Adaptive Order FEM Approach to Vibroacoustic Simulation in the Automotive Field

As the legislation for pass by noise (PBN) has recently become more stringent, car manufacturers face again a challenging task to reach the emitted noise targets (70dB(A)). It is clear that a good acoustic design of the engine bay is required to sufficiently attenuate the powertrain component in the radiated noise. For decades, engine bay treatments have been designed with the main purpose of "just absorbing" the noise radiated by the engine surfaces. The treatments were applied simply on the engine bay's walls and their NVH targets were expressed in absorption coefficients. This indicates that these parts were meant to influence the transmission path between source and receiver just by dissipating as much as possible the noise impinging on them. Since a few years, though, thermo-acoustic powertrain encapsulation systems, often mounted also directly onto the powertrain, have appeared on the market. Their popularity can be understood by considering the synergy they provide in tackling both a noise issue and providing re-duction of CO2 and fuel consumption, thanks to heat storage effects. The design of this type of innovative treatments, though, is somehow less obvious: next to pure acoustic absorption, also the panel transmission of the treatment will affect the noise finally radiated outside of the engine bay. It is therefore not so clear which metric should be used to assess the treatment's performance. In this article, this important aspect of the design of engine bay treatments is analyzed in detail using as a test case an engine bay mock-up for which external Acoustical Transfer Functions (ATFs) are simulated and measured with and without acoustic treatments. The numerical methodology includes use of Finite Elements Method Adaptive Order (FEM AO) technology in combination with standard poro-elastic FEM elements for the treatments. Both engine bay mounted and engine mounted treatments will be addressed.