The porosity of a sample of porous material is defined as the
ratio of the volume of air in the void space (Va) in the sample to the
total volume of the sample. The air flow resistance is the resistance
experienced by the air when it passes through open pores in the
material. The tortuosity or “shape factor,” as it is sometimes known,
is a measure of the shape of the air void passages (whether they
are almost straight or twisted and winding and slowly or rapidly
changing cross section area) and the effect this has on the material
sound absorption properties.8