ASA 127th Meeting M.I.T. 1994 June 6-10

4pSAb3. Reflection of a subsonic flexural wave on a plate at an air--water interface and far-field observations of the transition radiation in water.

Thomas J. Matula

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

An investigation of radiation by subsonic flexural plate waves due to a discontinuity in fluid loading is described. A tone burst of flexural waves propagates down a plate, the lower section of which is submerged in water. Measurements indicate that there occurs a branching of energy as the flexural wave passes through the air--water interface, with little reflected energy. A portion of the transmitted energy continues along the plate as a subsonic flexural wave with an associated evanescent wave. A second acoustic wave (which is referred to here as term transition radiation) originates at or near where the plate crosses the interface, and propagates in water to the far field. The near-field interference between the two acoustic waves in water results in a series of pressure nulls [T. J. Matula and P. L. Marston, J. Acoust. Soc. Am. 94, 1877 (A) (1993); T. J. Matula, Ph.D. dissertation (1993)]. Far-field observations of the transition-radiation angular pattern resemble that of a line quadrupole (approximating the farfield radiation due to a line moment on an uniformly loaded thin plate). The agreement between measured and modeled patterns is improved, however, by displacing the quadrupole horizontally behind the surface of the plate. [Work supported by ONR.]