Fig. 5. Reflection coefficient, , as a function of the porosity for the three samples according to the FMMM. The curves were calculated using Eq. [4] with a value of _s of 0.055 mol L^-1. For the frictional coefficient, R_wm, an initial value of 0.1 was taken which decreased linearly in the porosity range between 0 and 0.4. Above a porosity of 0.4 the value of R_wm was taken to be zero.