The Effect of Sample Edge Conditions on Standing Wave Tube Measurements of Absorption and Transmission Loss

The acoustical properties of isotropic, elastic porous materials are now conventionally specified by a set of nine, frequency-independent macroscopic parameters: e.g., flow resistivity, tortuosity, viscous and thermal characteristic lengths, etc. When these properties are known, it is possible to predict the absorption performance of the material, for example, in arbitrary geometries. Conversely, it has become popular to infer the macroscopic parameters of porous materials by finding the set of parameters that results in an optimal match of measurements and prediction. The software packages FOAM-X and COMET/Trim, for example, offer inverse characterization features of this type. However, here it is shown that that it may not be possible to represent large and small diameter samples of the same material as measured in standing wave tubes by using a single set of parameters. In practice, measurements in standing wave tubes can be significantly affected by sample edge effects, particularly gaps around the sample resulting from minor damage of the sample during cutting. Here it will be illustrated that it is necessary to model the sample inhomogeneity resulting from edge damage if both large and small tube results are to be modelled by using a single, consistent set of parameters