Broadband and adaptive noise control with MOSFET controlled shunt-electro-mechanical diaphragm

Abstract

Noise pollution is receiving attention from researchers worldwide. Practically, the spectra of noise sources vary with their working conditions, such as the fan noise spectrum which varies with its rotary speed. This poses additional difficulties on conventional noise control methods, which are effective in a limited frequency band. In this thesis, two strategies are explored for the control of noise with a varying spectrum: to expand the effective bandwidth, and to adapt the control to the varying spectrum. The studies are based on a shunted, electro-mechanically coupled diaphragm (SEMD), which is essentially a suspended diaphragm with a moving-coil immersed in a permanent magnetic field. Its acoustic impedance is coupled with the electrical impedance of the shunt circuit, which can be connected or disconnected by a metal-oxide-semiconductor field-effect-transistor (MOSFET) in a programmed manner. An incident sound induces a Lorentz force on the SEMD, whose magnitude and phase are determined by the electrical impedance of the circuit. When the Lorentz force is out-of-phase to the driving force, a “force dipole” is formed, resulting in a reduction of sound transmission through the SEMD. Experiments demonstrate switching between branches of the circuit constructs a broadband switchable sound isolation. Further reduction of electrical impedance achieves a transmission loss over 20dB for 18-763Hz. The analysis predicts a band gap of almost infinite bandwidth with superconducting wires. When the SEMD is backed by a cavity, it forms a resonator whose resonant frequency is determined partially by the circuit. Lumped-parameter modelling reveals an additional resonant peak induced by the circuit. Analysis shows both down-tuning and up-tuning of both resonant frequencies, and the reduction of resistance broadens the effective tuning range, even to infinite bandwidth of up-tuning with superconduction. The analysis is experimentally verified, where resonant frequencies shift is observed when switching between branches of the circuit. The response time of the MOSFET is up to one micro-second, enabling the SEMD resonator to divide its working time to multiplex between different shunted states. The sound absorption of a multiplexed resonator is calculated by weighted sum of absorption coefficient of each multiplexed state, which can smooth out peaks and troughs in the frequency spectrum. The rather flat spectral response by the multiplexed resonator is studied via validated time-domain computation, which revealed its transient effect of frequency scattering that induces additional dissipation and enhances sound absorption. Analytical and numerical analyses show the transient effect of the SEMD scatters energy to sidebands of sum and difference frequencies of the incident and multiples of switching frequencies, whose efficiency can be improved by reduction of circuit resistance. In one example, experiment confirms the prediction that the efficiency of energy scattering for a broadband incidence can be up to 55%. Further experiment demonstrates the conversion of a tonal sound to bandlimited white noise by randomized switching. Such effect of frequency scattering offers alternative strategy in noise control.