Making the horn walls move would be a very difficult objective to achieve if we were limited to the 'off the shelf' driver technologies. A very different type of loudspeaker diaphragm would be required in order to meet the 'normal' loudspeaker requirements and behave as a horn boundary.
Employing a rigid thermo-formed closed cell foam as the loudspeaker diaphragm enables the the shape to be precisely defined. The shape is directly inherited from the calculated horn surface permitting the HF section of the horn to operate very close to calculated optimum.
The mid frequencies are free from compression and more importantly performance has not been sacrificed to achieve the symmetrical closely spaced layout of the two frequency sections.