An extensive series of measurements has been made to obtain a systematic evaluation of the effects of glass thickness and interpane spacing on the sound transmission loss of windows. These included 93 laboratory measurements conforming to ASTM standard E90 and field measurements on a subset of 18 of these window types. The samples studied included glass thicknesses from 3 to 6 mm and interpane separations from 6 to 150 mm. In addition, the effects of absorptive treatment, cavity dividers, and nonparallel glass surfaces were investigated. The data contradict one widely held belief about window design.