Why's Horn Polarity Reversed in SWR G-II?

From http://www.diyaudioandvideo.com/FAQ/XOver/ -

"The last problem in designing a crossover is Phase Shift. Phase is the timing of a signal, and the shift is the degree of the delay that occurs on the signal when passed through a crossover. Each order of crossover introduces a 90 degree phase shift. A 180 degree shift is an inverse of the wave. If 2 speakers are 180 degrees out of phase then they will cancel each other where ever they produce the same frequencies. Even with crossovers, both speakers will produce sound for several octaves beyond the crossover point. If this problem occurs, there will be a noticable dip in the frequency response at the crossover point. To solve this problem, wire one, but not both, of the speakers backwards (+ to -). Usually, phase shift problems only occur with 2nd order (or 6th order) crossovers, but can also occur when using multiple 2-way crossovers in a 3-way (or more) speaker system. The only way to really find and fix a phase shift problem is trying all possibilities in reversing the speaker leads. If reversing the leads makes the system sound louder, then you know you have fixed the problem."

 

What Greenboy said is correct; the crossover can rotate the phase such that what looks like a reverse polarity hookup is needed to restore proper phase relationships in the crossover region. The drivers' inherent rolloff characteristics also rotate (shift) the phase, and add to whatever effect the crossover has.

Another factor is driver physical offset. Assuming a 2.5 kHz crossover point, if the horn driver's diaphragm is 2.7 inches farther away than the effective acoustic midpoint of the woofer's cone, that's the equivalent of 180 degrees phase shift at that frequency - which may or may not call for reversing the horn's polarity, depending on what else is happening in the system.

Duke