12 Expansion of Coupled Structural-Acoustic Systems 137 Fig. 12.6 Structural mode expansion (mode 12, 1047 Hz) from left to right: real structural mode, imaginary structural mode, real acoustic mode, and imaginary acoustic mode Fig. 12.7 Acoustic mode expansion (mode 38, 1969 Hz) from left to right: real structural mode, imaginary structural mode, real acoustic mode, and imaginary acoustic mode This expansion problem, just like all expansion problems, was sensitive to mode selection and truncation. Through preliminary expansion simulations, it was observed that omitting the rigid body modes produced an improved solution. In addition, preliminary analysis showed that modes up to 2500 Hz (the first 80 modes) were required to get a good match to the actual response at the n-DOF. While there are only a few structural modes in this bandwidth, there are many acoustic modes and coupled structural-acoustic modes. As such, a high number of modes were required. For other coupled systems with fewer acoustic modes, fewer total system modes and thus fewer a-DOF may be required.
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