Numerical Modeling of an Active Headrest

Active noise control (ANC) in headsets has been successfully implemented in numerous research and practical applications. In situations where it may be deemed unsafe or impractical for a person to use such hearing protection, active headrests have been proposed as an alternative approach. Using loudspeakers and microphones, the control system attempts to attenuate external disturbances and create quiet zones around the head of a listener. The purpose of this paper is to quantify the acoustic performance limitations of an active headrest using virtual sensors. The approach of virtual sensing is to estimate the pressure at remote locations based on measurements made by nearby microphones. Using a numerical model, the performance is evaluated for diffuse primary fields as well as primary fields consisting of an individual plane wave. In both cases the size and shape of the quiet zones are frequency dependent, becoming smaller at higher frequencies. In the case of a plane wave primary field, the performance is poor in certain frequency ranges due to sensor locations coinciding with pressure nodes, which are present due to the interference between the incident and reflected sound fields. This behavior is not observed in the case of diffuse primary fields since the pressure is more spatially uniform. The quiet zones resulting from conventional ANC at physical sensors are then compared with ANC at virtual sensors located near the ears. The virtual microphone arrangement is shown to perform poorly at certain frequencies due to the assumption of equal physical and virtual primary pressures. When the difference between the primary pressure at the physical and virtual locations is taken into account, using the remote microphone technique, performance is improved, provided accurate transfer functions are available.