The Johnson-Champoux-Allard Model command determines the static air flow resistivity, the tortuosity, the viscous characteristic length and the thermal characteristic length of the current 3D structure. These values are then used to predict the sound absorption of the material in a Kundt tube according to the empirical functions derived by Johnson, Champoux and Allard (cf. Allard and Champoux, Champoux and Allard). The model is valid for porous materials with a rigid frame and arbitrary pore shapes.