Increasing levels of low-frequency environmental noise have led to an increased need for appropriate mitigation in building construction. Windows are a common pathway for low-frequency sound transmission due to their light weight and low damping. In the work described in this thesis, the use of stiffening elements to decrease low-frequency noise transmission through lightweight panels was explored. The effect of elastic mounting as a possible contributor to low-frequency noise transmission was also considered. A finite element model allowing for both panel stiffening from clamping and the addition of variable-stiffness beams within the panel was constructed. The model allows for variable positioning of such elements as part of a stiffened panel configuration. Several double-panel configurations, representing double-pane windows typically seen in energy-efficient constructions were also tested with various clamped-stiffening elements. The change in panel vibration was also gauged through the use of a Rayleigh-Ritz model that allowed for construction of the modes of vibration of stiffened panels. In addition to the FEM and Rayleigh-Ritz models, experimental sound transmission loss testing of panels with variable edge stiffness was performed to verify the character of the predictions seen in the models. The results show that low-frequency sound below 150 Hz can be reduced by several dB through careful use of stiffening elements, and that resilient mounting can contribute to low-frequency sound transmission through windows.