Accuracy Requirements of Ear-Positioning for Active Control of Road Noise in a Car

Obtaining sound information at ear positions is essential to maintaining high noise reduction performance of active control in a car during head movement. In this paper, the accuracy requirements of ear-positioning in a local active noise control system with two secondary loudspeakers mounted on a seat shoulder are investigated. The variation of noise reduction at different locations is analyzed first based on a scattering model of a rigid sphere under various sound incident directions in a free field. The numerical results show that the average noise reduction loss due to the head translation and rotation increases with the head movement distance, and the value is larger at high frequencies or when the original noise reduction level at the initial location is high. By fitting the numerical simulation results, an empirical formula is developed to predict the noise reduction due to head movements. The experiment results in an electric car running at a speed of 50 km/h are presented to support the findings. Based on the measurements, the required distance of ear-positioning with an original noise reduction of about 9 dBA and the maximum allowable noise reduction loss being 3 dBA is no more than 2.0 cm.