Critical subwoofer tip

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.

I assume that everyone already knows the importance of phase matching a sub to the main speakers 

Um, no. Not at all. Physical impossibility. Neural impossibility too. Absolutely positively zero chance this is important at all. None. 

Flies in the face of physics. Sound travels at a rate of about one foot per millisecond. The wavelength of sound varies tremendously by frequency, from as short as an inch at very high frequencies to 50 feet or more for low bass. Sound travels in waves. Waves are reflected, refracted (bent, diffused), or absorbed according to their wavelength and the size, shape and composition of whatever they encounter.  

Got it? Okay, so we play music all these waves start bouncing around the room. Forgot to mention, when waves meet they either cancel out or reinforce each other. Very important. Sounds silly basic but crucially important you understand these points.  

So a sub, it puts out 80Hz and below. Waves 20ft, 30 feet and longer. Most of these waves are longer than the biggest dimension of your room. What this means in practice is a low bass wave emanates from the sub, encounters a wall, and is reflected right back to the sub, all of which happens before even one wave is complete! The sub is canceling- or reinforcing!-  its own output!

This never happens with midrange and treble frequencies by the way. Unless your room is really tiny, like closet size, then you have other problems but not the cancellation/reinforcement problem.  

So you see it cannot possibly matter where a sub goes, at least not in terms of what you think, phase matching. No such thing. Impossible. You can see that now, right?

Next problem, neural or psycho-acoustics. Very low frequencies do not register as sound at all at anything less than a full wave. At 20 Hz that means 1/20th of a second to hear that frequency at all. We know a 20Hz wave is more than 40 feet long. You can look it up, I'm getting a little bored explaining all this for the umpteenth time. Point is, it cannot possibly matter where the sub goes for the simple reason your ears cannot even register its output fast enough to locate where it came from.

Even if you don't buy the science, which a lot don't. Most people talk about how they respect science, when really they go with whatever the most people are saying. In other words not thinking at all but following. This information will do you no good whatsoever if you are a herd animal, it is only useful for thinking beings.  

All thinking beings know from experience you cannot tell where the sub is located. If you somehow screwed this up come on over, happy to demonstrate. Everyone is shocked to look around and see FIVE subs randomly spread around the room, all pointing away from the center, all within inches of the wall. No idea whatsoever that any bass at all is coming from them.  

If phase matching mattered at all the bass in my system would be horribly chaotic. When in fact it is gloriously clean, articulate, and absolutely beautifully integrated with the midrange and treble.  

But only because I, unlike "everyone", understand the unimportance of phase matching subs.