Air-flow resistivity is a main parameter governing the acoustic behavior of porous materials for sound absorption. The international standard ISO 9053 specifies two different methods to measure the air-flow resistivity, namely a steady-state air-flow method and an alternating air-flow method. The latter is realized by the measurement of the sound pressure at 2 Hz in a small rigid volume closed partially by the test sample. This cavity is excited with a known volume-velocity sound source implemented often with a motor-driven piston oscillating with prescribed area and displacement magnitude. Measurements at 2 Hz require special instrumentation and care. The authors suggest an alternating air-flow method based on the ratio of sound pressures measured at frequencies higher than 2 Hz inside two cavities coupled through a conventional loudspeaker. The basic method showed that the imaginary part of the sound pressure ratio is useful for the evaluation of the air-flow resistance. Criteria are discussed about the choice of a frequency range suitable to perform simplified calculations with respect to the basic method. These criteria depend on the sample thickness, its nonacoustic parameters, and the measurement apparatus as well. The proposed measurement method was tested successfully with various types of acoustic materials.

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In the course of a study for characterizing the acoustic properties of some loose granulates made of limestone chips, the authors have measured their resistivity in a very simple way by a steady-state air-flow technique. The results of these tests are compared with the corresponding values obtained by an acoustic technique. The latter relies on the assumption that at very low frequencies the real part of the normal-incidence flow impedance of a thin porous layer is very close to its steady-state air-flow resistance. The measurements results confirmed the above mentioned assumption to a certain degree. To the knowledge of the authors, no previous comparison of steady-state air-flow versus oscillating air-flow measurement of the resistivity of the considered granulates has been published yet.