Hi RMC,
As I have shown on the figure
http://www.audioheritage.org/vbullet...1&d=1480884646
re-scaling the frequency axis relative to the distance (here mentioned to 3m, e.g. by reducing the frequency 6 times) it can be seen that such effect called "mutual coupling" would occur for the frequencies LESS then about 50Hz, as in stereo sound reproduction LF speakers are producing almost the same sound program (especially "under 300Hz"). So that is real physics law of the sound. But such effect is "masked by" the sound reflections from the nearby large surfaces where "local" mutual coupling would produce +6dB up to +18dB, but a lot of my words have been 'said' about that in the previous posts here.
regards
ivica