In the last decades the continuously tightening limitations on pollutant emissions has led to an extensive adoption of after-treatment devices on the exhaust systems of modern internal combustion engines. While these devices are primarily introduced for reducing and controlling the emissions, they also play an important role influencing the wave motion inside the exhaust system and so affecting the acoustics and the performances of the engine. In this paper a novel approach is proposed for the modeling of two after-treatment devices: the catalyst and the Diesel Particulate Filter. The models are based on a fast quasi-3D approach, named 3Dcell, originally developed by the authors for the acoustic modeling of silencers. This approach allows to model the wave motion by solving the momentum equation along the three directions. The capability of modeling complex shape devices, allowing the description of three-dimensional wave action, makes this tool an interesting solution for studying the acoustic behavior of today's mostly used after-treatment devices. In this work the 3Dcell approach has been extended to the modeling of the catalyst and DPF monolith, in order to combine the capability of predicting the flow distribution in the monolith channels with a lower computational demand than traditional 3D CFD approaches. The developed models have been applied to the simulation of the acoustic behavior of these components, comparing the results to measured data. For completeness sake the results have also been compared to traditional 1D models applied to the acoustic simulation of after-treatment devices.