Attenuation of spiral modes in a circular and annular lined duct

An experimental and theoretical programme was undertaken to measure and predict the attenuation of plane and spiral modes within a cylindrical and annular lined duct. A standard duct modal theory was used for the cylindrical duct whereas a thin annulus model was evolved for the annular configuration. The experimental results show a considerable increase in attenuation rate for the higher order modes, especially in the cylindrical duct case. Excellent agreement was obtained between theory and experiment for the open cylindrical duct, and moderate agreement was found in the case of the annular duct. The results indicate that the theory can be used with reasonable confidence for the design of silencing devices for jet engine noise which is dominated by higher order mode effects.