| A passive and lightweight technology to cancel low frequency sound was investigated. Simple lightweight mechanisms were embedded inside a double walled panel to radiate destructively interfering waves. The embedded mechanisms induce out-of-phase motions that destructively interfere with radiation from the panel's in-phase motions. A panel embedded with these mechanisms was found to cancel more sound than heavier equivalent barriers. An optimization process was developed to configure the mechanism placements for maximum sound cancellation. A finite element direct coupling method was used to relate the acoustic cavity interactions to the panel's plates. This direct method was advantageous because it was compatible with all types of acoustic and structure finite elements, and did not require formation of a coupling matrix. Topology optimization was used to explore other possible geometries of the mechanisms. The results of the optimal placements, boundary interactions, and the new machine geometry, resulted in the construction of an optimized sound panel. Experimental studies of the panels were conducted with a laser-Doppler scanning vibrometer, from which the acoustic power was then extrapolated directly from the surface vibrations. Two optimized panels produced less sound than a baseline panel with the same number of mechanisms by an average of 15.7 dB and 5.6 dB over the low frequency range of 550 Hz. |
