The lower plate should be quite thick ~40 mm or more! to
avoid transverse oscillations of the structure in the kHz
range, and the upper plate should be relatively thin and light
to minimize the force on the piezos induced by lowfrequency
vibrations transmitted from the soft rubber support
to the upper plate. A suitable plate can be made from doublesided
circuit board, using the upper side as a grounded
shield. Such a system has been constructed by the author to
measure high-speed tennis ball impacts, and the results will
be presented elsewhere.