Statistical energy analysis is a technique ideally suited for the study of sound and vibration transmission through complex structures, but is unreliable at low frequencies due to the statistical uncertainties that occur when there are few resonant modes in each of the elements or subsystems. Experimental work has shown that this uncertainty is related to the mobility of the receiving subsystem, which enabled semi-empirical expressions to be derived for the upper and lower limits to the coupling loss factor. Finite element models do not suffer from these limitations at low frequencies and can be used to model such systems "exactly". A finite element model was therefore used to carry out "numerical experiments" to show in more detail the relationship between the properties of subsystems and the coupling between them. It is found that the semi-empirical expressions previously developed give very good agreement with the numerical results of the finite element model. © 1994 Academic Press. All rights reserved.