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Critical subwoofer tip

I assume that everyone already knows the importance of phase matching a sub to the main speakers but it’s a little more complicated than simple 90 degrees or 180. The B&W sub that I have has four choices. In every case there has been a definite correct position that can be non standard. My current setup shined at 270 degrees vs the std positions. It’s completely obvious and the other choices would not have been satisfying. 
From my lengthy experience I would want a subwoofer with several phase choices. I personally don’t see how one could seamlessly integrate the mains and the sub without this flexibility. No one asked but i thought this info might be useful to anyone purchasing a subwoofer. YMMV
4425

Showing 2 responses by mtmug

mtmug
14 posts
 
I think it is safe to say that I am one of the confused ones. If we ignore changes in amplitude over time (impulses) and just stick with a sustained tone at a given frequency, how are two wave fronts 180 degrees out of phase not the same as two with the polarities reversed?
If I had a sub with speaker level inputs and no phase control, I would expect swapping the speaker leads to produce the same effect as flipping the phase 180 degrees.
BTW, I am enjoying these posts and appreciate the OP for starting it and everyone who has contributed. It has caused me to think more about things I "knew", or should have known, and how poorly I have considered some of those things in practice.
mtmug
14 posts
 
That is achieved by time and phase alignment of the sub, LF, mid and HF drivers. Full Stop.

@ieales Yours is not the first mention in the discussion referring to the importance of time alignment. I don’t think I understand this in the context of low frequency behavior in a closed room.

For example, if my listening position is 10ft away from my main speakers, all frequencies above about 113Hz are arriving at the listening position from the speakers directly because all the frequencies at this level and higher have wavelengths of less than 10 feet. In contrast, frequencies below this threshold will require more than 10 ft to reach full amplitude because their wavelengths are longer than this. This means, to hear these lower frequencies, they have to bounce off at least one or more room boundaries. This would seem to indicate that all these lower frequencies arrive at the listening position later than the higher ones. In other words the higher frequencies would arrive at my listening position after traveling 10 ft, the 30 Hz frequency had to travel at least 37.5 ft, the 40 Hz traveled over 28 ft., etc. Since all the frequencies travel at about 1,125 ft/sec, the lower, reflected frequencies are all going to lag behind direct ones from the mains.

There was some suggestion that a delay could be applied to the subwoofer but it would seem for exact time alignment, you would have to apply a variable delay to all frequencies except for the lowest the system can produce to have them all arrive at the listening position at the same time. This seems pretty involved to me. Or have I missed the whole point again?


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