ASA 127th Meeting M.I.T. 1994 June 6-10

5aNS1. Passive-active isolator control of sound radiation from a raft-cylinder system.

C. R. Fuller

E. Toffin

Vibration and Acoust. Labs., Dept. of Mech. Eng., VPI & SU, Blacksburg, VA 24061-0238

This paper analytically investigates the active-passive control of sound radiation from an elastic finite cylinder containing a rigid raft mounted on active-passive isolators. The disturbance is narrow band and acts at various locations on the internal raft. The control cost function is constructed from radiated far-field pressure or supersonic shell structural wave-number estimates in contrast to the usual practice of minimizing vibration directly under the mount attachment point. A control effort term is included in the cost function to overcome the problem of an undetermined system when many isolators are used. The results demonstrate that, using an acoustic-based cost function in conjunction with a fully coupled control approach of the active-passive isolators leads to significant radiated sound reduction with a very low control effort. The mechanism is similar to ``modal restructuring'' observed in previous plate studies. The results are contrasted to direct minimization of vibration at the attachment points. The influence of passive variables such as mount damping and stiffness on the control forces and the associated sound reduction are also studied. [Work supported by ONR.]