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Acoustic materials are characterized according to their macroscopic and microscopic properties, among which the sound absorption coefficient and flow resistivity are of paramount importance. The dependence of a sound absorption coefficient on air-flow resistivity has been emphasized by various empirical models, and thus its measurement becomes very much essential. The evaluation of flow resistivity basically requires knowledge of the material’s intrinsic properties which can be obtained through the implementation ofthe indirect acoustic methods.The aim of this paper is to study the existing static flow resistivity measurement methods and then accordingly implement suitable indirect methods based on a standard impedance tube.The standard impedance large tube facilitates two microphone spacing configurations, namely, wide microphone spacing, which coversthe frequency range from 63-500 Hz and close microphone spacing covering the 400-1600 Hz frequency range. Surface impedance of the sample is measured using the pressure data. The measured surface impedance is used in the indirect methods. The indirect acoustic methods discussed in this paper include the two-cavity method (with arbitrary air-gap or quarter wavelength air-gap) and the two-thickness method. The minimum frequency for providing a quarter wavelength air-gap is above 500 Hz based on the available air-gap. For the 400-1600 Hz frequency range, the two-cavity method (with an arbitrary and quarter wavelength air-gap) and the two-thickness methods were employed, whereas for the 63-500 Hz frequency range, the two-cavity method (with arbitrary air-gap) and the two-thickness method could only be employed. The evaluation of intrinsic properties and hence flow resistivity is carried out for polyurethane foam material in the 63-500 Hz and 400-1600 Hz frequency range and compared for optimum results. In case of two-cavity method, the effect of change in air-gap and thickness on flow resistivity is studied. A comparison and suitable conclusion is drawn between the resistivity results obtained by the two-cavity and two-thickness method for the considered frequency range. The validation of flow resistivity is carried out by estimating the sound absorption coefficient through suitable empirical relations and then comparing it with the sound absorption coefficient values directly measured using the impedance tube. The indirect methods discussed could be suitably used for estimating the flow resistivity of acoustic materials using the standard impedance tube.
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