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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 parameter's 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

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.

This paper presents a description of the use of simplified numerical methodologies for the optimization of the low cut-off frequency of anechoic and hemi-anechoic chambers. The anechoic chamber is modeled as a cavity with proper surface impedance boundary conditions. First, the shape of the wedges is optimized by means of a minimization-based procedure of a finite element model of such elements in a "virtual" impedance tube for a plane wave field. An equivalent surface impedance of the wedges is determined from those data. An analytical procedure is then used to determine the complex reflection coefficient for spherical waves at oblique incidence. Finally, a complex image source approach is used to predict the sound field within the chamber. The methodology is applied to two anechoic chambers and the results are compared in terms of sound decay along fixed directions and surface pressure distributions.