Determination of porous material data via two-port measurements

Abstract

The classical model used to describe porous materials of fibrous type is the so-called equivalent fluid model. For typical fibrous materials this model is valid except for low frequencies where solid frame vibrations become important. Since porous materials are widely used in dissipative silencers, the trend for better and better modeling requires a good knowledge of acoustic data for such materials. This means that a complete characterization is necessary to provide input data for, e.g., FEM or BEM codes. As known from theory, this is possible by specifying the complex wave number (Ka) and complex characteristic impedance (Za) for a propagating plane wave. Of course, for a nonisotropic material, these data must be measured in more than one direction. The standard technique to determine Ka and Za is to use a standing wave tube and measure the complex impedance for two samples of different thickness. In this paper an alternative approach is suggested based on the measurement of the acoustic two-port for a single sample mounted in a duct with plane wave propagation. The paper describes the development of such a two-port test rig and discusses the advantages compared to earlier used techniques.