Machine acoustics focuses on the description and modification of airborne sound radiated by vibrating machine structures. Sound generation and its mechanisms can be described by the fundamental equation of machine acoustics. This meta-model implements the transfer function of structure-borne sound as a “black box”. Thus, machine acoustics lacks a scientific tool for designing and evaluating structures that are optimized with respect to structure-borne sound. The presented work addresses the evaluation of sound transfer through real structures by analyzing the energy flow represented by the structure-borne sound intensity (SSI). A methodology is developed that enables the modification of sound transfer within real shell structures by adjusting local distributions of impedances and damping based on SSI calculations. This method enables the acoustic evaluation of alternative structure designs based on the analysis of sound transfer paths. In a first step, the convergence of SSI results is examined analytically using KIRCHHOFF plates and EULER-BERNOULLI beams. It is shown that the error of the modal superposition approach depends not only on the number of included modes and the frequency but also on the location on the structure where the SSI is calculated. Modal superposition shows poor convergence in several cases where less than 1000 superposed modes lead to errors of up to 10 percent. Further numerical studies confirm that normal components of the SSI of shells, which are neglected in common literature, are indeed neglectable in most cases as they account for approximately one percent of the total energy flow for a wide range of shell structures. Transient SSI calculations show that the energy flow attunes to a steady state after 100 to 200 periods for a qualitative result and after 1000 periods for a quantitative result. Additional studies on curved shells show that the energy flow resulting from in-plane vibrations is in general not neglectable. In cases of real multiply-curved shell structures such as car bodies, even small in-plane vibrations can dominate the transfer of structure-borne sound. Therefore, structural modifications that influence the transfer of in-plane vibrations prove to be viable means of affecting the coupled out-of-plane vibrations, which are responsible for sound radiation. Eventually, the derived methodology is applied to a vehicle oil pan and an underbody. By analyzing the SSI, parts of the structures are identified where local variations of impedances caused by modifications of local masses and stiffnesses, reduce the transmission of structure-borne sound and vibration amplitudes as well. Finally, studies on numerically optimized distributions of damping layers on shell structures show that the effects of the additional mass and stiffness on the vibration amplitudes exceed the effect of the additional damping.