Case study: Correlation between boundary and finite element determination of large silencer transmission loss

Silencers used in the power generation industry generally have large ducts entering and leaving the silencer. With large cross-sectional dimensions, the plane wave cutoff frequency will be exceeded at a low frequency so that transmission loss can no longer be evaluated by assuming constant sound pressure over a cross-section. More sophisticated calculation and processing approaches are necessary. In this research, the boundary element method is used in conjunction with a reciprocal identity method to determine the transmission loss for rectangular and circular cross-sections: the two configurations that cover most real-world designs. The boundary element method is compared to a finite element method strategy where the transmission loss is determined using an automatically matched layer boundary condition at the inlet and outlet. This approach can be used in most commercial software. Although these two approaches have little in common, transmission loss results compare well with one other. Validation by comparison is helpful because analytical solutions are only available for simple axisymmetric cases. Methods are compared for practical configurations like parallel-baffle silencers and reactive silencers.