The estimate of the resonant sound reduction index has received attention over the years as the prediction method described in the standard, ISO 15712 has been applied to lightweight building constructions. A method of estimating the resonant sound reduction index involves the measurement of the total and the resonant radiation efficiencies of the building elements involved in the first order flanking paths. The radiation efficiencies of different lightweight wall constructions were evaluated as part of a study conducted at the National Research Council Canada are presented. The study focused on the measurement of the radiation efficiencies with the aim of developing guidelines for the measurements. Predicted values of the flanking transmission loss for each flanking path are compared to data which was measured in the National Research Council's eight room flanking facility.