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I wanted to see if I could get some response from Home Theater Shack forum readers about considering the passive crossover phase characteristics of main speakers(FL,C,FR,RR,LR) when calculating and making subwoofer phase adjustments since most powered subwoofers will use active crossovers and contain no passive circuit components. Thus, we assume the subwoofer to be at zero degrees and in phase using 24db/octave L/R crossover at 80 Hz. This is never true in real life but will be necessary as a starting point when making phase predictions, simulations, and adjustments for this discussion. By combining the phase of the main speakers passive crossover, (90 degrees caused by the first order X-Over in this example), and the natural phase differences between the main speakers and the subwoofer (45 degrees for example) this would give a total difference of 135 degrees. Therefore, setting the subwoofer phase adjustment to 135 would bring everything into proper phase alignment in this case. I expect this would be a fairly good starting place for most systems since most main systems will use a first order crossover. More advanced main speakers would likely use 2nd, 3rd, or, 4th order crossovers.
Depending on the natural phase characteristics of the main speaker’s passive crossover it may be good to include this when calculating/making the home theater subwoofer phase adjustments: for example, a First Order, or, 6db per octave slope on the woofer section of the main speakers will induce 90 degrees of phase shift automatically putting it 90 degrees out of phase with the subwoofer which as previously stated has no passive components to induce phase differences. The phase shift caused by a Second Order Linkwitz/Riley 12db per octave Xover on the low pass woofer section would be 180 degrees which would be reverse the phase of the subwoofer. This would indicate that the natural phase differences between the main speakers and the subwoofer would have to be added to the 180 degrees which is beyond the phase correction abilities of most sub amps. It may be possible to add the additional phase difference needed using the subwoofer distance setting on your AVR. By increasing the distance setting for the subwoofer from 10 feet to 13 feet would compensate for the phase difference of around 45??? degrees. A full discussion on how to make the distance settings on your AVR can be found at RhythmikAudio.com
 
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