Simply holding the rod in
one hand also provided excellent isolation, but the distance
to the laser beams was then not known accurately. In principle,
a much longer rod could have been used to delay the
reflected pulse, but a rod of length at least 10 m would have
been required to avoid the reflected pulse from a tennis ball.
A rod of length about 1.5 m is ideal for studying the impact
of small steel balls, and it also generates textbook examples
of compressional ~nondispersive! and transverse ~strongly
dispersive! wave modes that can be detected with a small
piezo at one or both ends.