Schemes for Noise Control in Three-Dimensional Enclosures

Abstract of Ph.D. Dissertation (July 1997)

Analytical and experimental investigations into active control of noise in a three-dimensional enclsoure are conducted with the long-term objective of developing viable schemes for noise control in rotorcraft cabins. Five rigid boundaries of the enclosure are constructed from acrylic material and a flexible boundary is constructed from aluminum material. This flexible boundary can be construed as a trim panel in a rotorcraft cabin. Active control is realized by using Lead Zirconate Titanate (PZT) actuators, condenser microphone sensors, and Polyvinylidene fluoride (PVDF) film sensors. Experiments are conducted for analog control of tonal disturbances and digital control of tonal and bandlimited disturbances. In the analytical work, frequency-domain models and time-domain (state-space) models are developed. For tonal disturbances, three different performance functions for global noise control and one performance function for local noise control are formulated and compared. Issues such as the number of actuators and actuator locations are investigated for symmetric and asymmetric plane-wave disturbances. The state-space model for bandlimited disturbances is implemented numerically and sound pressure levels (SPLs) are studied. The analytical predictions of SPLs are found to compare well with the experimental observations. The energy based performance function is found to give the best results. The control mechanisms are in general found to be different for panel and cavity controlled modes. The number of actuators and the locations of the actuators are found to have a significant influence on the magnitude of noise reduction achieved. For analog control of tonal disturbances, local noise reductions of 45 dB and 60 dB, respectively, are realized when one actuator pair and nine actuator pairs are used for control. In the experimental digital control investigations, the convergence coefficient had to be decreased over time in order to assure convergence of the performance function value. In the experiments, high local noise reductions up to 35 dB are obtained when only acoustic sensors are used. The results of the studies conducted thus far are indicative of the strong possibilities for carrying out active noise control in three-dimensional enclosures such as rotorcraft cabins.