Removing material from the centre of the xylophone bar causes the
frequency to drop.  This is because the majority of the oscillating
mass is at the extreme ends of the bar as it is vibrating in its
'free-free' mode, i.e., sitting on its two nodal points with the two
ends moving one way whilst the centre moves the other way.  Removing
material from the centre makes this area less stiff so the stiffness
of the system drops whilst the vibrating mass hardly changes.

The frequency is raised by removing mass from the extreme ends of the
bar.  It is obviously important to remove material from the back of the
bar in either case, or in the case of the ends, from each end equally.

Nicholas Simons, GB