Modulated wind noise in a road vehicle due to atmospheric fluctuations and reduction measures

In vehicle development, noise reduction is critical for ensuring passenger comfort. As electric vehicles become more common and engine and vibration noises are reduced, aerodynamic noise around vehicles becomes prominent. In particular, wind noise modulation caused by atmospheric turbulence, wind gusts, and the wake of preceding vehicles is an issue because it causes discomfort to passengers. However, there is no clear definition of the modulated wind noise, and the mechanism by which wind noise modulates is poorly understood. Furthermore, there is no established quantitative evaluation method for modulated wind noise, and reduction measures have not progressed. This is because previous studies have relied on on-road tests, which are not reproducible due to changing atmospheric conditions. To address this issue, automakers are developing devices that simulate natural winds by generating turbulence in the wind tunnels. However, these cannot simulate on-road wind conditions because large-scale fluctuations in natural winds are difficult to reproduce. In this study, we simulated the fluctuating winds experienced by vehicles on the road and reproduced the modulated wind noise. Using the modulation power spectrum, we confirmed that wind noise modulation can be quantitatively evaluated, and the contribution of on-road wind components to the modulation power was investigated. Furthermore, we clarified the mechanism of the flow field around the vehicle, which causes wind noise modulation. Modulated wind noise is an acoustic phenomenon in which high-frequency aerodynamic noise above 1 kHz is produced by the longitudinal vortex of the A-pillar, which becomes prominent because of crosswind and is amplitude-modulated by mainstream fluctuations. The modulation power peak of wind noise can be reduced by suppressing the A-pillar separation.