Analysis of thermal and viscous boundary layers in acoustic absorption by metallic foam

Abstract

A method for estimating acoustic absorption in foams is presented using a combination of micro-computed tomography, finite element analysis, and boundary layer loss theory. In the method, the foam is assumed to be rigid framed and the viscous and thermal boundary layers at the fluid and frame interface are assumed to be small compared to foam dimensions. The boundary layer losses are approximated using an infinite planar model. The method is demonstrated for a commercially available open-cell metallic foam and allows for absorption to be estimated without determination of any intermediate variables that are required in existing methods. Enhancement of sound absorbing properties by selection of foam properties, such as porosity and pores per inch, is discussed. Furthermore, predicted absorption trends agree with other published models and experimental data. A simplified, two-dimensional geometry is presented in which the assumptions of this method are analyzed.