Surface acoustic waves are propagated along piezoelectric material, in response to an electrical signal applied to an input transducer. During propagation along the material, the propagation surface of the piezoelectric material is flooded with a short burst of electrons, causing the emission of low energy secondary electrons. These electrons are immediately attracted from the negative to the more positive adjacent regions of the surface, neutralizing any potential variations on the surface. After the acoustic waves thereon have subsided, electrons trapped on the surface are left behind in a frozen pattern corresponding to the pattern of the acoustic waves. The acoustic waves can be reformed by again flooding the surface with a short burst of electrons. Output transducer means act to transform the reformed waves into an electrical signal.