Dealing with the coupling problem where structural vibration mutually interacts with acoustic fields, we have to solve them simultaneously. This fact makes us difficult to solve coupling problems. Many researchers have analyzed such coupling problems especially ones between a structure and either a finite or infinite size of an acoustic field. Recently a general coupling problem, which deals with a structure which interacts both finite and infinite acoustic fields have been studied experimentally and analytically. However, it is needed to introduce some measures which are easily understandable for the explanation of coupling mechanism. For this reason, we attempt to use of the concept of modal impedance. As an example of a coupled system, a partially open two-dimensional membrane-cavity system was used. As a result we found that impedance is highly dependent on the system characteristics.