ASA 126th Meeting Denver 1993 October 4-8

5aSA9. Comparison of dual surface and tri-surface approaches for acoustical holography measurements of radiated and scattered fields.

Michael A. Sartori Joseph A. Clark

Carderock Div., Naval Surface Warfare Ctr., Bethesda, MD 20084-5000

Using near-field measurements of the pressure field of an immersed structure, an estimate of the structure's radiated field and scattered field can be made. With the pressure and the pressure gradient measured at discrete locations in the near field, a discretized solution of the exterior Helmholtz integral is solved to obtain the far-field pressure. This approach has been shown to work for both acoustic radiation and acoustic scattering problems. Using two normal surfaces to approximate the pressure gradient, parametric simulations were performed to determine the scattered field predictability of the approach for an insonified rigid sphere. The results indicate that this dual surface approach better predicts the scattered field when the surfaces are very close, which implies that a good approximation of the gradient is needed. A different approximation of the pressure gradient uses three surfaces instead of two. Using a cylindrical shaped measurement surface, the radiated pressure field from a monopole and the scattered pressure field from a rigid sphere are simulated for the different cases of the approximated gradient, namely the dual surface approach and the tri-surface approach. The effects on the predictability of the radiated field and the scattered field are discussed. [Work supported by ONR.]