No description available.

This thesis examines the transmission of sound through lightweight parallel plates, (plasterboard double wall partitions and timber floors). Statistical energy analysis was used to assess the importance of individual transmission paths and to determine the overall performance. Several different theoretical models were developed, the choice depending on the frequency range of interest and method of attachment of the plates, whether point or line, to the structural frame. It was found that for a line connected double wall there was very good agreement between the measured and predicted results, where the dominant transmission path was through the frame and the cavity path was weak. The transition frequency where the coupling changes from a line to a point connection is when the first half wavelength is able to fit between the spacings of the nails. For point connected double walls, where the transmission through the frame was weaker than for line connection, the cavity path was dominant unless there was absorption present. When the cavity was sufficiently deep, such that it behaved more like a room, the agreement between the measured and predicted results was good. As the cavity depth decreases the plates of the double wall are closer together and the agreement between the measured and predicted results were not as good. Detailed experiments were carried out to determine the transmission into the double wall cavities and isolated cavities. It was found that the transmission into an isolated cavity could be predicted well. However, for transmission into double wall cavities the existing theories could not predict transmission accurately when the cavity depth was small. Extensive parametric surveys were undertaken to analyse changes to the sound transmission through these structures when the material or design parameters are altered. The SEA models are able to identify the dominant mechanisms of transmission and will be a useful design tool in the design of lightweight partitions and timber floors.

La méthode SEA (Statistical Energy Analysis) est une approche statistique de la vibroacoustique permettant de décrire les systèmes complexes en termes d'échanges d'énergies vibratoires et acoustiques. En moyennes et hautes fréquences, cette méthode se présente comme une alternative aux méthodes déterministes (coût des calculs dû au grand nombre de modes, de degrés de liberté, unicité de la solution) Néanmoins, son utilisation requiert la connaissance et le respect d'hypothèses fortes qui limitent son domaine d'application. Dans ce mémoire, les fondements de la SEA ont été examinés afin de discuter chaque hypothèse. Le champ diffus, l'équipartition de l’énergie modale, le couplage faible, l'influence des modes non résonants et l'excitation rain-on-the-roof sont les cinq hypothèses qui ont été abordées. Sur la base d'exemples simples (oscillateurs couplés, plaques couplées), les équivalences et leurs influences sur la qualité des résultats ont été étudiées pour contribuer à la clarification des hypothèses nécessaires à l'application de la SEA ct pour borner son domaine de validité SEA. / Statistical energy analysis is a statistical approach of vibroacoustics which allows to describe complex systems in terms of vibrational or acoustical energies. ln the high frequency range, this method constitutes an alternative to bypass the problems which can occur when applying deterministic methods (computation cost due to the large number of modes, the large number of degrees of freedom and the unicity of the solution). But SEA has numerous assumptions which are sometimes forgotten or misunderstood ln this thesis, foundations of SEA have been examined in order to discuss each assumption. Diffuse field, modal energy equipartition, weak coupling, the influence of non-resonant modes and the rain on the roof excitation are the five look up hypotheses. Based on simple examples (coupled oscillators, coupled plates), the possible equivalences and their influence on the quality of the results have been discussed to contribute to the clarification of the useful SEA assumptions and to mark out it's the validity domain.

Résumé : Ce projet fait partie du projet MFERD (Magnésium Front End Research and Development) qui vise à développer les technologies permettant de rendre les alliages de Magnésium (Mg) comme un principal matériau structural pour les voitures (aujourd'hui essentiellement constituées d'acier quatre fois plus lourd que le Mg) afin d'en réduire leur masse pour des raisons environnementales et sécuritaires. Dans ce travail de maîtrise nous avons regardé la partie acoustique dans le cas d'un tablier (structure métallique derrière le tableau de bord) en magnésium. En effet, le confort acoustique à l'intérieur des voitures est devenu un argument de marketing d'une grande importance. Le tablier en séparant le compartiment moteur, source de bruit, de l'habitacle, joue un rôle important dans l'isolation acoustique de l'intérieur de la voiture. Ainsi le passage d'un tablier en acier à un tablier en Mg ne doit pas entraîner une baisse de performance. Dans ce travail, nous avons d'abord effectué une revue de la littérature sur les types de traitements acoustiques utilisés dans l'industrie automobile ainsi que des différentes techniques de leur modélisation. Nous avons ensuite comparé les performances acoustiques du tablier en Mg sur lequel on a appliqué des traitements classiques (à une couche, deux couches et trois couches) à celles des tabliers en acier et en aluminium et ce, à masse surfacique, raideur et/ou fréquences de résonnances égales. Finalement nous avons optimisé différents concepts de traitements acoustiques innovants appliqués sur le tablier en Mg en vue d'avoir des performances acoustiques semblables ou supérieures à celles du tablier en acier classique. L'optimisation s'est faite à partir d'un modèle SEA (Statitical Energy Analysis) couplé à un code d'optimisation basé sur un algorithme génétique||Abstract : This work is part of the MFERD (Magnesium Front End Research and Development) project which goal is to develop enabling technologies for the use of magnesium alloys as a principal structural material for cars (mainly made in steel which is four time heavier than magnesium) in order to reduce their mass for both, environmental and security concerns. In this work we have focused on the acoustic part, in the case of a magnesium alloy dash panel. The dash board, by separating the engine compartment from the interior cabin, plays a critical role in the insulation of the car interior. Since the acoustic comfort inside the car has become a marketing argument of great importance, the passage from steel to magnesium dash panel should not deteriorate acoustic performances. In this work, we first conducted a literature review on the types of acoustic treatments used in the automotive industry as well as various techniques of their modeling. We then compared the acoustic performances of a Mg dash with attached traditional acoustic treatments (single-layer, two layers and three layers) to those of a steel and aluminum dash panels with the same mass density, stiffness and/or frequency of resonances. Finally, we optimized different concepts of innovative sound packages applied on the Mg dash panel to achieve a noise performance similar or superior to those of a conventional steel dash. The optimization was done using a SEA (Statitical Energy Analysis) model, coupled with an optimization code based on a genetic algorithm.

