mtmug-
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?
They aren't. The are the same. Your confusion stems from reading guys who sort of understand some of it, just enough to keep everyone else thoroughly confused.
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.
Correct. Same thing.
Think of a 40Hz wave propagating out of a sub. The wavelength at 40Hz is about 28 feet.
https://www.translatorscafe.com/unit-converter/en-US/calculator/sound-frequency-wavelength/ This means your room has to be at least 28 feet in every dimension, or else the wave will encounter a wall and reflect back and cancel itself. Before even one full wave.
Of course no one's room is this big, and 40Hz isn't even the bottom, we want 20Hz, which use the web page, is 56 feet! Nobody but nobody has a room 56 feet on a side with a ceiling 56 feet high! Nobody! So ALL rooms are small, from the perspective of a sub.
Take a room most would consider large, 17x30x9. Pretty good size room, right? Those dimensions in terms of wavelength work out to 66, 37 & 125 Hz, respectively.
You see what happens? No matter where you put a sub the waves are all much larger than the room, and so they wind up reflecting and canceling or reinforcing themselves. You can physically move the sub around. All this will do is move the locations of the modes. You can reverse polarity. All this will do is move the locations of the modes. You can adjust phase. All this will do is move the locations of the modes. Changing location, phase, and polarity are all different sides of the same coin. All any of them are doing is moving around the modes. Same exact problem. Never goes away.
Notice there is zero chance any of this has anything to do with the main speakers. The mains are either too high in frequency to matter, or if they are at the same bass frequency then phase still doesn't matter, because they are just two more sources of the same problem. They still cancel and reinforce because the same wavelength physics applies to them as well.
When people notice profound effects and improved bass by changing phase, it is not because they have matched anything. It is because they have found a setting that is less lumpy at their listening location - for certain frequencies! Same goes for EQ. You can get it pretty good at one spot, but only by making it worse everywhere else.
This is why multiple subs works so well. More subs in more locations results in more modes, more areas of cancellation or reinforcement. Same problem. No getting around the physics. With more subs however then the output of each one can be less. Because each one is less then each mode is less of a lump. Yet add them all together, the result is the volume you want without the lumpiness you do not want.
This distributed bass array concept by the way is one of the great audio developments of the last 50 years. This one simple move - four subs instead of one - enables anyone to have truly awesome bass in any room and for as little as $3k. Amazing. Yet hardly anyone is doing it, mostly as far as I can tell because the concept is hard to understand.
Well, do a search. Look around. The physics is rock solid. The results uniform and unimpeachable. Everyone who does it is blown away how good it works.
