An alternating air-flow method for measuring the resistivity of porous materials

Air-flow resistivity is one of the main non-acoustic parameters governing the acoustic behaviour of porous materials used in sound-absorbing systems. The International Standard ISO 9053 specifies two different methods for the measurement of the air-flow resistivity: a steady-state air-flow method and an alternating air-flow method. The latter is based on the measurement of the sound pressure at a very low frequency (2 Hz) in a small rigid volume closed partially by the porous test sample. A known volume-velocity sound source excites the cavity. This article reports experimental data concerning the air-flow resistivity of various porous materials obtained by an alternating air-flow method developed by the authors. It is based on the ratio of the complex sound pressures measured inside two cavities coupled through a low-frequency loudspeaker. The suggested measurement method was tested with fibrous materials, loose granular materials and samples constituted of a number of calibrated glass tubes. The resulting air-flow resistivity values are compared with the corresponding values obtained either with different measurement methods or given by material manufacturers or predicted by theoretical models as well.