Air radiation caused by vibrating structural systems is a fundamental problem in acoustic engineering with a wide application range. Exact analytical solutions are possible for some simple structures but for complex ones analytical expressions are not easily established. In this paper a methodology for computing the acoustic radiation caused by any type of flat structural system is presented. A general equation is established in modal terms, including the interaction between the structure and the surrounding fluid through pressure fluctuations. These are defined using a potential function, whose description is based on Hankel or Green baffled functions depending on the radiating surface dimension. Solving the obtained equation modal displacements caused by a specific excitation are known. Through these displacements the radiated power by the structure as well as the energy stored are quantified. Relating both the coupling loss factors between the structural system and the surrounding fluid are determined. In order to achieve the statistical diffuse field assumption coupling loss factor values are averaged both in frequency and in space. Some examples are analysed and it is shown that accurate results can be obtained applying the developed methodology in simple structures. In complex configurations it is validated using the computed coupling loss factors to estimate the transmission loss values and compare them with the experimental ones.