The dimensionless ratios governing sound propagation in acoustic boundary layers and capillaries, indicate that the specific acoustic impedance of Rayleigh solids can be made invariant under scaling, as required for model experiments involving sound reflection by, or diffraction over, absorptive boundaries. Specifically, if macroscopic dimensionsL, such as the acoustic wavelength or the thickness of the porous layer are scaled down toL/sin the model experiment, the capillary diameter and spacing must be scaled down by a factor of onlys1/2. More generally, a correct model of fibrous layers requires that intersticial dimensions be scaled ass1/2, the layer thickness ass, and that the porosity remain unchanged, i.e., that the number of interstices per unit area be scaled up by a factors.