Prediction of the Sound Isolation of Multi-layered Small Sized Elements

Often it is a problem to get a good estimate by prediction of the sound transmission loss R of multi-layered structures, like glazing, panels, etc.. A practical solution could be based on the transfer matrix approach for infinite plates for the prediction of the trans-mission coefficient t that is a function of the angle of inci-dence of sound. Since in reality a more or less diffuse sound field exists in rooms, we have random incidence. If an ideal diffuse sound field is assumed the prediction will underesti-mated grossly the sound transmission loss due to low values for grazing incidence. Beranek [1] suggests to limit the an-gles of incidence up to 78° for limp single layer panels. For other types of structures the cut-off angle usually varies between 70° to 85° to get a good estimate of the single num-ber rated value, but for the frequency dependence of stiffness controlled panels this ad-hoc correction does not give a satis-factory agreement at all. To improve the prediction, an ap-proach that considers the finite size of the structure is com-bined with a non-uniform directional distribution of incident power.