The most common mistake in this matter is the tendency to
optimize the system for on-axis to the HF horn. The roots for this practice are understandable since the HF horn provides the principal source of angular control for the system.
The vulnerability of this approach can be found by returning to the triangulation concepts discussed earlier.
Alignment to the horn axis introduces the right triangle
summation response and therefore cannot hold its synchronicity
over distance. The phase optimization can only
work at one distance. At distances closer than this point the
HF driver leads and at points behind, the LF driver leads.
Such approach also results in an asymmetrical response
at angles off the center line of the horn. This is due to the
different rates of change over angle. In one direction you
move away from both drivers, in the other you move first
toward the LF and then away from both.