The main objective of the work here presented is to extend the calculation of the radiation efficiency to plates whose characteristics differ form the simply supported isotropic rectangular plates. In this work the radiation efficiency is calculated by using the modal summation approach based on the far field sound intensity. The plate is considered isotropic or orthotropic. The orthotropy is introduced by the characteristics of the material or by the presence of ribs parallel to an edge. Different combinations of free, simply and clamped supported boundary conditions are considered for the isotropic plate, while the orthotropic plate is always considered simply supported. Free vibration of isotropic plates are calculated by using the classical Kirchoff-plates theory. The results obtained for the radiation efficiency are compared with a BEM model and show the influence of the constraints in the edges. Results show that the clamped supported plate is the the configuration for which the radiation efficiency is higher, while the freely supported configuration is the one with lower radiation efficiency. The difference between different boundary conditions is also summarized in terms of third octave bands. In this manner it has been shown that the difference between a clamped plate and a simply supported plate in the mid-frequency range in terms of radiation efficiency is not 3 dB as stated in literature by Leppington, but is variable and can reach up to 10 dB. The orthotropy has been introduced. First, a plate made of an orthotropic material is considered. Free vibration have been calculated analytically and have been compared with with a FE model, making the results a good agreement. The radiation efficiency has been calculated using the same formulae as the isotropic plate. Then the orthotropy is considered by the introduction of ribs. An analytical-numerical method is used to calculate the free vibration of a ribbed plate. Results have been compared with a FE model and make a good agreement. The radiation efficiency of a ribbed plate is calculated by modifying slightly the formulation presented for the calculation of the radiation efficiency of an isotropic plate. The formulation adopted for the radiation efficiency allows to investigate the effect of the modal damping ratio, the influence of the boundary conditions and the effect of the ratio of the stiffness along perpendicular directions.

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