<p>Generally, heavier noise control treatments are favored over lighter ones since heavier acoustical materials tend to insulate (block) noise sources more effectively than do lighter materials. In automotive applications, however, heavier materials cannot always be adopted because of concerns over the total weight of the vehicle. Thus, it would be useful to identify lightweight acoustical treatments that can mitigate vehicle interior noise. Automotive sound packages have both absorption and barrier characteristics, and there is inevitably a trade-off between these two. Therefore, it is important to study the exchange between the absorption and transmission of acoustical materials particularly as it pertains to weight. Here, a procedure based on plane wave analysis is described that can be used to identify weight reduction opportunities by adjusting the acoustical properties of a generic sound package, consisting of a fibrous layer and a flexible microperforated panel surface treatment, so that it meets a target sound pressure level in a downstream interior space. It has been found, for the configuration studied here, that there are lightweight sound package configurations that can maintain acoustical performance equivalent to that of heavier noise treatments, and further, it has been found that the lightest treatments tend to favor barrier performance rather than absorption. Further, the impact of acoustical leaks has been considered, and it has been found that even very small leaks can result in a very substantial weight penalty if a specified level of acoustical performance is to be ensured. Further, the impact of changing the underlying panel mass and altering the frequency weighting used in the optimization process has also been considered.</p> <p>The optimizer used in the proposed procedure requires considerable calculation time; hence, the acoustic pressure calculation time needs to be minimized to enhance the efficiency of the solution process. Thus, the transfer matrix method (TMM) for a two-dimensional case was used to calculate the interior acoustic pressure for a simple geometry as a starting point in the process of identifying the minimum-weight sound packages. The TMM is a widely used analytical approach to predicting the sound pressure (and particle velocity) for a system that can be represented as a series of subsystems. Although the TMM can offer fast and simple calculations for the acoustic system, its application is limited to a plane-wave-based model. Thus, the TMM is not the best option for the acoustic pressure prediction in a complex geometry such as a vehicle interior, that involves non-planar wave propagation. Therefore, a hybrid TMM-FEA method is proposed in this research to evaluate the acoustical performance of the sound package in more complex geometries (here, a vehicle-like cavity). So, in this research, the TMM was introduced to obtain the initial solutions that can be used in conjunction with the FEA tool to calculate the sound pressure field in the complex geometry case. The correlation between the results of these two approaches was then analyzed to develop a space-averaged pressure prediction model for various absorptive cases in the interior space. Finally, this SAP prediction model was used to generate an acoustic map that can be used to graphically estimate the SAPs in the complex geometry case.</p> <p>In order to validate the usage of the developed equation for different sets of boundary conditions, several case studies were performed to study the effects of the surface impedance arrangements, geometrical shapes, and, lastly, the presence of extra features in the interior space. Finally, the SAP difference between the area near the driver’s right ear and the total interior cavity was studied to show that the SAP of the total cavity can be adjusted to evaluate the acoustic performance of the sound packages along the lines of conventional industry practice. </p>

Résumé : Les véhicules aériens et terrestres sont constitués de systèmes bâtis complexes. La structure principale est généralement composée de panneaux légers renforcés par des éléments rigides. Cette solution de conception est répandue parce qu’elle allie la force et un faible poids. Cependant, on sait qu’elle offre des résultats vibroacoustiques médiocres, c’est à dire que l’effet des perturbations externes qui touchent le système peut générer un niveau de bruit excessif à l’intérieur de la cabine des passagers. C’est une préoccupation majeure chez les fabricants, parce que ce niveau de bruit nuit sensiblement au confort ressenti par les clients et peut causer de la fatigue chez les conducteurs et les pilotes. Pour cette raison, les composants passifs constitués de matériaux dissipatifs assemblés en mode multicouche sont généralement intégrés à la structure. Ces assemblées bordées intègrent surtout des matériaux poroélastiques, qui sont plutôt répandus, grâce à l’agencement intéressant de bonnes propriétés d’isolation sonore et de faible poids. L’intégration en