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.