ROAD VEHICLE EXTERIOR NOISE RECONSTRUCTION: A SIMPLE DIFFRACTION BASED ENGINE NOISE PROPAGATION MODEL

摘要

Advanced car engine noise control requires the simulation of the sound field in a receiver position at some distance from the vehicle. The aim of the presented model is to capture the salient physics of the radiation problem, allowing to extract a limited number of key-parameters governing the sound propagation. In general, the prediction of noise transmission from the engine surface through holes and apertures in the engine compartment implies the numerical solution of a coupled interior/exterior problem, involving time consuming modelling and high computational costs. Here some typical features of automotive engine compartments are used to develop a simplified propagation model. Due to the highly irregular shape of the air cavity, it is assumed that massive scattering will occur in the engine bay, substantially reducing coherent reflections in the frequency range of interest for pass-by noise (300-3000 Hz). Cavity modes are consequently considered of subordinate importance and most of the diffracted energy is assumed to be absorbed in the acoustic lining. The use of classical frequency domain expressions for outdoor sound propagation and diffraction over barriers have been attempted. Reflection on the road surface is included using the mirror source technique. Experimental validation has been carried out on a box-like laboratory mock-up and on a real car. Good agreement has been obtained between measurement and calculation results, including the assessment of interference phenomena between paths through different apertures, showing the validity of the model. It is furthermore shown that only those paths which, from the engine surface, directly reach the different apertures and are diffracted outside the vehicle towards the receiver have to be taken into account.