The ratio between the passage ways through the pores and the thickness of the po­rous material is called tortuosity.61 Tortuosity gives the extent of the deviation of the pores from the normal of material thickness.71 The sound absorption performance of porous materials is generally affected by tortuosity which does not allow the sound waves to follow straight paths.72 The air that is forced to follow a tortuous path suffers accelerations which cause momentum transfer from air to the material.35 The value of tortuosity determines the high frequency behavior of sound absorbing porous materials.61

Tortuosity can be measured by an electrical conductivity method.17 In this meth­od, the porous material is saturated with a conducting fluid such as a brine solution, and the electrical resistivity of the saturated porous material and the solution alone is measured and compared.

(28)

where F is the formation factor, and R, and R are the electrical resistance values of

7 ef es

the fluid alone and the porous material saturated with fluid, respectively, which have

the same dimensions, in ohm units. As the electrical resistance is proportional to the length, L, and inversely proportional to the area, S, of the conducting material the ratio between the resistances; formation factor, F, should be tortuosity, Ts, divided by porosity, h, as shown in the following equations:17

Tortuosity contributes to the ‘structure factor,’ Г, along with the effect of in­ner structure of porous materials.40 The structure factor, Г, (x in 18) accounts for an increase in the inertial mass density of the air. In other words, the irregularity of the structure causes an addition of induced mass to the density of air. For fibrous ma­terials, structure factor, Г, is generally between 1.2 and 2.3,13 but it is assumed to be unity in modeling studies by numerous researchers including Allard and Cham — poux,73 Ballagh46 and Cox and D’antonio.14 High structure factor leads to low propa­gation speed, which increases the effective thickness of the absorber; and thus, used in sound absorber design.13 Biocomposites are advantageous in terms of tortuosity due to the fact that the irregular shapes of constituent plant fibers prevent a straight flow of sound waves through the thickness of the absorbent material.