In order for a material to absorb sound, it must be porous, so that the sound waves can move through the material.17 Porosity is the ratio of open space volume to the total volume of the porous material. Only connected pores, which are accessible to air flow, should be included in porosity calculation.14

The gravimetric measurement of the porosity requires the knowledge of the vol­ume of the porous material and the density of the fibers. The porous material is weighed and fabric density is calculated. The porosity is calculated by using the following equation:

where h is porosity, rw is density of fibrous material, rf is the density of the fiber. Reciprocal of porosity, massivity, ¥, is the ratio of fiber volume to the total volume of the porous material:18
(27)

Another method of measuring porosity involves saturating the porous materials with a kind of fluid such as water or mercury and determining the porosity from the relative weights of saturated and unsaturated samples. The disadvantage with this method is the deformation of the pore structure due to the introduction of the fluid.17

A dry method of porosity determination is based on the measurement of the change in pressure within a container, which contains the porous material, with ad­dition of a small volume of air. It is advantageous as it only measures the connected pores, through which sound propagates.17

The range of porosity is higher than 0.95 for mineral and glass wools and porous plastic foams. The porosity values of fibrous mats have been reported to range be­tween 0.83 and 0.95 by Cox and D’antonio.14 Porosity determines the ratio between the average particle velocity in the channels and on the cross-sectional average par­ticle velocity of the absorber material.13 A number of models use the assumption that the porosity is close to unity including that of Delany and Bazley.28 The effect of porosity should be included in air flow resistivity terms that takes a significant role in predictive models as the quotient between the density of the biocomposite to its constituent fibers, , given in Eq. (25) corresponds to massivity, ¥, which is porosity subtracted from unity.