Modal coupling in acoustic waveguides: planar discontinuities

A new derivation for a scattering matrix for reflection and transmission of higher order modes at the planar junction of two waveguides is presented. The derivation is extended to include finite junction wall impedance and offset waveguides. The resulting matrix equations are analyzed and the physical significance of the matrices is explained. As an example of the theory, analytical expressions for the coupling coefficients at a size change in a rectangular duct are developed and the resulting reflection and transmission coefficients are computed. The results should be of interest to the HVAC noise control community. The paper also shows the effects of modal truncation on the accuracy and convergence of the solution. It is shown that the proper selection of the ratio of the number of modes on either side of the discontinuity is related to the ratio of the characteristic sizes of the waveguides. Finally it is shown that at least one higher mode should be included for reasonable accuracy in the computation of plane wave reflection and transmission coefficients except at the very lowest frequencies.