Relation Between Acoustic Impedance and Flow Resistance of Porous Acoustic Materials

Abstract

A theory is developed for the penetration of sound waves into uniformly porous acoustic material. An expression is obtained giving the acoustic impedance at the front surface in terms of the porosity of the material, its flow resistance, and the effective density of air in the pores, taking into account reflection from the back surface. Curves will be shown of acoustic resistance and reactance as functions of frequency for different values of flow resistance, for uniformly porous material with rigid backing. Comparison of these curves with the experimental data of Beranek shows that for a number of commercial materials the effective values of flow resistance, porosity, and air density are constant over the frequency range 100 to 6000 c.p.s. It is indicated how other types of backing change the curves, and how computations can be made for non-uniform material. The static flow resistance or “d.c. resistance” of porous materials can be measured directly. A method is developed using a very slow steady stream of air flow through the material and determining the pressure drop across the material. The air flow is furnished by siphoning water at a known rate from a container mounted several feet above a control valve and outlet; air drawn into the container first passes through the porous material. The air velocities and pressures used are of the same order of magnitude as those obtaining in actual sound fields. Flow resistance values for several commercial materials are reported and correlated with the effective values indicated by impedance measurements.