An Acoustic Evaluation of Circumferentially Segmented Duct Liners

Abstract

An evaluation of circumferenti ally segmented duct liners is performed by comparing attenuations of optimized segmented and uniform liners for a range of frequencies and source structures. Broadband suppression and the effects of changes to input modal structure are explored in these comparisons. Credence in the theoretical model was obtained by comparing predictions to results of a carefully controlled experiment performed on the Langley Spinning Mode Synthesizer Facility. Excellent agreement was obtained in these comparisons. It is shown that for the lower order spinning mode sources, the optimum segmented liner degenerates into an optimum uniform liner so that no increased suppression of the segmented liner over the uniform liner is obtained. In contrast, predicted results show that the optimized segmented liner is more effective than the optimized uniform liner for the higher order spinning mode sources with a hard-wall/soft -wall admittance variation representing an optimum configuration for the segmented liner. Predicted results also indicate that the segmented liner gives better broadband performance and that its suppression characteristics are not particularly sensitive to changes in input modal structure when compared to that of the uniform liner. It is concluded that the greatest suppression benefit of segmented liners occurs near mode cuton frequencies and that these benefits are obtained despite reductions in the total amount of acoustic treatment of 50% or more.