Searching for matching(?) Subwoofer solution

Hello Mike,

     I usually crossover my four Swarm subs at 40 Hz for music and HT but sometimes as high as 50 Hz.  According to your method of 1/2 the length of the crossover frequency, this would mean my subs should be no more than 14 feet apart at 40 Hz and no more than 11 feet at 50 Hz.
    My room is 23'x16'x8' with my main speakers (with bass extension rated at 35 Hz +/- 3db) along the 16' wall and run full-range. Two of my subs are along this 16' front wall with each about 2' away from the nearest corner and about 6' apart from each other.  The other two subs are located, one each, along the left and right 23' walls and each about 3' away from the corners along the rear 16' wall.  This results in my rear subs being about 16'apart from each other and about 20' apart from the nearest sub along the front 16' wall.  So, since some of my subs are more than 14' apart and exceed your rule of thumb, you're suggesting I should have bass phase issues in my room.
     The Swarm amp/control unit does have a continuous phase control adjustment.  I set this by inverting the phase on both of my main speakers and setting the phase control to the position at which the bass sounded the weakest at my listening seat.  I then reattached the speaker wires to in-phase and the bass sounded equally very good throughout my entire room and at the 6 seating positions in my room.  Do you think this is why I don't detect any obvious bass phase sound quality issues in my room?
    Although I've never heard one, I do understand from what I've read and from your comments that subs configured in a line array are capable of even better bass performance than subs configured in a distributed bass array.  However,  my music listening and HT room also doubles as my living room and devoting the entire 16' wall of my living room to a line of subs is not practical. I'm not stating I disagree with the high quality bass capability of a line array bass system.  I'm just stating that I'm willing to sacrifice optimum bass response performance in my room for the extremely good bass response performance of a distributed bass array that blends much more inconspicuously in my living room.  

Tim

Thanks Tim, Subs oriented in a line array are going to project more power because the volume drops off slower as you move away from the speaker than a point source. I can up with this approach because it is the best way to integrate subs with line source loudspeakers. I also cross over to my subs at 125 Hz which may be why Duke and I have somewhat opposing views.
Duke I need you to define what you mean by a minimum phase system.
Yes, I understand if the decay is slower at a certain frequency that frequency will be emphasized. Your ears register everything immediately just like any good microphone. It is your brain that takes time to register just about anything. I think you missed my point about impact. It is the visceral component that adds excitement to the music and why live performances can be so exciting. You not only hear sound but at lower frequencies you feel it. When you confuse phase like that the impact goes missing. When said individual reconfigured his system he described it as having more impact without my prompting him. The result I would have expected. 
I have to admit I also cheat because I am using very elaborate room control. Anyway there is more but my wife is pressing me to go out for a bike ride. Time to go out and assault the motoring public. Back later:)

Mike

Tim, I just read your full post. I will comment on it when I have a chance.

Mike

Ok Tim, first of all this is not my method. It a rule of acoustics and speaker design. I started applying it to subwoofers because I was having difficulty matching subwoofers to my Line source ESLs. So I created a horizontal line source subwoofer array which has the added benefits of being very efficient and greatly reducing reflected sound in the room because of the way a Line Source radiates. 
Now I have no experience doing Swarm systems around point source satellites. But I find it an interesting concept as I too have great performance using four subwoofers. Your numbers are about right. So, if you are crossing at 40 Hz you would want 14 or less feet from one sub to the next. Theoretically the drivers are now acting as one acoustically and are phase coherent. But, then you have to add the room into the equation which I think is Duke's message and I would like to understand what he is trying to describe better. With subs at various distances from walls and each other you have a very random pattern of reflections at different times (phase) and with dimensions in and around the wavelengths of the frequencies you are producing certain frequencies are going to resonate longer. Simplifying the situation, what Duke is saying is that placing subs randomly throughout the room creates a situation that smooths out the frequency response throughout the room. I am trying to understand how that happens. 
I would not say that the line array is better bass in all situations. It is better in my situation because I cross over much higher and I have line source satellites. If I put my subs around the room it would really screw up the image. In your case crossing over at 40 Hz  you can not tell where the bass is coming from. The question I have Tim is if you arrange your subs so that any gap is not longer than 14 feet can you detect any difference in the bass. 40 Hz is way down there. What you might try is playing a 30 Hz test tone. Many test records have test tones. You can even download them digitally. Play the tone and some music with deep bass before and after you rearrange things. It may not make any difference at all. 

Here is Earl Geddes explaining subwoofers
https://mehlau.net/audio/multisub_geddes/
Smoothing the bass frequency response in a room with multiple subs is a simple matter of overlapping nodes. He thinks that three subs is optimal the 4th adding little to the solution. He also wants the subs as far apart as possible. He would cross over somewhere below 80 Hz where he thinks bass becomes non directional. So his approach is radically different than mine. I cross over higher but still below the Schroeder frequency and create a line source. Funny thing is that he greatly favors directional speakers over omni directional speakers because they reduce room interactions in a way superior to room treatments. He does not say that in this paper but relates this strongly in other papers. My array does exactly this. Again, I have a sophisticated room correction system which is to some extent cheating. If I bypass it my speakers sound like crap because of the rising response of the transformers I am using. I can not separate the subs from the satellites in this regard. It is either all on or all off.
This paper was written for lay people so he does not use terms like minimal phase system. He may well use terms such as this in more scientific papers aimed at people who understand what he is talking about. I got as far as the Fourier wave equations and gave up.
So Tim, According to Earl Geddes I am wrong. You want to keep your subs as far apart as possible and throw the fourth one away. It is just taking up space. Or, you could try making a line array using all 4 subs and see what happens. Couldn’t hurt to try:)


Mike