Hybrid Feedforward-Feedback Active Noise Control for Hearing Protection and Communication

Research over the past decade has demonstrated substantial increases in noise reduction performance for circumaural hearing protectors through feedforward active noise reduction (ANR) based on least mean square (LMS) methods. However, commercialization of feedforward ANR hearing protection devices has yet to occur. This paper explores issues related to robust realization of feedforward ANR for hearing protection. Specifically, the dynamic range of noise sources, the potential for leakage around circumaural earcups, vibration conditions of high noise environments, and the need for communication requires ANR algorithms that are robust to large variations in the acoustical dynamics of hearing protectors. To meet this need, a hybrid ANR architecture that exhibits excellent stability margins and performance for both stationary and non-stationary noise sources is presented. The hybrid system is comprised of a Lyapunov-tuned leaky LMS feedforward component and a broadband digital feedback ANR system. The contribution of each component to ANR performance is adjusted by individual feedforward and feedback gain factors, and stability margins are defined by the net increase in these gains that can be accommodated while maintaining system stability. Stability and noise reduction performance of the hybrid system are validated experimentally using an earcup from a commercial circumaural hearing protector and using a prototype active earplug. Experiments are performed through flat plate or manikin testing using a DSP development system. Results demonstrate that stability margins are increased, in some cases, by several orders of magnitude.