Abstract: In coupled rooms,besides the common exponential decay,two other different decay styles can be observed sometimes.One decays fast in the beginning and slow down in the late part,while the other in the opposite manner. Each type of decay style occupies a region in coupled rooms.The popular opinion is that,two different styles of sound field are divided by the physical boundary such as the partition and coupling aperture between sub-rooms.By conducting numerical simulation and experiments,the divergence between this physical boundary and that of the sound field was found.The boundary which divides the sound field of different decay styles was provided in this article,and it is not located at the partition or coupling aperture,but infiltrate to the more ‘live’ sub-room.The degree of the infiltration depends on the dispersion of natural reverberation between the two sub-rooms,the large it is the more the infiltration appears.