Tortuosity and Acoustic Slow Waves

The experimental measurements of tortuosity of porous structures using either the acoustic index of refraction of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ or the electrical conductivity are shown to agree with each other. This and other measured parameters are used to calculate directly the acoustic speeds of water-saturated, fused-glass-bead samples; there are no adjustable parameters and agreement with experiment is excellent. The dependence of tortuosity on pore volume fraction, $\varphi$, is discussed.