Eighth-space radiation is defined as having directivity factor of 8, which corresponds to directivity index of 9dB. To me, it's a no-brainer that the room's walls confine sound within them. One can quantify this by describing the radiating angle. There are other things that occur in an enclosed room also, but we're talking specifically about directivity here. I don't see any reason why you would disagree that a room corner defines its maximum radiating angle, and that bass frequencies are therefore limited by this radiating angle.