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Aiming at accurately predicting the global Damping Loss Factor (DLF) for Highly Dissipative Structures (HDS), the current study uses the Wave Finite Element (WFE) methodology. It starts by deriving the forced responses of a Unit Cell (UC) representative of the periodic meta-structure. Then it computes the DLF of the wave via the power balance. The Bloch expansion is employed. The response to a point force applied to the periodic structure is decomposed in the Brillouin zone, allowing the prediction via integration over the wavespace. The global DLF is derived based on the Power Input Method (PIM). The accuracy of the methodology is demonstrated through several cases from simple panels to complex meta-structures. For HDS, results of General Laminated Model (GLM) is exploited for wave DLF and PIM based on Finite Element Method (FEM) data is provided as reference approach for global DLF. The study discusses the influence of bending waves on the DLF estimation for HDS. A final case study with a meta-structure is also offered. The later consists of a doubly periodic coated sphere in a host rubber, it demonstrates the importance of Bloch modes. • Wave-based methodology is developed by Bloch wave expansion to estimate DLF of HDS. • DLF computation confirms the reduced impact of bending wave on HDS forced response. • Higher Bloch modes are essential for DLF estimation of meta-structure with large UC. [ABSTRACT FROM AUTHOR]

This paper presents and studies the performance of a sub-structuring method that employs a patch transfer approach (PTF) to couple the standard finite element schemes of the structures and cavities with an analytical model of the sound package. First, the approach is presented followed by a convergence study to define a patch mesh criterion. Then, the accuracy of the proposed methodology is assessed for single and double wall transmission problems with two different attached noise control treatments. The obtained results are systematically compared to three models, namely full finite element/boundary element strategies (FEM/BEM), and to two sub-structuring approaches based on the modeling of the sound package by (i) a locally reacting model and (ii) FEM. It is observed that the proposed method predict accurately and efficiently the dynamic behavior of flat trimmed vibroacoustic systems. [ABSTRACT FROM AUTHOR]

An acoustic impedance model to characterize micro-perforated panels at high sound pressure levels is proposed. The model uses a rigid frame porous medium, where the micro-perforated panel is modeled with an effective density, function of the frequency following the approach of Johnson-Allard and the equivalent fluid parameters such as the tortuosity and the flow resistivity are expressed as function of the incident sound pressure which is considered as a main variable. Unlike existing models which are limited to micro-perforated panels coupled to air cavity, the present model predicts correctly the acoustic response of micro-perforated panel backed by porous media which can be air cavity, porous material or resistive screen. Micro-perforated panel backed by porous media involves a distortion of the flow caused by the perforations through the porous media, thus the micro-perforated panel is modeled in this case using an equivalent tortuosity where a correction term is proposed to account for the flow distortion effect and it depends on the dynamic tortuosity of the porous layer and the incident sound pressure. The proposed impedance model is compared numerically with other existing nonlinear impedance models for different configurations of the micro-perforated panel. The results show a good agreement among each other for sound pressure levels up to 150 dB. In addition, experimental measurements were performed on several micro-perforated panels backed by air cavities or porous material using the classical impedance tube. A good correlation between theoretical and experimental results is obtained. Some validation and benchmarking results are illustrated and discussed in this paper. It is shown that the high sound pressure levels decrease the tortuosity and increase the flow resistivity of the micro-perforated panel. Furthermore, the acoustic energy dissipation by the micro-perforated panel absorber with a large perforation diameter which is low in the linear regime can become important and interesting at high sound pressure level. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, sound transmission through an aircraft sidewall representative double panel structure is investigated theoretically and parametric and validation studies are conducted. The studied configuration is composed of a trim panel (receiver side panel) attached to a ribbed skin panel (source side panel) with periodically spaced resilient mounts. The structure is considered infinite in order to use space harmonic expansion. The partition is also assumed planar for simplicity. The model allows for a 3D incident field and the panels can be metallic and/or composite. A four-pole formulation is employed for modeling of the mounts and the absorption provided by the fiberglass that fills the cavity between the leaves is addressed with an equivalent fluid model. The investigation of mount stiffness, damping and spacing show that properly designed mounts can increase the TL significantly (up to 20dB of difference between rigid and resilient mounts). However, they can create undesirable resonances resulting from their interaction with the panels. The influence of cavity absorption is also studied and results illustrate the fact that it is not worth investing in a highly absorbent fiber if the structure-borne transmission path is not adequately insulated, and likewise that it is not worth investing in highly resilient mounts without sufficient cavity absorption. Moreover, the investigation of panel damping confirms that when structure-borne transmission is present, raising skin damping can increase the TL even below coincidence, but that on average, greater improvements are achieved by raising trim damping. Finally, comparison between the periodic model and finite element simulations for structure-borne transmission shows that the average level of transmitted energy is well reproduced with the periodic approach. However, the modes are only captured approximately due to the assumption of an infinite structure. [Copyright &y& Elsevier]

