Effect of lining non-linearity on realized attenuation of tonal noise

An iterative model based on an FEM propagation code is introduced for prediction of attenuation in a duct in the presence of a two degree of freedom acoustic lining for which impedance depends on local grazing flow speed and overall local Sound Pressure Level. A general approach is described; however this investigation is limited to the case of a source Sound Pressure Level spectrum dominated by a single tone for which the lining is optimized. The tone is specified by its circumferential mode and a statistically represented radial modal amplitude distribution. It is assumed that the underlying broad band spectrum is diffuse and relatively unaffected by the lining. Lining impedance is computed locally and is described by an iterative model imbedded in the propagation code. It is determined how local variation of Sound Pressure Level affects realized attenuation by comparing attenuation predictions when impedance depends on source Sound Pressure Level and when impedance depends on local Sound Pressure Level. Predicted attenuation is dependent on local variation in lining impedance when the tone level is significantly above the overall broadband level. When suppression of the tone has little effect on the overall Sound Pressure Level there is little effect of local impedance variation on attenuation. (C) 2010 Published by Elsevier Ltd.