Influence of anisotropic liners on the attenuation of sound in circular ducts

An analysis is presented of the sound propagation and attenuation in a circular duct carrying a uniform mean flow and lined with an anisotropicporous material backed by cellular cavities. A combination of a fourth-order Runge-Kutta routine and a Newton-Raphson procedure is used to determine the effects of the liner properties, the flowMach number, and the sound frequency on the attenuation of spinning and nonspinning modes. The results show that low-frequency noise is better attenuated by anisotropic liners. The optimum liner is the one whose axial resistivity increases with increasing frequency.