Evaluation of the acoustic and non-acoustic properties of sound absorbing materials using a three-microphone impedance tube

This paper presents a straightforward application of an indirect method based on a three-microphone impedance tube setup to determine the non-acoustic properties of a sound absorbing porous material. First, a three-microphone impedance tube technique is used to measure some acoustic properties of the material (i.e., sound absorption coefficient, sound transmission loss, effective density and effective bulk modulus) regarded here as an equivalent fluid. Second, an indirect characterization allows one to extract its non-acoustic properties (i.e., static airflow resistivity, tortuosity, viscous and thermal characteristic lengths) from the measured effective properties and the material open porosity. The procedure is applied to four different sound absorbing materials and results of the characterization are compared with existing direct and inverse methods. Predictions of the acoustic behavior using an equivalent fluid model and the found non-acoustic properties are in good agreement with impedance tube measurements

Acoustic metamaterial for low frequency sound absorption in linear and nonlinear regimes

Acoustic metamaterial absorbers have been built and tested with focus on low frequency airborne sound absorption in linear and nonlinear regimes. The absorbers are made up of a series of piled up flat cavities, separated by thin walls and traversed by a perforation at their centre. A model for absorber effective properties is developed and compared with experimental data. The model is used to derive simple formulae for the frequency and the peak value of the absorption coefficient at the lowest frequency resonance, depending on the geometrical parameters of the structure. Different absorbers have been built with several cavity thicknesses to allow comprehensive comparisons with the model. Nonlinear properties of the absorbers are investigated experimentally using sine wave excitation around the resonance frequency with the amplitude of the incident wave up to 250 Pa. Flow resistivity measurements at low flow rates show that the periodic set of cavities does not modify resistivity significantly when compared to a simple perforated cylinder with same thickness. As flow rate increases, the flow resistivity grows linearly according to Forchheimer's law and has a significant dependence on the absorber thickness. A numerical model is developed accounting for the linear growth of flow resistivity with particle velocity amplitude in the central perforation and compared with the measurements at high amplitudes of the incident wave

Concevoir la ville à partir des gares, Rapport final du Projet Bahn.Ville 2 sur un urbanisme orienté vers le rail

Expérimenter de nouvelles façons de faire de l'aménagement et du développement urbain autour des gares ? C'est l'objectif du projet franco-allemand Bahn.Ville 2, recherche-action qui vise à promouvoir « un urbanisme orienté vers le rail ». Valoriser les investissements faits sur les lignes ferroviaires régionales périurbaines par des mesures d'accompagnement dans le domaine de l'urbanisme, optimiser les conditions d'accessibilité aux gares de ces lignes, améliorer la qualité du service rendu aux usagers dans les lieux d'échanges autour de ces gare telles sont les ambitions de ce projet réalisé sur la période 2007-2009. Il s'agit de tester les conditions de la mise en œuvre d'un urbanisme orienté vers le rail

Periodic unit cell reconstruction of porous media: Application to open-cell aluminum foams

Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.International audienceIn this article, the issue of reconstructing an idealized periodic unit cell (PUC) to represent a porous medium is examined by means of microcomputed tomography (μCT). Using μCT, three-dimensional images of open-cell foam are collected and used to characterize the representative parameters of its cellular morphology. These parameters are used in order to reconstruct the porous medium by means of an idealized PUC: a tetrakaidecahedron with ligaments of triangular cross sections, whose characteristic dimensions have been measured on the μCT images. The proposed reconstruction of the idealized PUC is applied to four aluminum foams. The averaged macroscopic properties of the foams (open porosity and thermal characteristic length) are deduced from their respective PUC model and compared to experimental measurements and literature data. Good correlations are obtained. For each of the foams, this provides a parameterized idealized periodic unit cell on which the partial differential equations governing sound dissipation and propagation in foams can be solved with a view to studying the microphysical basis of the acoustical macrobehavior

Computation of the dynamic thermal dissipation properties of porous media by Brownian motion simulation: Application to an open-cell aluminum foam

Copyright 2007 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.International audienceThis paper reports simulation results of frequency dependent heat conduction through three-dimensional reconstructed unit cells of an open-cell aluminum foam under acoustic excitations. First, a three-dimensional random walk based algorithm is proposed to calculate the dynamic thermal permeability or dynamic bulk modulus of periodic complex porous geometries. Second, the error and convergence of the implemented calculation algorithm are quantified in terms of the random walk population, normalized trapping distance, and type of geometry. Finally, the algorithm is applied to the calculation of the dynamic bulk modulus of an aluminum foam and compared to laboratory measurements. Good agreement is obtained between simulations and measurements

Computation of the dynamic bulk modulus of acoustic foams

International audienc

Linking microstructure with acoustic properties of open-cell foams

Structured session: "New materials and systems in building acoustics"International audienceA research program has been initiated in January 2002 in order to link microstructure of high porosity open-celled foams with their acoustic properties. This paper is intended to highlight the main results of this study. The general objective of this work is the determination of the acoustical macro-behavior at the local scale of real rigid-framed porous media. In this purpose, one needs first to determine the local geometry of the media, and second to solve in it the partial differential equations which govern dissipation phenomena by thermal and viscous effects. The first step has been overcome by the modern technique of Computed Micro Tomography (CMT). This leads to experimental identification of the parameters of an idealized periodic unit-cell. The second step, the resolution of the linearized heat and viscous fluids equations under harmonic regime, is performed using Brownian motion simulations and finite element methods respectively, in three-dimensional and equivalent two dimensional unit-cells. Macroscopic behavior is obtained by spatial averaging of the local and frequency-dependant thermal and velocity fields. Results are presented in terms of only two frequency-dependant functions, taking into account the long-wavelength sound absorption and propagation properties of an air filled media; and compared with standing waves tube measurements. This computational methods combined with physically realistic cellular models will make optimal design of real open-celled foams a reality in the future

Bottom-up approach for microstructure optimization of sound absorbing materials

International audienceA major issue in building acoustics concerns the need to increase or adapt the absorption spectrum of commonly used sound absorbing materials. However, the most advanced models used to characterize and predict sound absorbing material performances are based on macroscopic parameters, which are inter-dependant, and do not take explicitly into account the local geometry description of the porous media, i.e. microstructure. For these reasons, optimizing sound absorbing material performances firstly relies on our ability to predict acoustic properties of porous media from the description of their local geometry, and secondly to propose pertinent physically realistic modifications of the microstructure having predictable impacts on the absorption spectrum. Starting from an open-cell foam sample having poor sound absorption properties, this paper exposes how the link between microstructure and macro-behavior is achieved, and how its sound absorption is increased from the optimization of its microstructure