An examination of "smart" foams for active noise control
It is well known that acoustic foams are good high frequency sound absorbers but poor low frequency sound absorbers. It is also known that active noise control systems do not typically work well at high frequency due to the increasing order of the physical system model and increased computational demands. Therefore, it is sometimes necessary to use both passive and active systems in combination. In this thesis, the use of the passive absorber (acoustic foam) as the secondary sound source for an active noise control system is examined. The combination of the passive and active noise control systems should provide space and weight savings. Potential applications include aircraft and automobiles. Simple analytical models are used here to examine several basic "smart" foam configurations to determine the effects of foam thickness and actuator location. More complicated analytical models using spatial Fourier series representations of the sound fields are then used to model more practical "smart" foam features. Models are presented for foam backed, perforated panels. One of these models has been used to calculate the random incidence absorption coefficients for a foam backed, perforated panel. Two experiments are described--one with adaptive control and one without adaptive control. In the adaptive control experiment, a two coefficient adaptive controller was used to achieve a 40 dB reduction in the error signal proportional to the reflected sound pressure. In the other experiment, the control requirements for a "smart" foam were measured and compared with analytical model results. The measured and predicted results agreed with reasonable precision at high frequency, but at low frequency the measured control effort is higher than that predicted. This behavior was attributed to nonlinear specimen behavior. Several methods of "smart" foam control have been examined, including the incorporation of the error microphones into the foam. Actuation methods such as PVDF films, piezo-foam, ferro-foam, ceramic piezoelectrics, electromechanical shakers, and electrostatic drivers were examined.
Bolton, Purdue University.
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