Underwater Focussing of Sound by Umklapp Diffraction

Abstract

Scholte modes that are localized between a submerged axisymmetric-structured elastic plate and surrounding fluid can undergo mode conversion via Umklapp diffraction into radiative modes; this radiative response is verified by experiments that show focussing of underwater sound across a broad range of frequencies. The diffracted beams, that form a cone, are engineered to exist at a desired spatial position, associated with an abrupt change in the patterning of the plate. These structures take the form of grooves present only on one side of the plate, yet the focussing phenomena is achieved on both sides, even as viewed from the flat surface.