On Implemintation of Loudspeakers for Feedback Control, Open-Air, Active Noise Control Headsets

Abstract

The loudspeakers used in active noise reduction (ANR) headsets are generally identical to loudspeakers used in commercial headphones. Unfortunately, the frequency response characteristics of these loudspeakers are not particularly well suited for open-air active noise control (ANC). Open-air headsets float outside the ear with no contact between the system and the user and allow for regular conversation with others in the environment. This study has identified three limitations on the closed-loop performance of open-air headsets: the distribution of gain and phase in the loudspeaker's open-loop frequency response function, manufacturing variations in loudspeakers that can deviate from design specifications by up to 40%, and the variations in acoustic impedance coupling (ear-to-speaker) among users. This thesis explores the mechanisms that underlie these limitations with the goal of designing open-air headsets that are robust to manufacturing and user variations. Methods are introduced on ways to minimize the effects of manufacturing and user variations and are proven by experiment. With these variations minimized, the controller's design is only limited by the frequency response of the loudspeaker. A comprehensive examination of techniques to model moving-coil loudspeakers is presented followed by detailed studies on how each parameter affects the system's frequency response. A review of frequency domain control system design is then included to help the reader understand loop-shaping techniques. Finally, a compensator is designed for an open-air ANR headset using loop-shaping techniques and the robustness of the closed-loop performance is verified experimentally.