This paper presents a straightforward application of an indirect method based on a threemicrophone 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. Doutres et al.3

This work presents a method to directly determine the open porosity of porous solids, and more particularly those used in sound absorbing liners. The method is based on the measurement of four masses at four static pressures from which the open porosity and true mass density are deduced using the perfect gas law. The precision of the method in relation with the used experimental setup is studied, and a simple expression is derived to predict the experimental error in function of the bulk volume of the sample to test. For a given experimental setup, this simple expression can be used to select the amount of bulk volume to test to reach a given precision. The method and its error prediction are tested experimentally on different samples of theoretically known open porosity. Good correlations are obtained for low and high porosity samples.

Complement to standard E2611-09 of the American Society for Testing and Materials [Standard Test Method for Measurement of Normal Incidence Sound Transmission of Acoustical Materials Based on the Transfer Matrix Method (American Society for Testing and Materials, New York, 2009)] is proposed in order to measure normal incidence sound transmission loss of materials in a modified impedance tube using a three-microphone two-load or one-load method. The modified tube is a standard two-microphone impedance tube, where a third microphone is mounted on a movable hard termination. This method is conceptually identical to the four-microphone two-load or one-load method described in the standard; however, it requires fewer transfer functions and one microphone less. The method is validated on (1) symmetrical homogeneous and (2) non-symmetrical non-homogeneous specimens.

Several methods for measuring the complex wave number and the characteristic impedance of sound absorbers have been proposed in the literature. These methods can be classified into single frequency and wideband methods. In this paper, the main existing methods are revisited and discussed. An alternative method which is not well known or discussed in the literature while exhibiting great potential is also discussed. This method is essentially an improvement of the wideband method described by Iwase et al., rewritten so that the setup is more ISO 10534-2 standard-compliant. Glass wool, melamine foam and acoustical/thermal insulator wool are used to compare the main existing wideband non-iterative methods with this alternative method. It is found that, in the middle and high frequency ranges the alternative method yields results that are comparable in accuracy to the classical two-cavity method and the four-microphone transfer-matrix method. However, in the low frequency range, the alternative method appears to be more accurate than the other methods, especially when measuring the complex wave number.

Two types of general methods can be found in the literature for the determination of the normal incidence sound transmission loss (nSTL) of acoustical elements. The first one is based on the transfer matrix (TM) approach, and the second one is based on the wavefield decomposition (WD) theory. From all the techniques proposed in the literature, the general TM methods (two-load or two-source location) are the only methods yielding the exact nSTL of an acoustical element without any assumptions on its symmetry and on the termination (i.e., the load). Except for the case of an anechoic termination, there is no method based on the WD theory which yields exact nSTL. This paper presents a general WD method to measure the exact nSTL of an acoustical element without any assumptions on its symmetry and on the termination. Similar to general TM methods for non-symmetrical elements, four microphones and two loads will be required. As a first validation of the method, symmetrical and non-symmetrical porous materials are investigated. Results are discussed and compared with some existing methods and with the classical two-load method. A perfect agreement is found with the classical two-load method.