Flow resistivity

It is rather amazing (but very convenient) that for most common sound absorbing materials the principal acoustic properties can be predicted over the most important frequency range (50 Hz to 4 kHz) just by knowing the flow resistivity of the material. From the flow resistivity one may use theoretical or empirical models to predict the characteristic impedance and propagation coefficients of the material, which then tells you most of what you need to know about that material. From these two properties one may predict the acoustic impedance and for a given thickness of material the normal incidence absorption coefficients. Furthermore provided the material can be considered locally reacting the random incidence absorption can be estimated accounting for diffraction by the sample edges.

The flow resistance of a porous material such as fibreglass is defined as the pressure drop across a sample of material when a steady airflow is passed through the material, There is an ASTM standard (C522) for measuring flow resistance. The unit of flow resistance is the Rayl (or Pa·s·m-1) . Beware that older studies have used a unit of flow resistance based on the cgs system, this is usually spelt with a lower case r as rayl, or referred to as cgs rayl (1 rayl = 10 Rayls). The flow resistivity is the flow resistance for a metre thickness with units of Rayl/m. In some instances it may be estimated from the materials density and there is a built in calculator in Zorba that can be used to estimate the flow resistivity from the absorptive materials density for fibreglass, mineral fibre, and polyester materials. Many manufacturers of acoustic materials measure their products flow resistivity and either publish this data or have it available on request.