Generation of modally pure acoustic surface waves in a tapered partially filled duct

Abstract

The acoustic modes are predicted in a rectangular duct partially filled with rigid-porous material. When the gap and porous material layer are both thin, the duct supports only quasi-surface wave propagation. By tapering the layer and duct cross-sections, the quasi-surface wave is transformed into an infinite-plane fundamental surface wave mode corresponding to the pole of the planewave reflection coefficient. The surface wave is modally pure; consequently it is possible to perform experiments on these modes without interference from other scattering phenomena. Moreover, by use of a tapered partially filled duct, it is possible to make surface wave measurements on narrow strips of material even for frequencies of the order of 1 kHz. Measured surface wave data from a prototype acoustic surface wave cell are presented which validate this measurement strategy.