amont des traitements de contrôle du bruit au processus de conception est la clé de succès d’un produit. Pour ce faire, des outils pratiques numériques en mesure de capter le comportement dynamique des systèmes vibroacoustiques impliquant les structures, les cavités et les matériaux d’insonorisation sont requis. D’une part, la modélisation de ces systèmes couplés en utilisant des procédés à base d’éléments finis peut être, bien que précis, irréalisable pour des applications pratiques. D’autre part, les approches analytiques telles que la méthode de matrice de transfert sont souvent préférées grâce à leur facilité d’utilisation, même si elles manquent de précision en raison des hypothèses rigoureuses inhérentes au cadre analytique. Dans ce contexte, les procédures de structuration hybrides sont récemment devenues très populaires. En effet, les différentes techniques de modélisation sont généralement recherchées pour décrire les systèmes vibroacoustiques complexes arbitraires sur la plus large gamme de fréquences possible. L’objectif du projet proposé est de mettre au point un cadre hybride offrant une mé- thodologie simple pour tenir compte des traitements de contrôle du bruit dans l’analyse vibroacoustique par éléments finis. A savoir, le modèle de calcul qui en découle conserve la souplesse et la précision de la méthode des éléments finis en bénéficiant de la simplicité et de l’efficacité de la méthode de matrice de transfert pour obtenir une réduction de la charge de calcul pour la modélisation de composants acoustiques passifs. La performance de la méthode pour prédire la réponse vibroacoustique de structures planes homogènes avec des traitements acoustiques attachées est évaluée. Les résultats démontrent que la méthode hybride proposée est très prometteuse, parce qu’elle permet une réduction de l’effort de calcul tout en conservant suffisamment de précision par rapport à l’analyse complète par éléments finis. En outre, la méthode de matrice de transfert proposée de modélisation des traitements de contrôle des bruits est générale, comme on peut l’appliquer dans d’autres cadres outre l’application de l’élément fini considéré dans ce travail. // Abstract : Aerial and terrestrial vehicles consist of complex built-up systems. The main structure is typically made of light panels strengthened by stiffer components. Such design solution is widely used as it combines strength and low weight. However, it is known to give poor vibroacoustic performances, i.e. the effect of external disturbances acting on the system may generate an excessive noise level inside the passengers cabin. This is a main concern for the manufacturers, as it significantly affects the comfort experienced by the costumers and may fatigue drivers and pilots. For this reason, passive components consisting of dissipative materials assembled in a multilayer fashion are typically integrated within the structure. These lined assemblies mainly involve poroelastic materials, which are commonly used thanks to the appealing combination of good noise insulation properties and low weight. The early integration of noise control treatments in the design process is the key to a successful product. For this purpose, practical numerical tools able to capture the dynamic behavior of vibroacoustic systems involving structures, cavities and noise proofing materials are demanded. On the one hand, modeling such coupled systems using finite element based methods can be, albeit accurate, time consuming for practical applications. On the other hand, analytical approaches such as the transfer matrix method are often preferred thanks to their ease of use, although they suffer from a lack of accuracy due to the stringent assumptions inherent within the analytical framework. In this context, hybrid substructuring procedures have recently become quite popular. Indeed, different modeling techniques are typically sought to describe arbitrarily complex vibroacoustic systems over the widest possible frequency range. The aim of this thesis is to devise a hybrid framework providing a simple methodology to account for noise control treatments in vibroacoustic finite element analysis. Namely, the resulting computational model retains the flexibility and accuracy of the finite element method while taking advantage from the simplicity and efficiency of the transfer matrix method to obtain a reduction of the computational burden in the modeling of passive acoustic components. The performance of the method in predicting the vibroacoustic response of flat structures with attached homogeneous acoustic treatments is assessed. The results prove that the proposed hybrid methodology is very promising, as it allows for a reduction of the computational effort while preserving enough accuracy with respect to full finite element analysis. Furthermore, the proposed transfer matrix based methodology for noise control treatments modeling is general, as it can be used in alternative frameworks besides the finite element application considered in this work.