The elements will overlap in the crossover region. If the
elements combine at their 6 dB points, the center will
sum to 0dB (a unity class crossover), provided the phase
response is matched. At frequencies above and below
the crossover point, the amplitude and phase responses
will both begin the process of moving apart. The result is
successively lesser amounts of addition, which is exactly
what is required to maintain a constant summed level.
Our design intent is that the remaining addition will fill
in for the individual response rolloffs. Once isolation has
been achieved, the extent of the addition tapers such that
the individual nominal response matches the combined
responses in the crossover region. This can only come
about if the amplitude isolation occurs before the relative
phase response has moved into the cancellation side.
With non-coaxial two-way systems the drivers are
physically displaced in at least (and hopefully at most)
one direction: vertical, or horizontal. The crossover point
can only be perfectly optimized for a single point within
the coverage of the system. The choice of this point, subject
to discussion, is not one to be selected arbitrarily. The
best choice for such point is the one that yields the highest
degree of usable crossover coverage without cancellation.
It is given that any point off-axis to the optimal point will
have less than a perfect phase addition, and that even the