Christophe MARCHETTO : "Experimental characterization of the vibro-acoustic response of panels under random excitation by measuring sensitivity functions "

Thesis submitted on February 14, 2018

Defense of a doctoral thesis from the University of Lyon in international co-supervision between the University of Sherbrooke (Sherbrooke, Canada) and INSA Lyon Laboratory: LVA Doctoral School ED162 MEGA

Abstract:

The experimental vibro-acoustic characterization of panels submitted to random pressure fields is of great interest in the industry as well as in research laboratories. For the transport sector, this type of excitation can be found when a turbulent flow develops at the wall of a moving vehicle for example. The pressure fluctuations induced by the turbulent boundary layer excite the panels which radiate a noise inside the cabin. The experimental reproduction of those pressure fluctuations requires test means which can be very costly (i.e., wind tunnel, in situ tests) and whose physical parameters can hardly be controlled. The repeatability of measurements can thereby be questioned which makes it hard to compare different technological solutions. A second example of random pressure field is the diffuse acoustic field. This latter is usually reproduced in a reverberant room which is often coupled with an anechoic chamber by means of the panel whose acoustic insulation is to be tested. A pressure field is assumed to be diffuse if the acoustic energy comes from every direction with an equiprobable intensity of the incident waves. This assumption is never fully reached in practice (lack of grazing incident waves, strong modal behavior of the room at low frequencies, etc.). A laboratory tool which allows reproducing the effect of those random excitations in a controlled environment is therefore of great interest.
In this context, this thesis aims at developing an experimental method to characterize the vibro-acoustic behavior of panels under random pressure fields without using the common test means (wind tunnel, reverberant room, in situ tests, etc.). For relevance sake, this approach must compensate for the previously stated issues. The approaches studied in this work are based on the mathematical formulation of the problem in the wavenumber domain. This latter allows an explicit separation of the contributions of the excitation via the wall-pressure cross-spectrum, from those of the vibro-acoustic behavior of the panel via so-called ‘sensitivity functions’. Assuming the wall-pressure cross-spectrum of the excitation is known, it is only required to experimentally determine those sensitivity functions, on the panel or in the acoustic medium, to determine the response of the panel to the considered excitation by post-processing. Two methods aiming at determining the sensitivity functions will be numerically and experimentally studied: the source scanning technique and the method based on the reciprocity principle. Results obtained with those method are compared to measurements using standard test means to attest the validity of those methods. Several vibro-acoustic indicators will be confronted while considering the two previously mentioned excitations and for two types of panels: an academic panel and a ‘complex’ from the aeronautic sector. This latter shows the applicability of the method in an industrial context.