The Measurement of Flow Resistance of Porous Acoustic Materials

Abstract

The flow resistance is one of the three physical parameters of a uniform, rigid, porous material which completely determine its acoustic behavior; for more complicated materials and sound absorbing structures the flow resistance gives at least some acoustic information. The specific flow resistivity or flow resistance per unit cube is given by the equation r = pAt/Vt, in which p is the pressure drop in dynes per cm2 across the thickness l cm of a sample of area A cm2 when a volume V cc of air flows steadily through the sample in a time t sec. An accurate method has been developed for obtaining r by direct measurements of the quantities p, A, t, V, and l. Using this method flow resistance measurements have been made on twenty-four materials over a wide range of velocities. The measurements show that for nearly all materials there is a variation of flow resistance with velocity. For most materials the resistivity is constant below 0.1 cm per sec. but increases considerably at higher velocities. It is shown that previously described comparison methods, which have the advantages of simplicity and speed, might yield results in serious error unless certain suggested precautions are observed. As indicated previously, a direct correlation is found between the steady state flow resistance values and the resistive component of acoustic impedance for several materials. Analysis of the data also yields additional information bearing on the basic physical processes involved in the absorption of sound in these materials. In addition, the measurement of flow resistance has practical applications, such as the rapid routine checking of uniformity of samples of porous materials.