A test program was recently instituted by the United States Gypsum Company—Research Center, with the cooperation of the Riverbank Acoustical Laboratories, to determine the effect of ambient humidity on the sound-transmission loss of a double-leaf gypsum-board partition system. The results of this test series were quite explicit. As the humidity was increased, a degree of freedom was introduced that allowed for lateral movement of the gypsum core and paper. These two skins were then able to perform independently of one another, causing the eventual shift of control frequency from 2800 to 350 cps—i.e., from coincidence to resonance control. In addition, these test data reveal that, at some frequencies, a difference of as much as 6 dB occurs between the high and low humidity levels, while at others the differential is 0 dB. These differences clearly indicate the need for standardization of the humidity conditions at the present commercial testing laboratories.