### ASA 126th Meeting Denver 1993 October 4-8

## 4aSA5. Material tailoring of structures to achieve a minimum radiation
efficiency.

**Koorosh Naghshineh
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*Acoust. and Radar Technol. Lab., SRI International, 333 Ravenswood Ave.,
Menlo Park, CA 94025
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**Gary H. Koopmann
Ashok D. Belegundu
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*Penn State Univ., University Park, PA 16802
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A strategy is developed for designing structures that radiate sound
inefficiently in light fluids. The problem is broken into two steps. First,
given a frequency and overall geometry of the structure, a surface velocity
distribution is found that produces a minimum radiation efficiency. This
particular velocity distribution is referred to as the ``weak radiator''
velocity profile. A finite element adaptation of the integral wave equation is
combined with the Lagrange multiplier theorem to obtain this surface velocity
distribution. Second, a distribution of Young's modulus and density
distribution is found for the structure such that it exhibits the weak radiator
velocity profile as one of its mode shapes. Extensive use of structural finite
element modeling as well as linear programming techniques is made to find this
distribution. The result is a weak radiator structure. When compared to a
structure with uniform material properties, the weak radiator structural
response is found to exhibit lower wave-number content in the supersonic
region. The effect of modal overlap on the performance of the weak radiator
structures is found to be negligible. The example of a simple beam radiating in
a rigid baffle is used for the purpose of illustration.