The aim of this study is to develop a sound transmission model that can be used to predict the vibration and noise levels of a submerged vessel. The noise transmitted from the mechanical vibrations of the hull of a submarine and the turbulent boundary layer excitation on the submarine are investigated. A simplified physical model of the submarine hull including the effects of bulkheads, end enclosures, ring stiffeners and fluid loading due to the interaction of the surrounding medium is presented in the study. An energy approach, i.e., Statistical Energy Analysis (SEA) is used for the analysis because the characterization of the hull of the structure can be done by a very large number of modes over the frequency range of interest and the deterministic analysis methods such as finite element and boundary element methods are limited to low frequency problems. The application consists of the determination of SEA subsystems and the parameters and the utilization of power balance equations to estimate the energy ratio levels of each subsystem to the directly excited subsystem. Through the implementation of SEA method, the sound pressure levels of the hull of the structure are obtained. In terms of military purposes, the sound levels of the submarine compartments are vital in the aspects of the preserving of submarine stealth.

La méthode SEA (Statistical Energy Analysis) permet de prévoir les échanges d'énergie entre sous-systèmes d'une structure lorsque celle-ci est soumise à une excitation. La SEA est théoriquement très simple à mettre en place et permet de faire des variations paramétriques sur un modèle de la structure en phase de conception. Cette méthode est utilisée de manière pertinente dans le domaine du bâtiment mais peine à s'implanter dans le secteur des transports où les systèmes étudiés sont beaucoup plus complexes. Plusieurs difficultés apparaissent dans ces cas de figure et empêchent une modélisation correcte de la structure. La première difficulté vient de la nécessité de découper la structure en sous-systèmes respectant les hypothèses de la SEA. Ce découplage, presque trivial dans le bâtiment, devient particulièrement complexe dans le cas d'une caisse de voiture par exemple. L'estimation des puissances injectées par les sources dans la structure est une deuxième difficulté. En effet, les énergies des sous-systèmes sont directement proportionnelles à la puissance injectée. Ainsi, une mauvaise estimation de la puissance entraîne une erreur sur les échanges d'énergies. Le but de ce travail de thèse est double. Dans la première partie, une modèle de puissance injectée dans une plaque rectangulaire soumise à une couche limite turbulente (CLT) établie. Ce modèle simple permet de souligner l'influence des différents paramètres agissant sur la puissance injectée et d'estimer avec le temps de calcul très faible la puissance injectée par bande de fréquence. Une validation expérimentale est aussi exposée. Dans la deuxième partie, une méthode de sous-structuration automatique d'une structure en sous-systèmes SEA est proposée. Elle est basée sur l'analyse et la classification de fonctions de transfert énergétiques simulées par éléments finis. Un algorithme de classification permet de diviser la base de données en N sous-systèmes (N allant de 2 à Nmax). Un indice, validé sur des assemblages simples, indique la sous-structuration optimale pour une application SEA. Des applications industrielles, démontrant l'applicabilité de la méthode à des structures complexes, seront présentées.

