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The use of lightweight complex heterogeneous structures increased during the last years principally in the transportation sector (i.e., aviation and trains). This sector's technology enhancement pursues reducing long-term CO2 emissions and increasing efficiency. Lightweight structures may have poor vibro-acoustic behavior and in designs with complex shapes and material heterogeneities, its vibro-acoustic modeling brings new challenges in terms of accuracy and computational cost. Techniques such as model order reduction, homogenization, mesh and meshless methods (with and without periodicity conditions) and energy methods are typically employed to tackle this problem. Within homogenization techniques, an equivalent properties strategy can be utilized to equivalently represent complex structures into more simple ones (for example, a single layer panel).

Stochastic Finite Elements Method (SFEM) is applied in many fields. For instance, in frictional systems, it helps quantify uncertainties about the parameters controlling the involved process and thus, provides a more reliable prediction of the dynamic instabilities. Usually, SFEM is coupled with sensitivity theory to investigate the effect of a given input on the output. However, the available methods which often couple Monte-Carlo (MC) algorithm with the Finite Element (FE) method have a computational cost that scales linearly as a number of stochastic iteration N and input parameters k (i.e., t ~ N x k). To achieve convergence, the magnitude of N must be on the order of thousands or even millions. Hence, for a frictional system with 5 random variables and requiring 15 min of CPU time per run, the computational cost will exceed 52 days (!). Such a method cannot be applied in an industrial design framework with a high number of random variables since its CPU time becomes prohibitive.

The input mobility is an important vibro-acoustic parameter used by engineers in the industrial design process. In fact, this information guides the choice of the connection between the vibrational source and the receiver. To select the most effective connection points, the input mobility is characterized at every possible location of the receiver structure leading to a mapping of the input mobility. Several works propose to compute the full map by averaging the input mobility in a given frequency bands over a Finite Elements (FE) mesh of the receiver structure. By nature, the input mobility is a Frequency Response Function (FRF); consequently, it does not consider the frequency content of the source. This paper presents a method to compute a full map of input power instead of input mobility.

This paper presents a case study on the full inverse characterization of the material properties of an open-cell poroelastic foam using impedance tube measurements. It aims to show the importance of controlling the lateral boundary condition in the impedance tube, and selecting an appropriate acoustic model to obtain the most accurate material properties. The case study uses a four-inch thick melamine foam and a 100-mm diameter tube. The foam is mechanically cut to fit within the circular tube. However, the cutting process is not perfect and a tiny lateral air gap exists between the material and the tube (i.e. the foam diameter is 99.5 mm for a 100-mm diameter tube). The typical characterization procedure is to mix direct and indirect measurements to retrieve the material properties of the foam. First, open porosity, bulk density, and static airflow resistivity are directly measured.

This article aims to assess and discuss the performances of a hybrid methodology by considering the radiation of a curved structure-cavity system with attached noise control treatments. The hybrid method uses a Patch Transfer Functions (PTF) approach to couple the standard finite element method of the curved structure and cavity with an analytical model of the sound package, i.e. Green functions based model. First, the used approach is presented. Then, the accuracy of the proposed methodology is assessed for two different curved noise control treatments, namely (i) light foam and (ii) light foam with a mass layer. The obtained results are systematically compared to three models, namely full Finite Element/Boundary Element (FEM/BEM) strategies, and to two sub-structuring approaches where the sound package is modeled by (i) a locally reacting model and (ii) FEM.

A poroelastic characterization of open-cell porous materials using an impedance tube is proposed in this paper. Commonly, porous materials are modeled using Biot’s theory. However, this theory requires several parameters which can be difficult to obtain by different methods (direct, indirect or inverse measurements). The proposed method retrieves all the Biot’s parameters with one absorption measurement in an impedance tube for isotropic poroelastic materials following the Johnson-Champoux-Allard’s model (for the fluid phase). The sample is a cylinder bonded to the rigid termination of the tube with a diameter smaller than the tube’s one. In that case, a lateral air gap is voluntary induced to prevent lateral clamping. Using this setup, the absorption curve exhibits a characteristic elastic resonance (quarter wavelength resonance) and the repeatability is ensured by controlling boundary and mounting conditions.

A method for estimating the sound absorption coefficient of a material under a synthesized Diffuse Acoustic Field was recently proposed, as an alternative to classical sound absorption measurements in reverberant rooms (Robin O., Berry A., Doutres O., Atalla N., ‘Measurement of the absorption coefficient of absorbing materials under a synthesized diffuse acoustic field’, J. Acoust. Soc. Am., 136 (1) EL13-EL19, 2014). Using sound field reproduction approaches and a synthetic array of acoustic monopoles facing the material, estimation of the sound absorption coefficient under a reproduced Diffuse Acoustic Field in a hemi-anechoic room was shown to be feasible. The method was successfully tested on a few samples of melamine foam of close thicknesses and areas, but the influence of several parameters such as the source height, or the samples dimensions together with the nature of the porous material was not fully investigated.

