~2! The length of the brass rod was not sufficient to avoid
reflections off the far end of the rod. The transit time of a
pulse from the upper surface of the piezo to the lower end of
the rod was 30 ms, resulting in a standing wave of period 60
ms or frequency 16.7 kHz. This mode was not excited with
any significant amplitude by any of the balls tested since the
ball contact time was longer than 120 ms in all cases.