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Greg Berchin
 
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On 30 May 2005 23:26:36 -0700, wrote:

I don't see how being in phase at all frequencies could eliminate
off-axis anomolies ? Surely going off axis vertically will introduce a
time delay that will cause problems no matter what type of filtering
you use. Or are you only refering to the effects of horizontal off axis
response where the drivers are still equidistant from the listener, but
the narrower dispersion of the larger driver causes an additional
change in the response that causes incomplete summing of the response.


Consider the "ideal" (but impractical) case, in which the woofer driver
and the tweeter driver have identical characteristics. If the crossover
is in-phase at all frequencies, then as the frequency is increased the
signal will gradually be diverted from the woofer to the tweeter. But
the only difference between the woofer signal and the tweeter signal,
regardless of frequency, will be amplitude. There will be off-axis
cancellation (in the vertical direction if the drivers are stacked; in
the horizontal direction if the drivers are side-by-side), but it will
be predictable and stable.

Now consider the same "ideal" drivers, but with a crossover that puts
the drivers, say, 90° out-of-phase at the crossover frequency.
Depending upon the distance between the drivers, at some locus of
horizontal and vertical angles the drivers will be perfectly in-phase,
resulting in an increase in output at that frequency and those
directions. Similarly, at some other locus of horizontal and vertical
angles the drivers will be 180° out-of-phase, resulting in no output at
all at that frequency and those directions. Furthermore, the crossover
that puts the drivers 90° out-of-phase at the crossover frequency likely
also puts them at different relative phase angles at other frequencies,
so the off-axis interference pattern varies not only with direction, but
with frequency.

To make matters even worse, as you mentioned, the off-axis
characteristics of the woofer and tweeter are never the same. So the
"ideal" situation described above, as bad as it is, is never even
achieved because the off-axis driver responses never sum or cancel
perfectly. In one's living room this may be a minor annoyance. In a
theater or an auditorium or a stadium it is a real problem. In
practical terms, the way to minimize (but not eliminate) the problem is
to ensure that the drivers are in-phase at all frequencies.

Greg