David E. P. Lawrence
Charles G. Don
Dept. of Phys., Monash Univ., Melbourne, Victoria 3168, Australia
Impedance and wave-number predictions from a rigid-frame model of a porous medium, where only a single wave is assumed to propagate through the permeating fluid, are compared with results from a dual-wave theory where there is both a fast and a slow wave present. Both results are matched against experimental data obtained by acoustic impulse techniques that allowed both the characteristic impedance and the layer impedance to be deduced from 200 Hz to 10 kHz. The complex wave number was determined by placing two microphones a known distance apart inside a foam block and recording the waveforms as a pulse passed. The high-frequency wave-number limit permits the tortuosity to be established, while the effect of a pore shape factor is more evident in the attenuation data. The position and magnitude of the maxima in the layer impedance are very sensitive to the tortuosity and the shape factor. Both models adequately predict the experimental impedance curves, however, for a foam with a flow resistivity of 25100 N s m[sup -4] the presence of a slow wave is required to fit the low-frequency attenuation and wave speed results. An improved incorporation of the shape factor into the dual wave model is suggested.