Joseph E. Bondaryk
Ira Dyer
Leo Chiasson
J. Robert Fricke
Dept. of Ocean Eng., MIT, 77 Massachusetts Ave., Rm. 5-435, Cambridge, MA 02139
The MIT Structural Acoustics Group is making experimental measurements to determine the structural loss and radiation efficiency of an undamped, three-dimensional truss structure, constructed of 0.5-in. aluminum tubing connected by massive aluminum joints. The truss was excited with a shaker driven by wideband white noise out to 20 kHz. The total vibrational energy in the truss was calculated by spatially integrating the energy estimated from acceleration measurements made over the entire truss. This value and the total input power, as measured with an impedance head, allow estimation of the total loss factor for the truss as a function of frequency. Total radiated sound power was calculated by spatially integrating a set of average acoustic intensity measurements made over a grid surrounding the truss. Since the total radiated sound power is known, the total loss factor can easily be broken down into a structural loss factor and a radiation loss factor. These measurements show that the radiation loss is smaller than structural loss, but larger than radiation from an equivalent length single strut would perdict. The additional radiation is accounted for by a factor due to multipath in the truss. This factor is frequency dependent and can be estimated from the data by comparing to theory. [Research supported by ARPA and ONR.]