The SEA equations involve various assumptions and approximations, one of which is that the various subsystems are weakly coupled. This paper considers what is meant by strong coupling and how it can be quantified in terms of the modes of the system. The modes of the system as a whole are used to determine the subsystem energies and hence, when averaged over a suitably wide frequency band, the coupling loss factors. These are seen to depend on the modal overlap M: for low M (strong coupling) they are proportional to the damping loss factors while for large M (weak coupling) they are independent of damping. There are non-zero indirect coupling loss factors except under two circumstances: if all the modes of the system are local, and in the high M limit of weak coupling. A measure of the coupling strength is found in terms of the statistics of the mode shapes of the system. A numerical example is given.
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