Quote Originally Posted by Pharos View Post
"When the bass energy from the driver units reaches the room boundaries, then a proportion of that energy is absorbed by that surface. This is turn is transmitted outside of the room. The walls effectively become "sound panels""

That is entirely consistent with my thinking that the sound is transmitted via the substrate rather than through air to another flat.
Sound vibrates the molecules in the wall the same as it des the molecules in the air - is my understanding anyway. So airborne and structure borne transmission are essentially the same thing.