In the automotive development process the use of numerical simulation has been established for many years, what lead to a reduction of the effort of experimental analysis and number of prototypes. Even in vibro-acoustic design of cars the computing of NVH (Noise Vibration Harshness) behavior with FEA (Finite Element Analysis) now is a common approach. For the calculation of acoustic performance of vehicles it is necessary to model the fluid as well as the fluid-structure coupling. This leads to a large computational effort. Therefore a reduced calculation estimating the the acoustic behaviour without a fluid, brings some advantages, especially in an early stage of the development. One of the most used methods for this case, as well as for numerical optimization is the ERP. Based on the underlying assumptions of plane waves and perfect radiation it is shown that the classical ERP has limitations. To overcome some of these limitations-without loosing the benefits-an advanced ERP is derived with an inclusion of the analytically correct radiation factor. This enhanced ERP is compared with the classical ERP and also with a real acoustic simulation and the improvement is shown. Finally suggestions for an advantageous use of the new ERP are given.