ASA 128th Meeting - Austin, Texas - 1994 Nov 28 .. Dec 02

4aPAa3. Selected topics in computational ray methods.

Roger H. Hackman

Lockheed Palo Alto Res. Lab., Palo Alto, CA 94304-1191

Gary S. Sammelmann

Coastal Systems Station, Panama City, FL 32407-5000

Several selected topics intimately involved with the application of ray theory to acoustic scattering are discussed. The first topic is the application of quantitative ray theory to the low-frequency acoustic scattering from large aspect ratio solids. In this approach, the scattering amplitude is developed as a time-ordered perturbation series. The series for the elastic response is explicitly summed to obtain a closed form expression that is analogous to results obtained for spherical and infinite cylindrical geometries through application of the Sommerfeld--Watson transformation. Emphasis is placed on novel features that have no counterpart on these simpler geometries (e.g., ``bipolar'' coupling phenomena) and their implications for resonance excitation. A second topic deals with a near-field/far-field target strength model for more complicated geometrical shapes that is under development at CSS. But theoretical and experimental results are presented. A third topic is the high-frequency ``quasi-resonance'' phenomenon of thin shelled structures. The structure of the ``quasiresonance'' mode is obtained for both complex wave number (real frequency) and complex frequency (real wave number) extensions of the reflection coefficient for a fluid loaded flat plate. The results are used to synthesize the scattering amplitude for spherical shells using forms previously derived for the Sommerfeld--Watson transform.