The prediction of the complex characteristic acoustic impedance of porous materials

Modeling the complex characteristic acoustic impedance and complex wavenumber of porous materials allows the prediction of the complex specific acoustic impedance of a system consisting of porous absorbers and air cavities in front of a rigid surface. By using the transfer matrix method, the complex characteristic acoustic impedance and complex wavenumber of a porous material can be predicted by using the measured complex specific acoustic impedance of two different systems of the porous material and an air cavity, performed in a two-microphone impedance tube. Depending on the method, the material can be measured with either a rigidly terminated back plate at the back of the material, or a rigidly terminated air cavity at the back. This paper looks at why predictions using the single and double thickness method break down for thinner, less dense materials.