Francesco Pompoli

Reproducibility experiments on measuring acoustical properties of rigid-frame porous media (round-robin tests)

This paper reports the results of reproducibility experiments on the interlaboratory characterization of the acoustical properties of three types of consolidated porous media: granulated porous rubber, reticulated foam, and fiberglass. The measurements are conducted in several independent laboratories in Europe and North America. The studied acoustical characteristics are the surface complex acoustic impedance at normal incidence and plane wave absorption coefficient which are determined using the standard impedance tube method. The paper provides detailed procedures related to sample preparation and installation and it discusses the dispersion in the acoustical material property observed between individual material samples and laboratories. The importance of the boundary conditions, homogeneity of the porous material structure, and stability of the adopted signal processing method are highlighted.

Inversion Problems for Determining Physical Parameters of Porous Materials: Overview and Comparison Between Different Methods

Summary In modeling the acoustical behavior of porous materials, the determination of the physical parameters (that are airflow resistivity, open porosity, tortuosity and viscous and thermal characteristic lengths) is a fundamental issue. As an alternative to measuring some of these quantities directly, it is possible to use inverse strategies to calculate them once some of the acoustical parameters are experimentally known. In this work both analytical and minimization-based methods will be investigated to determine the physical parameters of porous materials. Among the analytical approaches (that are derived from dynamic density and bulk modulus expressions governing viscous and thermal dissipation of sound waves in rigid-framed porous media), several formulae have been proposed in literature and some will also be considered in the proposed analysis. In this paper only the procedures that are able to provide a complete set of physical data from acoustical tests will be investigated. The results will be compared with theoretical and experimental data and details related to the accuracy and reliability of the inversely determined parameters will also be reported and discussed.

How reproducible is the acoustical characterization of porous media

There is a considerable number of research publications on the characterization of porous media that is carried out in accordance with ISO 10534-2 (International Standards Organization, Geneva, Switzerland, 2001) and/or ISO 9053 (International Standards Organization, Geneva, Switzerland, 1991). According to the Web of ScienceTM (last accessed 22 September 2016) there were 339 publications in the Journal of the Acoustical Society of America alone which deal with the acoustics of porous media. However, the reproducibility of these characterization procedures is not well understood. This paper deals with the reproducibility of some standard characterization procedures for acoustic porous materials. The paper is an extension of the work published by Horoshenkov, Khan, Bécot, Jaouen, Sgard, Renault, Amirouche, Pompoli, Prodi, Bonfiglio, Pispola, Asdrubali, Hübelt, Atalla, Amédin, Lauriks, and Boeckx [J. Acoust. Soc. Am. 122(1), 345-353 (2007)]. In this paper, independent laboratory measurements were performed on the same material specimens so that the naturally occurring inhomogeneity in materials was controlled. It also presented the reproducibility data for the characteristic impedance, complex wavenumber, and for some related pore structure properties. This work can be helpful to better understand the tolerances of these material characterization procedures so improvements can be developed to reduce experimental errors and improve the reproducibility between laboratories.

A single measurement approach for the determination of the normal incidence transmission loss

Several techniques for the determination of the normal incidence complex transmission coefficient and transmission loss have been proposed in literature. It has been shown that two different measurements have to be carried out for a correct evaluation of the above-mentioned quantities. However, single measurement approaches can be used at the condition that energy contributions are correctly included in the implemented formulation. This paper presents a single measurement approach based on a transfer matrix formulation, taking into account reflection contribution from the end termination and phase shift introduced by the material. The transmission performances of different kinds of layered porous materials are investigated by means of a four microphone technique in a plane wave tube. By using the decomposition technique, incident, reflected, and transmitted contributions are separated and transmission coefficient is easily calculated. Results are discussed and compared with similar techniques and with the two-load method.

Quasistatic evaluation of mechanical properties of poroelastic materials: static and dynamic strain dependence and in vacuum tests

A complete description of the vibro‐acoustical behavior of a poroelastic material requires the knowledge of both geometrical quantities, related to the structure of the fluid‐filled pores to account the sound propagation within them, and mechanical parameters (i.e. Young Modulus, Poissons ratio, and loss factor) in order to model the wave propagation through the elastic structure constituting its skeleton. Because the nonlinear nature of poroelastic media, those mechanical properties are shown depending on static preload and dynamic strain applied to them. In the present work a well established quasi‐static method, based on the measurement of mechanical impedance and the use of adequate polynomial relations, has been used to determine the dependence of the mechanical properties on the applied deformations. Furthermore, tests have been also carried out in a vacuum chamber in order to evaluate the real contribution of the filling fluid on the total vibro‐acoustical response of the material.

