Reducing radiation power from a noise source can be done by changing radiation impedance of the noise source. One possible way to change the impedance is attaching a structure around the source. In that case structural-acoustic coupling mechanism occurs. Actually many studies on structural-acoustic coupling have been done to find out the effect of a structure on acoustic fields or to estimate noise transmission through a structure[1-2]. Recently, Kim and Brennan studied the coupling problem by using modal impedance. However, the studies were restricted to the interaction only between a structure and the acoustic fields which are located one or either side of the structure. In this paper we investigate the system which represents a more general structural-acoustic coupling system: a finite cavity contacting both a structure and an infinite exterior field. The results demonstrate not only direct interaction between a finite interior field(cavity) and a semi-infinite acoustic field but also the structural-acoustic interaction among them. The coupling phenomena have been attempted to be analyzed by using total surface impedance which is directly related to the radiation efficiency. We found that this measure provides easier way to understand structural-acoustic coupling systems than other conventional measures of impedance.