Plane Sound Waves on Boundaries

Abstract

Analysis of a wave in an infinitely extended substance is possible only theoretically because in practice every substance terminates somewhere, i.e. it has a boundary. There the propagation of the wave is disturbed. If the material concerned borders on an empty space, no wave can go beyond this boundary because the transmission of such a wave always requires the presence of particles of material. At such a free boundary the wave will therefore return in one form or another. On a smooth boundary one then speaks of reflection, on a rough boundary of scattering. In this connection the roughness, i.e. the irregularities of the boundary, should be measured in terms of the wave length. If another material is behind the boundary, and adheres to the first material so that forces can be transmitted, the wave can be propagated in it, although usually in a more or less changed direction, intensity and mode.