The after treatment devices (ATD) used in internal combustion engine (IC-engine) exhaust systems are mainly designed with emphasis on emission control, i.e. chemical efficiency, while paying less attention to the acoustic performance. In automotive applications, the duct diameters are so small that studying the acoustic wave propagation only in the plane wave frequency range is usually sufficient. In the case of medium speed IC-engines, used for example in power plants and ships, the three dimensional acoustic phenomena must also be taken into account. The main elements of the medium speed IC-engine ATD are the selective catalytic reducer (SCR) and oxidation catalyst (OC), which are based on a large amount of coated channels, i.e. the substrates. The number and type of the substrates depends not only on the regional environment legislations but also on the engine type. In this study the acoustic attenuation of a medium speed IC-engine ATD is simulated and the results are compared with measurements. The focus is in the low and mid frequency range. That is, the three dimensional waves inside the ATD are taken into account whereas only the plane waves are assumed to propagate at the inlet and outlet ports. The paper also discusses how to extend the classical one dimensional two-port solution into the higher frequencies. Using the computationally effective two-port solution in a wider frequency range is of great importance, e.g., when searching the acoustically optimal substrate configuration.