MEASUREMENT AND CALCULATION OF ACOUSTIC PROPAGATION CONSTANTS IN ARRAYS OF SMALL AIR-FILLED RECTANGULAR TUBES

An experimental and theoretical investigation of sound propagation in a porous sample composed of capillary tubes with rectangular cross sections is described in this paper. An experimental technique valid for low flow resistivity and high porosity porous samples was developed to measure the attenuation and phase velocity in the porous material. This technique uses transmission of a short pulse in a large tube through the porous sample and subsequent frequency domain analysis in the range 200-1300 Hz. Good agreement was obtained if an anomalous tortuosity factor of 1.1 is used in the theory. A scaling factor for relating cylindrical and square tube capillary theories, known as the dynamic shape factor, was investigated. Propagation constants computed from use of a near unity dynamic shape factor in the cylindrical pore theory agree favorably with calculations based on the square pore theory for the frequencies and pore radii used in the experiment.