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

## 3aSA8. The exterior acoustic modes of structures.

**Noelle Curry
Kenneth A. Cunefare
**

**
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*The George W. Woodruff School of Mech. Eng., Georgia Inst. of Technol.,
Atlanta, GA 30332-0405
*

*
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The use of a modal-style approach for the analysis of the exterior
radiation characteristics of structures continues to receive increasing
attention. This approach generally seeks to find a set of orthogonal functions,
or acoustic modes, that diagonalize a radiation operator in the exterior domain
of an extended radiator. These acoustic modes are found through an
eigenfunction/eigenvector analysis or singular value decomposition analysis of
the radiation operator. The eigenvalue or singular value associated with a
given mode is directly proportional to the radiation efficiency of that mode.
Here, the frequency dependency of the acoustic modes of a baffled beam is
examined. Further, the dependency of the radiation efficiencies and mode shapes
on the number of degrees of freedom permitted in the derivation of the
radiation operator is investigated. It is demonstrated that the accuracy of the
acoustic modal representation depends on the number of degrees of freedom
permitted in the derivation of the radiation operator. The most efficient
acoustic modes are least sensitive to increasing degrees of freedom. The least
efficient acoustic modes are most sensitive to changes in degrees of freedom.
This behavior has significant impact on applications of the exterior acoustic
modal approach that seek to exploit the least efficient modes.