Effect of liner thickness on acoustic wave propagation in ducts

Abstract

The acoustic characteristics of a point-reacting duct liner that consists of a porous facing sheet backed by cellular cavities are derived by examining the wave propagation within the liner. The relation between the derived expression for the liner acoustic admittance and a semi-empirical formula that is widely used in the literature is discussed. The influence of the liner on acoustic propagation in a duct is examined for the case of a plane duct that carries a uniform mean flow. Numerical results for the attenuation rates vs. frequency are presented. These results. are of three types: (1) comparisons with previously published results for no backing cavity and no mean flow are made, and these results are extended to include the effects of the mean flow; (2) results of parametric variations of the liner dimensions are presented to assess the relative influence of the facing-sheet thickness and the cavity depth; (3) results from the derived expression for liner specific admittance and from the semi-empirical formula are compared in order to determine the significance of the wave propagation within the porous material and to determine the range of validity of the semi-empirical formula.