A reduced-order integral formulation to account for the finite size effect of isotropic square panels using the transfer matrix method

The transfer matrix method is a well-established prediction tool for the simulation of sound transmission loss and the sound absorption coefficient of flat multilayer systems. Much research has been dedicated to enhancing the accuracy of the method by introducing a finite size effect of the structure to be simulated. The aim of this paper is to present a reduced-order integral formulation to predict radiation efficiency and radiation impedance for a panel with equal lateral dimensions. The results are presented and discussed for different materials in terms of radiation efficiency, sound transmission loss, and the sound absorption coefficient. Finally, the application of the proposed methodology for rectangular multilayer systems is also investigated and validated against experimental data.

Hybrid inversion technique for predicting geometrical parameters of porous materials

In prediction of acoustical behavior of porous materials, five geometrical parameters play a very important role, but some of these geometrical properties are very difficult to measure directly. So many authors have suggested different inversion strategies for getting these properties from directly measured both characteristic and surface properties of the material using standing wave tube. These approaches can be divided in two different categories: analytical (based on the limit behaviour of the bulk properties) and minimization based methods (which make use of searching algorithms to determine the best solution that minimizes a cost function calculated by means of a prediction model). Recent studies have shown the reliability of the analytical methods for the determination of the airflow resistivity and the minimization based approach by using genetic algorithms for getting the other physical parameters. Consequently, a hybrid inversion technique can be proposed for the complete calculation of the geom...

A New Apparatus for Measuring the Effective Coupling of Acoustic Absorption of Materials Used Inside Cabins

The new apparatus developed in this research consists of a measurement tube with circular cross section which is equipped at both ends with sample holders. The sound power is injected by a small aperture on the side and the sound field is captured by a single microphone placed on the side as well. By calculation of the reverberation time inside the tube it is possible to derive the sound absorption coefficient for normal incidence both for a single and for a combination of two test samples. This apparatus is thus suitable to study the acoustic coupling of different types of sound absorbers in order to maximize their effect when used in combination. In the work the theory underlying the measurement method is expounded and the procedure of calculating single and double absorption figures is discussed. A set of experimental data is presented and compared with the available standardized process for the calculation of single absorption values. Then the coupled absorption coefficients are measured and it is shown how the behavior of coupled systems can significantly deviate from the prediction formulas.

Dynamic mechanical response of foams for noise control

In this work, the viscoelastic behavior of open cell polyurethane foams used in noise control applications is investigated through dynamic mechanical analysis in compression. Several levels of static strains superimposed on a small dynamic one were considered in order to assess the effect of material non-linearity on the mechanical response. Further, a wide range of frequencies and temperatures was explored. For each static strain a different master curve for conservative component of complex modulus, E’, could be determined. Interestingly, the loss factor was the same at all static strains, indicating that the relative contribution of energy dissipation and conservation is unaffected by the static strain. Moreover, shift factors (and thus the bulk material relaxation times) turned out to be independent on static strain level. These results suggest that the non-linearity of the foam is linked to the change in foam structure with strain rather than to a non-linear behavior of the viscoelastic constituent m...

Frequency-dependent mechanical modelling of poro-elastic materials for sound transmission loss simulations

Multilayer panels made of sheet steel, a poro-elastic layer and an impervious mass are widely used in noise control for increasing sound insulation. A complete description of the vibro-acoustical behavior of the poro-elastic material used in these panels requires knowledge of mechanical parameters (i.e. complex modulus and Poissons ratio) in order to model the wave propagation through the elastic structure constituting its skeleton. Although many of the materials used for noise and vibration control have viscoelastic behavior, prediction models currently make use of the linear elasticity theory. As a consequence, acoustical quantities, such as sound transmission loss, are predicted by using static or quasi-static values of mechanical properties. In this research, an experimental time domain approach in combination with an analytical model for linear viscoelasticity is utilized for determining the frequency-dependent complex modulus and increasing the accuracy of sound transmission loss simulations in practical applications.