Acoustic plate mode touch screen

This touch screen consists of a plate of glass provided with transducers which divide the plate into two perpendicular sets of n and m acoustic delay lines. The intersections of these delay lines define a menu of n*m squares. The acoustic wave propagates in a symmetric mode at a frequency chosen to give a maximum attenuation in the presence of water on the plate. Each delay-line is part of a loop which oscillates periodically except when a finger is pressed onto the surface of the delay line.