### 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:
fricke@mit.edu