The tortuosity, or identically the structure factor, is a geometrical measurement of the deviation of the actual path followed by an acoustical wave from a direct path.
Equipment: Ultrasound Tortuosity Meter
Method: Evaluation of tortuosity in acoustic porous materials saturated by air [1, 2]
Property: α∞ – tortuosity
 J.F. Allard et al., “Evaluation of tortuosity in acoustic porous materials saturated by air,” Rev. Sci. Instrum. 65, 209-210 (1994).
 C. Perrot, F. Chevillotte, R. Panneton, J.-F. Allard, and D. Lafarge, “On the dynamic viscous permeability tensor symmetry,” J. Acoust. Soc. Am. 124(4), EL210-EL217 (2008).