The air conditioning (AC) unit is one of the dominant sound sources for exterior and interior noise of a railway vehicle. The design of such devices is a compromise between major functions like efficiency, fluid mechanics, thermodynamics and acoustics. The vibrational and acoustical behavior of an air conditioning unit needs to be quantified in every state of the development process, starting in an early design phase. Therefore preferably simple and fast calculation tools are needed, which can be easily built up and adapted in a short period of time. Within a research project at the Fraunhofer Institute for Machine Tools and Forming Technology (IWU) an acoustical calculation model, based on transfer matrices of the different components, has been created to give a sufficiently accurate description of the sound power radiated by the air conditioning unit. In a first step the sound power of relevant sources has been characterized by measurement data. Furthermore the acoustical transfer behavior of ducts, chambers and additional components like condensers or evaporators have been individually investigated and implemented in the model using -port elements. Finally, the sources and transfer components were combined to get a complete representation of the acoustical properties of the air conditioning unit, which can be used for parameter variation studies.