Hybrid analytical/numerical method for mode scattering in azimuthally non-uniform ducts

Sound-absorbing liners for ducts are often made in several azimuthal sections with acoustically hard strips, known as splices, between them. These can alter the liner's performance by causing scattering between mode orders, usually to the detriment of overall attenuation. Three-dimensional finite element methods, often involving specialised codes, have previously been used to predict the performance of such spliced liners. Here, an alternative approach is presented which uses a readily available two-dimensional finite element solver to find the modes of the spliced liner and then matches them to the analytical modes of the hard duct. The results are compared with three-dimensional finite element calculations to verify their accuracy. The method as described is for ducts with no flow, but can be extended to flow ducts if necessary.