Boundary element analysis of packed silencers with protective cloth and embedded thin surfaces

Bulk-reacting porous materials are often used as absorptive lining in packed silencers to reduce broadband noise. Modelling the entire silencer domain with a bulk-reacting material will inevitably involve two different acoustic media, air and the bulk-reacting material. A so-called direct mixed-body boundary element method (BEM) has recently been developed to model the two-medium problem in a single-domain fashion. The present paper is an extension of the direct mixed-body BEM to include protective cloth and embedded rigid surfaces. Protective cloth, an absorptive material itself with a higher flow resistivity than the primary lining material, is usually sandwiched between a perforated metal surface and the lining to protect the lining material from any abrasive effect of the grazing flow. Two different approaches are taken to model the protective cloth. One is to approximate sound pressure as a linear function across the cloth thickness and then use the bulk-reacting material properties of the cloth to obtain the transfer impedance. The other is to measure the transfer impedance of the cloth directly by an experimental set-up similar to the two-cavity method. As for an embedded thin surface, it is a rigid thin surface sandwiched between two bulk-reacting linings. Numerical modelling of an embedded thin surface is similar to the modelling of a rigid thin surface in air. Several test cases are given and the BEM results for transmission loss (TL) are verified by experimental TL measurements.