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Noise inside vehicles decreases the passengers comfort. Most vehicles use lining materials to absorb this noise. Since the volume cabin is rather reduced, these liners must be light and thin, thus absorbing just in the high frequency range of the noise spectrum. Microperforated Panels (MPPs) provide a tool to develop thinner absorbers at lower frequencies. The utility of MPPs as wideband absorbers has been already proved. In this paper we discuss the design of even thinner MPPs capable of affording absorption inside vehicles in the frequency range of the cabin noise. A model is proposed which allows predicting the performance of such MPPs.

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.