Between 1990 and 1998, the author published five conference papers which described the gradual development of a simple theoretical model for predicting the sound insulation of building partitions. The first aim was to extend Sharp's model for cavity walls to cavities without sound absorption. The second aim was to remove the reported over prediction of Sharp's model for cavity walls. The third aim was to explain the five decibel empirical correction in Sharp's model for stud walls. The fourth aim was to produce a more theoretically valid model than Gu and Wang's steel stud wall model. Although the simple theoretical model has been reasonably successful, several concerns have since arisen. This paper describes how these concerns have been addressed and gives the current version of this theoretical model for predicting sound insulation. The theoretical model is compared with a number of experimental measurements and produces reasonable agreement.