Sound vs. pseudo-sound contributions to the wind noise inside a car cabin: A modal approach

We develop in this paper an analytical solution for the sound transmitted into a rigid rectangular cavity by a flexible plate loaded by the turbulent boundary layer of a grazing external flow. This simplified geometry aims to represent the vibro-acoustic response of a car front door window excited by the turbulent air flow that forms along the exterior surface of the vehicle. The modal expansion method is used to describe the plate displacement as well as the acoustic pressure in the cavity. The fluid loading is simulated using a direct noise computation code in the wake of a car rear view mirror. This code solves the full compressible three dimensional Navier-Stokes equations with highly accurate space and time algorithms and is able to capture the low amplitude acoustic part of the wall pressure fluctuations. The advantage of the modal approach is that the structural modes of the plate are represented in the wavenumbers' space as an array of dots. It is thus possible using a wavenumber diagram to separate the plate modes which are spatially coupled to the propagative acoustic component from the modes which are spatially coupled to the aerodynamic convected component of the wall pressure fluctuations. Although the acoustic component is of small magnitude compared to the convected component of the fluid loading, it is shown that its contribution to the interior sound pressure level is crucial above the acoustic critical frequency of the plate.