This paper describes a study of the development of a computer model based on the impedance theory for calculating the sound transmission loss (TL) of single and multiple elements. The impedance model developed by Au and Byrne, validated by laboratory measurements by using the intensity technique, allows predictions for elements consisting of various layer materials with different order, such as solid and solid-damped materials, porous material and air between the layers. The model including a complex calculation procedure was improved by developing a simple algorithm for building elements and was applied to various layered-structures to investigate the effects of some physical parameters on TL and R-w values. During the implementation of the model, the max.angle of incidence which is inversely related to TL at mid and high frequencies was found to be the most sensitive parameter in the calculations and its effect is considerably high for the multilayered structures. The flow resistivity R of porous layers has also an increasing effect on TL at high frequencies; about 4 dB per doubling of R. The calculated results agreed perfectly with the data of the original work, as well as of the other investigations published so far. The compatibility of the calculated results with those measured in the laboratory by using the two-reverberation chambers method, was also investigated in the study. The comparisons displayed satisfactory agreement for single elements, however revealed some discrepancies for multi-layered structures, due to the finite-size and the radiation efficiency. Variation of the discrepancies at third octave bands as well as in terms of insulation ratings were statistically analysed and the model was modified so as to give also the TL values of finite-size multilayered structures.