Abstract: In several automotive and aircraft applications there is a need for simple tools to assess quickly and accurately the performance of sound packages. Statistical energy analysis (SEA) and the transfer matrix method (TMM) are examples of such methods. The used methodology (for modeling sound packages) is well validated for acoustic excitations (airborne). However, a simple and reliable methodology is still lacking for mechanical excitations (structure-borne). This work concentrates on the latter. It presents and compares three different simple approaches to model the vibration and acoustic response of a mechanically excited structure with an added noise control treatment. Various examples are presented to confirm their relevance and accuracy in comparison to more exact and costly methods, such as the finite element method. In particular, it is shown that the TMM with a size correction (FTMM) is accurate enough to eliminate the classical assumption of low coupling classically assumed in SEA modeling of sound packages and/or compute efficiently the structure-borne insertion loss of sound packages used in SEA and FEM models. [Copyright &y& Elsevier]

Abstract: This paper examines various models predicting the influence of periodically spaced structural links on sound transmission through lightweight double panel structures. A baseline configuration made up from two aluminum plates connected through aluminum C-section channels and a fiberglass filled cavity has been specifically built and its TL measured for comparison and validation of the investigated models. First, classical decoupled approaches are outlined and adapted for the studied case. Next, a periodic model based on existing formulations is presented. The model allows for a 3D incident field and accounts for the absorption in the cavity with the help of an equivalent fluid model for the fiberglass. Three cases of coupling conditions are considered for the links: a mass¿spring¿mass approximation, a beam-type approximation and a beam-type approximation where the rigidity and the inertia of the beams are neglected. The measurements show that the bridged configuration strongly reduces the TL at mid and high frequency and exhibits pass/stop bands characteristic of periodic structures. The predictions of decoupled approaches capture the physics of the problem only approximately and with the integration of the mass of the connectors in the context of thin lightweight panels, their agreement with experimental data is further reduced. On the other hand, the results obtained with the periodic model are excellent over most of the studied frequency range. However, in the vicinity of the critical frequency of the thicker panel (around 6kHz), an overestimation of the TL is observed. This suggests that the model will have to account better for damping and resonant transmission. [Copyright &y& Elsevier]

There are several models and approaches for predicting the acoustic response of perforated plates and screens. Classical models are devoted to specific configurations: particular geometrical parameters including the size and shape of the perforations, excitations type, mounting of the screen (bonded vs. free), and interfacing media involved in multilayers sound packages. This paper reviews these models and presents a simple and general model that can handle easily and automatically the miscellaneous configurations in the context of the transfer matrix method. In particular, it is shown that a perforated plate or screen can be modeled as an equivalent fluid following the Johnson–Allard approach with an equivalent tortuosity. This equivalent tortuosity is shown to depend on the media interfacing with the perforated system. Experimental results depicting various practical configurations are shown to corroborate the validity of the proposed model and to evaluate its performance compared to classical ones. [Copyright &y& Elsevier]

Abstract: This paper is concerned with the modelling of diffuse field transmission into composite laminate and sandwich composite infinite cylinders. Two models are presented and compared: Symmetrical Laminate composite and discrete thick laminate composite. The latter is shown to handle accurately, as a particular case, the first model, and the important case of sandwich composite shells. In both models, membrane, bending, transverse shearing as well as rotational inertia effects and orthotropic ply angle of the layers are considered. Starting from the dynamic equilibrium relations and stress–strain–displacement relations, a dispersion system is given in a wave approach context. Next, expressions for the matrix systems governing the structural impedance, critical frequencies and ring frequency are given. The developed equations are applied to the calculation of the diffuse field transmission of an infinite cylinder. Predictions with the presented models are compared to results presented in the literature for both laminate composite and sandwich composite configurations. They confirm the accuracy of both models and the general nature of the presented discrete thick laminate composite model. [Copyright &y& Elsevier]

Acoustic performances of two concepts based on the integration of an elastic membrane in the front wall of a cavity and Helmholtz resonator embedded in a melamine matrix are studied. Membranes are made of acrylic material, this latter is first characterized with a uniaxial test in order to extract the mechanical properties needed to model the membrane. Analytical, numerical and experimental investigations are performed so as to determine the acoustical potential of the proposed systems in terms of absorption and transmission loss. The contribution to the sound absorption can be positive or negative at the resonator resonances, an analytical model is proposed to highlight the physical phenomena related to these effects. In addition, a multiphysics numerical model combining the acoustic-mechanical interaction has been developed and validated by comparison with the experimental data measured using an impedance tube. The studied concepts could offer significant enhancements of sound transmission loss properties in the low frequencies but also on acoustic absorption in specific frequency bands. [ABSTRACT FROM AUTHOR]

