Crosstalk-cancelled playback yields externalized sound only in the frontal hemisphere

Evidence

A nice feature of this technique is that the sound is typically externalized. This may be because head movements of the listener produce somewhat relevant cues, and because the sound is reproduced using far-field loudspeakers generating correct monaural directional spectral cues. However, although the sound is externalized, it is hard to reproduce the virtual sources in all directions with this technique. With a stereo dipole in the front, the reproduced sound scene is typically perceived only in the frontal hemisphere. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1946

Categorization

• QUESTION: Why does a front stereo dipole only render the frontal hemisphere of the scene — is this a hard physical constraint of the two-loudspeaker geometry?