Multilayered structures have many application in industry and society: they have peculiar properties and serve a variety of purposes, like structural support, thermal insulation, vibrational and acoustic isolation. This thesis concerns the prediction of sound transmission in multilayered structures. Two problems are herein investigated: the transmission of energy through structures and the transmission of energy along structures. The focus of the analysis is on the mid to high frequency range. To predict sound transmission in these structures, statistical energy analysis (SEA) is used.SEA models are devised for the prediction of the sound reduction index for two kinds of multilayered structures, double-walls used in buildings and trim-panels in vehicles; the double-walls comprise an air cavity in between flat plasterboard or glass plates, whereas the trim-panels a porous layer in between curved aluminium and rubber layers. The SEA models are based upon the wave-types carrying energy. The novelty in these SEAs is an element describing the waves in the air cavity, or in the porous layer, fully coupled to the mass-impeded external layers. Compared to measurements, the proposed SEA performs well: for double-walls, it performs better than previous models; for trim-panels, it is an original result. The parameters of the new SEA element, such as modal density, are derived from the coupling equations describing the fully coupled waves. For double-walls, these equations are derived via Newton's laws. For trim-panels, a variational approach based upon a modified Hamilton's principle valid for non-conservative systems is preferred, because it is a powerful machinery for deriving equations of motion and coupling conditions of a medium as complex as the porous layer. The modified Hamilton's principle for non-conservative systems is based upon a self-adjoint functional analogous to the Lagrangian, inspired by Morse and Feshbach's construction. A self-adjoint variational principle for Biot's equations in the displacement formulation is devised. An equivalent mixed formulation is obtained changing the coordinates of the displacement formulation via Lagrange multipliers. From this mixed formulation, the Lagrangian for a porous material with a limp frame is derived, which yields the continuity of the total displacement of the porous layer. Lagrange multipliers help to obtain the correct coupling functionals between a porous material and a solid. The Lagrange multipliers introducing the continuity of the frame and the solid displacements equal the traction of the in-vacuo frame, thus disappearing if the latter is limp. Measurements to gather material parameters for a Biot model of the porous layer have been conducted.The effects of spatial energy decay in the transmission along structures predicted by SEA is studied: a major effect is the increased relevance of indirect coupling loss factors between SEA elements. This may jeopardize the usefulness of SEA at higher frequencies. / <p>QC 20130218</p>

Two different Statistical Energy Analysis (SEA) models of a single turbo-prop engine propeller aircraft have been developed to predict the interior and exterior noise levels. The commercial software VA One is used for the analysis. First model is a pure SEA model developed with ribbed plates on the aircraft exterior. Second model is a hybrid model which employs finite element (FE) modeling of aircraft components with low modal density. These models have been analyzed for three different flight conditions, namely, take-off, cruise and climb for three different damping loss factors in each condition. Wind tunnel measurements are used to estimate the turbulent boundary layer (TBL) information on the surface of the aircraft. Propeller noise together with TBL loading are then used as the excitation for the models. Flow paths of energy are identified and cabin interior noise levels are predicted for the developed models. Results of analyses are comparatively evaluated.

Two sample statistical energy analysis (SEA) models are generated for a section of the fuselage panel of an aircraft, namely the uniform panel model which includes the frames and stringers, and the ribbed panel model in which the frames and stringers are smeared into the skin. Turbulent boundary layer (TBL) excitation is used as the primary acoustic excitation source. Stress levels are estimated from the average velocity data of the panels. The stress results are found comply with those obtained by the AGARD method. Effect of radiation from panels to exterior and interior of the sample skin panel as well as the pressurization of the skin panels are investigated separately to analyze their effects on the stress levels. The method is then used in the analysis of center fuselage skin panels on a basic training aircraft. Two models are generated for the aircraft analysis, namely the complete aircraft model and the simplified model which excludes the wings and the empennage. In addition to TBL, propeller noise is used as the primary acoustic excitation source. The effects of the wings and the empennage on the stress levels in the center fuselage skin panels are also investigated along with the radiation from panels to the exterior and interior of the aircraft and pressurization of the pilot cabin.

This research demonstrates the usage of numerical acoustics to model sound and vibrational energy propagation in HVAC ducts and plenums. Noise and vibration in HVAC systems propagates along three primary paths that can be classified as airborne direct, airborne indirect and structure-borne. The airborne direct path was simulated using acoustic FEM with special boundary conditions to handle the diffuse acoustic field loading and the baffled termination. The insertion loss for a number of different plenum geometries was compared to published measurement results. Results were in good agreement both below and above the cutoff frequency. Additionally, the airborne indirect path, often termed breakout noise by the HVAC community, was assessed using Statistical Energy Analysis (SEA). This path was examined experimentally by placing a loudspeaker inside the air handler and measuring the sound power transmitted through the walls. SEA results compared favorably with the measured results in one-third octave bands even at low frequencies. Finally, the structure-borne path was considered by exciting the walls of the aforementioned air handler using an electromagnetic shaker. The panel vibration and the sound power radiated from the panels were measured. Results were compared with the SEA with good agreement provided that SEA loss factors were determined experimentally.