This paper presents a damping loss factor estimation method of two-dimensional orthotropic structures. The method is based on a scanning laser vibrometer measurement. The dispersion curves of the studied structures are first estimated at several chosen angles of propagation with a spatial Fourier transform. Next the global damping loss factor is evaluated with the proposed inverse wave method. The method is first tested using numerical results obtained from a finite element model. The accuracy of the proposed method is then experimentally investigated on an isotropic aluminium panel and two orthoropic sandwich composite panels with a honeycomb core. The results are finally compared and validated over a large frequency band with classical methods such as the half-power bandwidth method (3 dB method), the decay rate method and the steady-state power input method. The present method offers the possibility of structural characterization with a simple measurement scan. [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates the effects of finite dimensions on the vibro-acoustic response of orthogonally stiffened panels. Three types of excitations are considered: acoustical excitation, point force excitation and random excitation by a turbulent boundary layer. In each case, a spatially windowed periodic model is compared with a Rayleigh–Ritz model where the modes of the un-stiffened panel are used as the basis functions. The latter model accounts for the reflected wave field generated at the boundaries by assuming that the panel is simply supported. On the contrary, the windowed periodic model only accounts for finiteness on sound radiation (the assumption of an infinite periodic structure is used to calculate the panel response). Numerical studies show that when the bending wavelength becomes comparable or smaller than the stiffener spacing, the periodic model is able to reproduce the results obtained with the Rayleigh–Ritz model. To complement the study, the developed models are compared with numerical simulations (finite element method) and with experimental results. [Copyright &y& Elsevier]

In this paper, the validity of using the Transfer Matrix Method (TMM) to account for the effect of practical curved sound packages (i.e. spring-mass treatment) in vibroacoustic systems is assessed. For this purpose, a sub-structuring method that employs a Patch Transfer Functions (PTF) approach is used to efficiently couple standard Finite Element Method (FEM) of the structure and the acoustic cavity with the sound package model. Three models of the noise control treatment are compared, namely (i) a locally reacting model, (ii) a non-locally reacting (NLR) model based on the TMM, and (iii) a FEM. The FEM model considers the actual curved geometry of the SP in order to estimate its dynamic behavior while the simplified TMM based models assume the SP to be flat (i.e. unwrapped if curvature is present), homogeneous and of infinite lateral extent. In addition, these results are systematically compared to a full Finite Element/Boundary Element Methodology. The accuracy and the limitations of adopting the unwrapped approximation in TMM based models are shown through two parametric studies. The first considers the radius of curvature of the system as a parameter while the SP thickness is constant. The second considers the thickness of the SP as a parameter while the radius of curvature of the system is held fixed. For the considered configurations, the first parametric study allows to conclude that the curvature of the SP does not affect the dynamic behavior of the system due to its large dissipation and softness and it can be practically assumed flat if the NLR model is used to model the SP. However, the second parametric study shows that the thickness of the curved SP may affect the efficiency of the NLR model. Moreover, the limitations of locally reacting models are confirmed. [ABSTRACT FROM AUTHOR]

The Inhomogeneous Wave Correlation technique has been used to estimate experimentally the wavenumber of simple and complex structures. However this method is essentially focused on the real part of the wavenumber and is not always robust to measure the damping loss factor related to its imaginary part. Another correlation technique able to capture the damping effects of complex structures as function of the heading angle accurately is presented here. Instead of using an inhomogeneous plane wave, the correlation is made with a Green's function to better take into account the behavior of the structure excited by a point force. Several results, with simulated and measured data, are compared with an analytical discrete laminate model and show the accuracy of this Green's Function Correlation technique to recover the damping loss factor of complex structures with high damping and in the mid to high frequency range. • A new Green's function-based model correlation (GFC) method is developed. • This new method estimates accurately the damping loss factor of complex planar structures. • The approach is more accurate compared to the plane wave hypothesis. [ABSTRACT FROM AUTHOR]

The application of shift cell technique is presented and discussed for periodic porous media described with equivalent fluid models: as it can be found in literature, it consists in a reformulation of classical Floquet-Bloch (F-B) conditions, in which the phase shift of the boundary conditions, related to wave propagation, is integrated into the partial derivative operator. Consequently, the periodicity is included in the overall behavior of the structure, while continuity conditions are imposed at the edges of the unit cell. Its major advantage stands in allowing the introduction of a generic frequency dependence of porous material behavior, through the resolution a quadratic eigenvalue problem, providing an efficient way to compute the dispersion curves of a porous material modeled as an equivalent fluid. A validation and a computational cost comparison are performed between the shift cell technique and the classical F-B approach, pointing out that the first can provide, among its other advantages, a sensible computational time reduction for this kind of analyses. The derivation of the equivalent acoustic properties of the unit cell from its dispersion characteristics is also investigated. To this aim, group velocity matrix formulation and a branch-tracking algorithm are described. Some test cases are used for validating the proposed methodology. [ABSTRACT FROM AUTHOR]