In passenger aircraft the most important noise control treatment is the primary insulation attached to the fuselage. Next to its acoustic properties the primary insulation main purpose is the thermal insulation and the minimization of condensed water. In general it consists of fibrous materials like glass wool wrapped in a thin foil. Due to stringent flame, smoke and toxicity requirements the amount of available materials is limited. Furthermore the amount of material installed in aircraft per year is much smaller compared to needs in the automotive industry. Therefore the best lay-up of the available materials is needed in terms of acoustics. This paper presents a tool for numerical optimization of the sound insulation package. To find an improved insulation the simulation tool is used in interaction with a measurement database. The databank is constructed from aircraft grade materials such as fibrous materials, foams, resistive screens and impervious heavy layers.

A novel processing method for fabricating high porosity microcellular ceramic foams for sound absorption applications has been developed. The strategy for fabricating the ceramic foams involves: (i) forming some shapes using a mixture of preceramic polymer and expandable microspheres by a conventional ceramic forming method, (ii) foaming the compact by heating, (iii) cross-linking the foamed body, and (iv) transforming the foamed body into ceramic foams by pyrolysis. By controlling the microsphere content and that of the base elastomer, it was possible to adjust the porosity with a very high open-cell content (ranging between 43 - 95%), high microcellular cell densities (9 × 108 - 1.6 × 109 cells/cm3) and desired expansion ratios (3 - 6 folds). Sound absorption testing has been performed using ASTM C-384 standard test. The preliminary results show that ceramic foams are candidate sound absorption materials.

This paper discusses the development of lighter weight, superior acoustic performance and cost effective viscoelastic microcellular foams for the use in automotive passive noise control panels. The study incorporates the control of the foaming process for production of variable microcellular structures and morphologies for the novel foams under investigation. For that purpose, the foaming process was controlled for production of foam samples with various microcellular structures. Cross linked LDPE was used as a base material for the produced foams. Very high open-cell content (ranging between 43 - 95%), high microcellular cell densities (9E108 - 1.6E109 cells/cm3) and desired expansion ratios (3 - 9 folds) were successfully obtained. While the material is overly porous, it is noted that the unfoamed skins on the outer surfaces of the samples have prevented sound waves from penetrating the samples. Manual skin removal resulted in slight improvement in sound absorption testing.

Statistical Energy Analysis has become extremely popular over the last decade in the transportation industry. As a prediction tool, it offers appealing advantages such as, its wide frequency range and short computational time, which conventional methods do not offer. Prediction of the vibrational response of dynamical systems, whether it is by means of analytical methods, numerical methods or, as in our case, by statistical methods, requires in particular the damping characteristics of the structure’s components. This work proposes a comparison study of the three techniques widely used to determine the loss factors of different complex structures ranging form simple flat plates to ribbed panels and sandwich composite panels in different mounting configurations. The studied structures are classically met in cars, aircraft and trains. They span both low and high damping configurations.

Typical automotive sound package components are usually characterized by their absorption coefficients and their acoustic power-based insertion loss. This insertion loss (IL) is usually obtained by subtracting the transmission loss (TL) of a bare flat steel plate from the TL of the same plate covered with the trim material. While providing useful information regarding the performance of the component, air-borne insertion loss is based solely on acoustic excitations and thus provides very little information about the structure-borne performance of the component. This paper presents an attempt to introduce a standard procedure to define the power-based structure-borne insertion loss of sound package components. A flat steel plate is excited mechanically using a shaker. Different carpet constructions are applied on the plate and tested. Based on velocity measurements, a force transducer and intensity probe, the mechanical input and the acoustic radiated power are obtained.

A new mixed finite element formulation very well adapted to analyze the propagation of elastic and acoustic waves in porous absorbing media is presented. The proposed new formulation is based on modified Biot's equations [1,2,3] written in terms of the skeleton displacement and the acoustic pressure in the interstitial fluid. It generalizes the previous formulation proposed in reference [11], and has the great advantage over existing formulations [5, 6, 7, 8, 9, 10, 11, 12 and 13] of automatically satisfying interior and exterior boundary conditions without having to compute surface coupling integrals at porous sub-domain interfaces. When elastic forces in the skeleton are neglected, the formulation automatically degenerates to an equivalent fluid model taking into account inertial coupling with the skeleton.