ABSTRACT
Porous materials are often used for the purpose of absorbing sound. It is the porous nature of many surfaces, such as grass-covered ground, that determines their sound reflecting properties. The property of a porous material which determines its usefulness for acoustical purposes, is the resistance of the material to induced flow through it, as a result of a pressure gradient. Flow resistance, an important parameter, which is a measure of this property, is defined according to the following simple experiment. A uniform layer of porous material of thickness, ℓ, and area, A, is subjected to an induced mean volume flow, U0 (m3/s), through the material and the pressure drop, ∆P, across the layer is measured. Very low pressures and mean volume velocities are assumed. The flow resistance of the material, Rf, is defined as the induced pressure drop across the layer of material divided by the resulting mean volume velocity per unit area of the material:
(5.1)
The units of flow resistance are the same as for specific acoustic impedance, ρc; thus it is sometimes convenient to specify flow resistance in dimensionless form in terms of numbers of ρc units.
