Improving near-field spatial uniformity of secondary sources for active noise control headrests

In an active noise control (ANC) headrest system, the near field uniformity of the secondary sound field affects the size of the generated quiet zone because the secondary sources are closer to the control zone than primary sources. In this study, a compound source with uniform near-field radiation is proposed to improve noise reduction performance of the ANC headrest system by using the sound pressure matching method. The compound source is driven with only one signal but consists of two loudspeaker units, whose amplitude ratio and phase difference are optimized with an ideal uniform target field and the secondary paths measured in the free field. In the paper, a numerical model in a diffuse sound field is developed first and the proposed compound source is demonstrated to be able to generate a larger quiet zone than a single loudspeaker. Then, the simulation results based on measured secondary paths and road noise data in a vehicle cabin are provided to show that the proposed compound source improves the average noise reduction of road noise by 2.0 dB in the 100-500 Hz frequency range compared to that with a single loudspeaker. Finally, it is shown that the proposed compound source is universally applicable and does not depend on specific application scenarios.