Measurement of tortuosity of anisotropic acoustic materials

Tortuosity, i.e., the high-frequency limit of the squared propagation index, has been evaluated with narrow-band piezoelectric transducers on reticulated plastic foams. The basic procedure was recently reported by J. F. Allard, B. Castagnède, M. Henry and W. Lauriks [Rev. Sci. Instrum. 65, 754 (1994)]. Further experimental work related to the anisotropic nature of these materials has been done by probing the propagation index versus angle. A simple numerical routine has been implemented in order to recover the propagation index along principal directions. These predicted values compare well with measurements taken directly along the principal geometric axes of the samples. Slight angular deviations of the principal axes themselves have been observed. This is the very first account of the anisotropy of tortuosity measured by ultrasonic methods in air-saturated porous materials.