### 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.]