Thomas J. Matula Philip L. Marston
Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814
The diffraction of subsonic flexural plate waves due to a discontinuity in fluid-loading is experimentally investigated. A tone burst of flexural waves propagates down a plate, the lower section of which is submerged in water. Observations indicate that there occurs a branching of energy as the flexural wave passes through the air--water interface. A portion of the energy continues along the plate as a subsonic flexural wave with an associated evanescent wave. A second acoustic wave (which is termed transition radiation) originates at or near where the plate crosses the interface, and propagates in water to the far field. In the near field of the interface there exists an interference between the two acoustic waves in water that results in a series of pressure nulls. The pressure nulls are associated with a (pi) phase change in the wave field and are indicators of wave front dislocations [P. L. Marston, ``Geometrical and Catastrophe Optics Methods in Scattering,'' Physical Acoustics (Academic, New York, 1992), Vol. 21, pp. 1--234]. A computation of the wave field in an unbounded fluid due to a line-moment excitation of a plate is comparable with the null pattern observed but differs in certain details. [Work supported by ONR.]