The tortuosity or structure factor of a porous medium represents the increase in resistance to normal air flow due to the air following an indirect path relative to the macroscopic acoustic pressure gradient. Along with flow resistivity and porosity, tortuosity is one of the important parameters required in many of the theoretical models used to predict the acoustic properties of porous mateirals. A commonly used technique for measuring the tortuosity involves saturating the material with an electrically conducting fluid, however, this is inappropriate for unconsolidated granular media such as soils. Alternatively, the tortuosity can be estimated from the high-frequency asymptote of the measured phase speed. In practice, the maximum frequency at which propagation measurements can be made will be experimentally limited. This paper outlines a simple method by which the tortuosity of a medium can be readily estimated, with the aid of a simple theoretical calculation, from experimental data that have not reached the asymptotic value. The method assumes a prior knowledge of the flow resistivity and porosity of the medium. To validate the technique, estimates of the tortuosity of plastic foams and soils are compared with values obtained through a more time consuming curve-fitting approach.
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