To predict the sound transmission loss of double partitions with absorptive layer, a theory is developed. The theory used in this paper agree well with experimental results. During the experimental procedure, the necessity of testrooms leads us to construct the twin reverberation chamber. Many requirements 6.17 are satisfied by the test facility, but the small size of it limits the lowest frequency that can be measured to 400 Hz. Experiments are also carried out for some effects that have not been investigated before. Result for double panels with internal corrugations of plastic shows considerable improvement on the sound transmission loss below the frequency of 1,000 Hz.