The Noise Reduction Loss Due to Head Movement in an Active Noise Control Headrest

Local active noise control systems are preferred in practical implementations due to their simple structure and high effective frequency; however, the amount of noise reduction at the human ears changes with the head movement in local active noise control. In this paper, the variation of noise reduction in an active noise control headrest with two secondary loudspeakers at both sides of the headrest is investigated. Firstly, an analytical model with a scattering rigid sphere in free field is established. Then the noise reduction loss caused by the translational and rotational movements of the sphere is analyzed by numerical simulations. Finally, an empirical formula is developed based on the simulation results to predict the noise reduction due to the head movement. It is found that when the noise reduction at the initial position is 10 dB, the noise reduction loss below 1000 Hz will exceed 3 dB if the translation distance of the head becomes larger than 1.0 cm or the rotation angle is larger than <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.5^{\circ}$</tex>.