ABSTRACTExpression for the transmission loss (T.L.) of composite plates in which flexural waves exist in each layer can be given in the same form as that for single panel. In the case of composite plates the surface mass is equal to the summation of the surface masses of the individual layers. The critical frequency of the composite plate may be simply related with the critical frequencies and the masses of the individual layers. The theoretical transmission loss of the composite plate is calculated from this critical frequency and also by the plateau curve method. The theoretical transmission loss agrees well with the experimentally obtained values.Experimental results of five more typical constructions with their sound transmission class (STC) numbers are also presented here.