The Effects of Unsteady On-Road Flow Conditions on Cabin Noise 2010-01-0289

At higher speeds aerodynamic noise tends to dominate the overall noise inside the passenger compartment. Large-scale turbulent conditions experienced on the road can generate different noise characteristics from those under steady-state conditions experienced in an acoustic wind tunnel. The objective of this research is to assess the relationship between on-road flow conditions and the sound pressure level in the cabin.

This research, covering links between the unsteady airflow around the vehicle and aeroacoustic effects, is a natural progression from previous aerodynamic studies. On-road testing was undertaken using a current production vehicle equipped with a mobile data logging system. Testing was carried out on major roads at typical highway speeds, where wind noise is very significant. Of particular interest are high-yaw conditions, which can lead to a blustering phenomenon. Flow data were collected from a five-hole aerodynamic probe mounted above the roof of the vehicle, which recorded both flow speed and direction. Sound data were recorded using an acoustic head located in the passenger seat of the vehicle. GPS data were also recorded, allowing information about the vehicle's speed, location and heading to be logged. Video recordings of each run were also taken, allowing probe and audio data to be correlated against the road surroundings.

It was found that the variation of in-cabin sound pressure level with yaw in the wind tunnel was in close agreement with on-road data at low lateral turbulence intensities. However, at higher lateral turbulence intensity there was a discrepancy between the wind tunnel and on-road sound pressure level, specifically to noise levels near the leeward sideglass. Sound pressure levels in this area were quieter than expected, indicating that transient flows around the leeward side of the vehicle do not have time to fully develop to their steady, high SPL condition. This is not necessarily beneficial from the point of view of perceived noise.