VIRTualization of Experimental facilities in structural aCoustics by wall pressure syntHesis – VIRTECH

Using a network of loudspeakers or acoustic sources close to the wall to be tested has several advantages: cost, versatility and reproducibility. The challenge with these technologies is to be able to reproduce characteristics specific to the desired pressure fields, these characteristics evolving with the frequency, the flow speed, etc. During the project, two very different technologies were tested. The first consists of simultaneously driving a large number of loudspeakers (64 in our application) arranged in a regular grid network facing the structure. The difficulties of this type of approach lie in the development of real-time algorithms and in the correction of diffractions and non-linearities brought by the network itself. But thanks to this test method, it is possible to switch from a «diffuse field« excitation to a «turbulent flow« excitation instantly! The second technology is even simpler in terms of the equipment required: it uses only a single acoustic source that moves relative to the structure. The measurement itself takes longer and requires delayed post-processing, but the method allows for testing more complex structures with, for example, pronounced curvatures.

The project has produced numerous results: design of a 64-loudspeaker array optimized for pressure field synthesis, design of a robotic synthetic antenna, successful applications for the synthesis of pressure fields with the characteristics of a plane wave, a diffuse field or a turbulent flow, development of an experimental method for identifying a pressure field by indirect measurement of the structure's vibrations, development of MEMS microphone antenna technology on a flexible support for measuring a wall pressure field. The activities carried out during the VIRTECH project also led to the initiation of numerous projects and collaborations, notably through the International Research Project «Centre Acoustique Jacques Cartier« but also through a regional project jointly funded by the CTTM and Le Mans Acoustique. The VIRTECH project also helped strengthen ties with industrial stakeholders, particularly in the aeronautics industry.

The approach taken by the VIRTECH project may be of interest to large industrial groups. This includes transport manufacturers in the aviation, rail, and automotive industries. For example, the approaches developed could, in some cases, eliminate the need for wind tunnel measurements, such as the S2A wind tunnel (PSA, Renault), which is highly resource-intensive. However, it is also possible to consider developing experimental platforms (such as the LMA's PSIVA platform or the CTTM's) to integrate the developed approaches and provide services.

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Sustainable transports require the use of lightweight structures to improve their energy efficiency. New materials (steel with very high strength, polymeric materials, hybrid polymer / metal or organo-mineral, biocomposites etc ...) are both very rigid and lightweight, with the drawback of poor acoustic performance. Improvement requires very expensive in-situ tests, especially in the case of turbulent boundary layer noise. Manufacturers must revolutionize their characterization and measurement protocols to transpose their knowledge from metallic materials to these new technologies. Currently available experimental facilities are very expensive, have a heavy carbon footprint and do not allow mastering all the physical parameters (noise, field temperature, homogeneity, etc.). The reproducibility of measurements can then be questioned, making it difficult to compare different technology solutions. It is therefore of considerable interest to have a tool usable in production, in order to reproduce random excitations characteristics and to evaluate systems with a high accuracy. The VIRTECH project brings an approach and tools for pressure field synthesis for integrating acoustic feature in new materials, replacing in-situ campaigns, significantly reducing costs without compromising realism. Under the terms "pressure fields synthesis" are grouped experimental methods (and optionally computational) that reproduce a pressure field complying with prescribed properties (homogeneity of the field, temporal and spatial correlations) using reproduction sound systems (speakers, mobile sources). These methods can be used in real-time or in a later post-processing step. Applications are as varied as characterization of acoustic performance, studies on aging or resistance in extreme conditions or even for sound quality applications. Potentially, the "field synthesis" could be used to test systematically strategic parts (in aeronautics or space activities in particular) directly on the assembly line. The project will address the following issues:
(1) the identification of useful components of the acoustic field by direct measurements (using an antenna of MEMS microphones), by indirect measurement by observing the vibro-acoustic behavior of the tested structure and by analysis of a numerical modeling of the pressure field. These analyzes will be coupled with a perceptive study for a comprehensive assessment of the filtering process of the exciting pressure field.
(2) optimization of a network of sources and its management to synthesize a pressure field having the characteristics of a diffuse field or a turbulent boundary layer by leveraging the filtering phenomena observed previously. The optimization will focus on various criteria: speed of implementation, spatial resolution, error sensitivity, cost, reproducibility.
(3) The confrontation between the obtained results and those obtained by standard test means including direct measures of the generated field and perceptual studies. Finally, the capacity of these alternative means will be illustrated with applications on "vibro-acoustic metrology" or "real-time quality control test bench"