2r}{\tau} \)