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Transmission loss (TL) is a good performance measure of mufflers since it represents the muffler's inherent capability of sound attenuation. There are several existing numerical methods, which have been widely used to calculate the TL from numerical simulation results, such as the four-pole and three-point methods. In this paper, a new approach is proposed to evaluate the transmission loss based on the reciprocal work identity. The proposed method does not assume plane wave propagation in the inlet and outlet ducts, and more importantly, does not explicitly apply the anechoic termination impedance at the outlet. As a result, it has the potential of extending TL computation above the plane wave cut-off frequency.

Discontinuous boundary elements, such as constant elements, have been used in many engineering applications for years. In acoustics, the primary variables are sound pressure and normal particle velocity, and they can be defined at the center of each constant element. Since the acoustic variables do not need to be continuous across the element boundary, it is easy to write an algorithm to automatically refine the mesh by subdividing inside each element as frequency goes up. To take one step further, one may also choose to use a geometrically incompatible mesh, in which not only the primary variables but also the geometrical nodes do not need to be continuous across the element boundary. The main advantage is that a model can be quickly assembled from different sources without paying too much attention to the node-to-node continuity at junctions. As such, it is easy to add or remove a component at will to see its effect at the concept design stage.