Boundary element prediction of sound propagation in ducts containing bulk absorbing materials

In this paper the sound propagation in a rectangular duct containing a bulk absorbing foam liner is investigated. The foam is characterized by the complex characteristic impedance and propagation constant which are measured using an improved two-cavity method. These quantities are used to predict the normal incidence impedance and absorption coefficient of the foam which is compared to measured data obtained using the ASTM E-1050 test method. The complex characteristic impedance and propagation constant are also used to predict the attenuation through a layer of foam in a plane-wave tube for which measured results are also available. Finally, the boundary element method is used to predict the attenuation of sound in a rectangular duct where one surface is replaced by a foam-filled cavity. Comparison with experiment is shown for several frequencies.