A metal rod rests on supports in a tank of water. A sound pulse, of some initial intensity, is generated at one end of the rod. Assume that the sound cannot leave the rod except as a result of partial reflections at its ends; and that there is no loss of intensity otherwise. After 2 round trips, the pulse travels back along the rod for a third round trip. How many end-reflections have occurred?

If the acoustic impedance of the metal is 2.00×10⁷ Pa·s/m, what fraction of the initial intensity remains at the start of this third round trip?