Simulation of the effect of the absorbing treatments in the engine compartment on the acoustic transfer functions from the power-train surface to exterior receivers

During the last 10/15 years, in the automotive field the development of quieter and quieter engines has progressively reduced the contribution of the power train to the exterior noise radiated by a vehicle and measured according to the ISO-362 standard. This fact has made the targets set by the ISO-362 standard less and less challenging for the power-train noise emissions and, together with the experience gained through the years, has put most OEMs in the position of being able to define the counter-measures suitable for the achievement of such targets simply in an empiric way. This is likely to be one of the reasons why, during the last years, while there has been a very big effort towards the development of progressively more and more refined numerical methods suitable for the analysis of power-train interior noise (both in the low and in the high frequencies' range), relatively little attention has been dedicated to the use of simulation methods for power-train exterior noise. As a matter of fact, even though there exist numerical methods (even commercially available ones, like the well-known Boundary Elements Method) that are in principle applicable to the simulation of power-train exterior noise, their use in the industrial design practice was seldom considered as a use-ful option during the last years. Even in the few cases in which an attempt was made, the application of numerical methods to exterior noise was hindered either by computational issues (for the determin-istic methods) or by theoretical issues (for the statistical methods). In view of the foreseen update of the ISO 362 standard in a more restrictive sense, this situation is likely to change and power-train exterior noise is likely to become again a relevant issue in the auto-motive industry. This will lead to a strong focus on the design of the acoustic trim parts positioned inside the engine bay and, in turn, the possibility of designing in an efficient and cost-effective way such parts will call for reliable simulation methods. This might be, then, the right time for a critical assessment of the numerical methods available for the simulation of exterior noise and their capability of tackling real industrial cases. This article intends to focus, in particular, on deterministic methods (Boundary Elements, Finite Ele-ments, Infinite Elements). These methods have been applied to the simulation of the exterior Acoustic Transfer Functions of a simplified (but not trivial) engine-bay mock-up and have been pushed to cover all the frequency range up to 3.5kHz. These methods have been assessed both for what concerns the quality of the correlation with experimental data and for what concerns their computational efficiency. A special focus has been kept on the possibility that these methods offer to analyze in an efficient way the effect of acoustic trim parts and the effect of the positioning of the apertures around the engine bay.