Importance of the evaluation of structure-borne NVH performance for lightweight trim design

The design of automotive parts for the control of interior NVH involves the analysis of the noise contribution from interior carpet and dash insulator systems. The interior trim is usually designed with respect to insulation and absorption performance, for the control of air-borne noise that is commonly admitted to dominate the overall vehicle NVH for frequencies above approximately 800 Hz. Below this frequency range, it can be shown that the interior noise level is more dominated by structure borne phenomena that are transmitted through the vibration of the body structure, and for which the influence of the insulators is usually considered in a simplified way. In this low frequency range, it is preferred to exploit stiffness and damping countermeasures that act directly on the panel vibration, assuming that the vibration reduction will result in interior noise reduction. From this stand point, it sounds natural to privilege the selection of insulators, which on top of their air-borne primary function can also influence the panel vibration reduction at low and medium frequencies by means of added stiffness and/or damping. This is driving to favour stiffer, thus heavier porous decouplers. In this paper, we first of all show by means of FE simulations on simple flat samples the influence of decouplers on the panel vibration. Concurrently, it can be shown that the interior structure borne noise performance – that is the relevant customer annoyance – is also directly influenced by the insulator transmissibility. Due to the spring-mass nature of insulators, this may lead to the selection of softer and therefore lighter decouplers. Therefore, in the context of light weight insulator products, it is crucial to demonstrate the influence of material selection on the transmission of structure borne noise from the structural vibration to the interior acoustics. In this paper, the flat sample test case is extended to a vibro- acoustical plate-box case. By means of FEM simulation, we show the relation between the decoupler mechanical properties and the final structure borne noise performance. The results on flat samples offer an overview of different technical solutions, including felt, light foam or viscoelastic foam. The validation on a plate-box test rig confirms the simulated tendencies. At the same time, due to the manufacturing limitations of the decoupler materials, it is shown that the mechanical properties driving the final performance can come into conflict with other functional requirements, such as the density, or static compression. For light weight technologies, a proper analysis leads to comprehensive strategies for the design of 3D parts. In this article, we propose a particular focus on injected fibre technologies, where the simulation can be used to find the best material density selection versus thickness, with a final demonstration on a vehicle floor application.