Investigation of Extended-Tube Liners for Control of Low-Frequency Duct Noise

Abstract

Existing models of extended-tube liners are mainly applicable to the normal-incidence case, and the influence of the grazing-flow effect is often ignored. In the current paper, an impedance model is developed based on the transfer matrix method, and the grazing-flow effect on the surface impedance is taken into account in terms of the end corrections. The validity of the model is examined on a flow-duct facility, and the liner impedances are obtained from an impedance eduction method. The proposed model shows a reasonable agreement with the educed data, and better accuracy is found in terms of the transmission loss. Geometric parameters including the length of the extended tube and the grazing-flow speed are then investigated. It is found that the frequency for maximum attenuation is shifted to lower frequencies for a longer extended tube, but it is less sensitive to the grazing-flow speed. The effects of the sound pressure level and nonlinearities are also investigated.