ASA 130th Meeting - St. Louis, MO - 1995 Nov 27 .. Dec 01

2aSA8. Acoustic radiation from a 3-D truss: Direct global stiffness matrix modeling results.

J. Robert Fricke

Leo Chiasson

Joseph E. Bondaryk

MIT, Rm. 5-218, 77 Massachusetts Ave., Cambridge, MA 02139

This study was initiated to evaluate the normalized sound power level radiated by a driven truss. The coincidence frequency for the beam elements was sufficiently high that global truss modes are not a concern. Radiation from local beam modes dominate the field. Acoustic radiation was modeled using a combination of the direct global stiffness matrix (DGSM) method and models based on radiation efficiency of cylindrical beams. Use of a nominal structural loss factor of 10[sup -4] for the beam material (aluminum) overestimates the measured field by an order of magnitude. Earlier studies suggest that a combination of structural loss in the joints and multipath wave-type conversion in the truss leads to a loss factor of order 10[sup -3]. For this study experimental data were inverted to estimate a frequency-dependent loss factor, which confirms the prior estimate of order 10[sup -3]. When the DGSM based model was run with the estimated frequency-dependent loss factor, the model results match the measured data closely. A power balance shows that the structural loss factor dominates the total power dissipated in the system, as one might expect for a lightly fluid-loaded structure. [Research sponsored by ARPA/ONR.] [sup a)]E-mail: