Far Field Extrapolation of the Noise Radiated by Vehicles to the Near Field

Our research group is currently involved in the identification of aggressive drivers by means of the near field vehicle noise picked up by two onboard microphones, one for the rolling noise and another for the engine noise. We have demonstrated that aggressive drivers radiate to the near field noise levels 5-9 dB higher than the average of normal drivers [1]. In order to relate these near field measurements with the vehicle noise radiated to the far field, we must provide the corresponding extrapolation filter. This filter should contain the effects of geometrical spreading, air absorption and ground interaction. Whilst the first two are easily implemented taking into account soundly based international standards, the effect of the ground on the propagated sound from the near field to the far field depends on its intrinsic characteristics such as the type (local or extended reaction), layering and acoustical impedance of the soil. Two of the most common type of soils, asphalt and land, are characterized in this paper and the corresponding extrapolation filters are calculated accordingly. It is demonstrated that the ground effect may be significantly relevant at medium frequencies. Modeling predictions are then compared with experimental results. The ground impedance is measured following the ANSI S1.18 Standard. Near and far field noise levels are concurrently measured with vehicles running at constant velocities between 40 and 90 km/h.