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Les antennes paraboliques ultra légères des satellites de télécommunication, sont fabriquées en matériaux compositesà partir de coques multicouches très minces obtenues par tissage. Pendant la première phase de vol du lanceur, lors de la traversée des premières couches atmosphériques, les excitations vibro-acoustiques dues au système de propulsion et aux forces aérodynamiques sont les plus critiques. La charge acoustique de type aléatoire induite sur la structure de l'antenne, devient alors très importante et peut provoquer des dommages aux structures et auxéquipements. Pour réduire les charges acoustiques, les concepteurs utilisent des coques micro-perforées pour permettreà la structure de (( respirer )) et de réduire ainsi la charge acoustique. Le dimensionnement et l'optimisation de ces structures nécessitent des outils de calcul numérique. La prise en compte de la perforation du matériau pour le calcul de la charge induite par l'excitation acoustique aléatoire n'est pas classique. L'objet de cetteétude est de proposer un modèle d'impédance locale représentatif de la micro-perforation pouvantêtre utilisé dans un code de calcul vibro-acoustique. Mots clés : Structures micro-perforées / vibro-acoustique / calcul numériqueAbstract -Vibro-acoustic analysis of micro-perforated composite structures used in space craft industry. In tele-communication satellite, the light weight parabolic reflectors are made with multi layered composite materials. During the lift off of the launcher and the first stage of the flight, the vibroacoustic excitations due to the thrust system and to the aerodynamic forces are the most critical. The random acoustic load applied on the antenna structure becomes very important and can cause damage to the structure and their equipments. To reduce the acoustic loads, micro perforated thin shell structures are used in order to reduce the acoustic load applied to the structure. CAD and numerical software tools are needed to design and optimise such structure. The computation of the acoustic load induced by a diffuse field on a micro perforated structure is not classical. The aim of this study is to develop a numerical local impedance model to represent the effect of the perforation to be used within vibro-acoustic software.

ESI Group (Formerly STRACO) has been involved during the last two decades in the numerical prediction of noise reduction index of ARIANE 5 fairing. During the 1990's, STRACO developed an axi-symmetric, boundary element model of the fairing where the fairing protection made of distributed Helmholtz Resonator is modeled by equivalent local impedance. Recently, the fairing acoustic protection has been replaced by a foam-made insulator. Such porous materials are widely used by transportation industries to improve the payload acoustic comfort. In collaboration with automotive industry, ESI group developed RAYON-VTM, a powerful tool allowing the predicting of fully trimmed vehicle vibroacoustic response up to 500 Hz. RAYON VTM model the porous-elastic material using a 3D Finite Element (PEM), based on the Modified-Biot-Equations. This new module of RAYON software has been applied to predict the vibroacoustic response of the ARIANE 5 fairing allowing a detailed modeling of the acoustic protection. A fully 3-D model of the fairing has been developed. The results show the influence of intrinsic modeling of the porous-elastic protection as well as the influence of non-axi-symmetric details usually neglected in the axi-symmetric approach.