146 R. Schultz and D. Joffre Fig. 13.7 Example expanded response results with mode truncation, using only modes 1,2, and 3 in the expansion. Left: Time response at one b-set DOF. Right: PSD at one b-set DOF Fig. 13.8 Acoustic chamber model showing the three input locations and a-set DOF (left), b-set DOF located on the surface of a cylindrical test article (center) and c-set DOF distributed throughout the chamber volume (right) 13.4 Transmissibility Expansion for Acoustic DOF in a Large Domain Model Expansion of a large domain using transmissibilities is demonstrated using a model of a 21x25x30 foot reverberation chamber. Due to the size of this domain, the element size was limited to 4 inches, which limits the maximum frequency of the analysis to around 500 Hz. Three independent inputs were provided by surface velocities at small patches in three of the bottom corners of the chamber model, which could represent three loudspeaker inputs, as shown in Fig. 13.8. Locations of 20 a-set DOF are shown as red dots in the left image of Fig. 13.8. These locations were chosen at random in this case, rather than with the EFI algorithm. Two sets of expansion DOF were chosen, representing two different use cases. The first, the b-set DOF, represents points on the wetted surface of a cylindrical test article located in one corner of the chamber, useful for cases where the as-tested pressure on a test article is desired. The second, the c-set DOF, are just random points throughout the entire chamber domain, which provides an indication of the quality of the expansion over the entire space. These two sets of expansion DOF are shown in the center and right images in Fig. 13.8. The chamber has modes, just like the shell cavity, which can be viewed in terms of a pressure mode shape. Figure 13.9 shows two such modes of the chamber. The difference between the shell cavity and this chamber is the modal density. Where
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