However a few things need to be borne in mind by the user. The accuracy seems to decrease as a function of the number of elements involved in the construction, and so for a triple panel construction the accuracy is appreciably less than for single and double panel constructions.
From the available data that we have to compare the results of INSUL predictions against laboratory tests, the accuracy (90% limits) is estimated to be 4 dB for the Rw or STC index. A
further warning is also in order, for some triple panel constructions (e.g. masonry walls that
have plaster board linings on both sides) the STC rating is controlled entirely by the low frequency dip in the transmission loss curve due to the 8 dB rule. The accuracy of the STC rating is then even less because you do not get the averaging effect across a number of frequencies normally associated with the Rw or STC rating method. The STC is completely determined by that single frequency with consequently only the lesser accuracy of a single band.