When a muffler is modeled by the boundary element method (BEM), the transmission loss (TL) can be evaluated by either the conventional four-pole method or the recently developed three-point method. The three-point method produces only the TL, and nothing else. On the other hand, the four-pole method has the advantage of retaining the transfer matrix of the muffler, which contains important parameters when the muffler is connected to another muffler or other components in the exhaust system. However, the major drawback of the conventional four-pole method is that it requires two separate boundary element runs due to the two different boundary conditions imposed on the outlet boundary. Therefore, it can take twice as much of the CPU time as the more efficient three-point method. In this paper, an improved four-pole method is introduced. This improved four-pole method does not require solving the boundary element matrix twice. Therefore, it is as efficient as the three-point method and it also retains the important four-pole transfer matrix. The boundary element analysis is done by the direct mixed-body BEM. Two different transfer impedance formulas for perforated tubes are used. Numerical results are compared to experimental data.

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