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

## 3aSA12. Active control of sound power using acoustic basis functions.

**Koorosh Naghshineh
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*Acoustics and Radar Technol. Lab., SRI International, 333 Ravenswood Ave.,
Menlo Park, CA 94025
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**Gary H. Koopmann
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*Penn State Univ., University Park, PA 16802
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An improved method of active structural acoustics control is presented
that is based on the minimization of the total power radiated from any
structure expressed in terms of a truncated series sum. Each term of this sum
is related to the coupling between the orthogonal eigenvectors of the radiation
impedance matrix (referred to as ``basis functions'') and the structural
surface velocity vector. The basis functions act as surface velocity filters.
These acoustic basis functions are found to be weak functions of frequency but
their corresponding weighting coefficients increase monotonically with
frequency. The minimization of the radiated power is shown to result in a
structural surface velocity vector that couples poorly to those acoustic basis
functions that account for high efficiency sound radiation. This strategy is
demonstrated numerically for a clamped-clamped baffled beam in air. An
unexpected benefit of the control strategy described is that it provides a
rational procedure for selecting the number and placement of actuators and
sensors on a structure for effective control. This development is significant
since this procedure doe not require a priori knowledge of the dynamics of the
structure.