Distributed Bass Array configuration


Please, I don't want to debate the merits of the DBA nor of those who espouse it. I am considering adding two more subwoofers to a system that has two already. To those who use a DBA,I am interested in how you have configured them, specifically--
  1. Do you run in mono, or do you split the array to run in stereo?
  2. What is your approach to setting phase (delay) among speakers that may be facing different directions and are different distances from the listener?
Thanks!
mike_in_nc
     The following quote was copied from a Stereo Times interview with Duke Lejeune that took place many years ago, There was no date listed on the article that I could find.  So, I'm not certain of the exact date but I believe it was published around the year 2000 give or take a few years.
    I thought the entire interview might be of interest to members and this is a link to the complete interview:

 http://www.stereotimes.com/comm081710.shtml

     
    The interview begins with a fairly lengthy portion about his regular speaker designs.  The copied portion below is a portion near the end of the interview, which I thought was more directly relevant to this thread since it is the portion at which "LB", Stereo Times interviewer Larry Borden, first asks Duke, "DL", about his "SWARM" 4-sub distributed bass array system:

"LB: Let’s turn now to bass. What is “SWARM,” and how does it work?
DL: The concept behind the Swarm arises from a brief conversation with Earl Geddes, as I was driving him to the airport after CES in 2006. I had been trying for years to come up with a subwoofer system that would match up well with Magneplanars and Quads, trying various enclosure types in a quest for very good “pitch definition” in the bass region, along with good impact (good planars excel at the former but not the latter,). Anyway, Earl mentioned that scattering multiple subs asymmetrically around the room resulted in a net smoothing of the in-room bass, as each sub would interact with the room differently so that the sum would be smoother than any one alone. The lightbulb went off in my head, and I asked him for permission to use the idea. He said yes. By the time we got to the airport, I was already designing the Swarm in my head. Let me digress for a minute into acoustics and psychoacoustics. The ear/brain system tends to smooth out peaks and dips that are fairly close to one another, but if they are more than 1/3 octave or so apart then the peaks and dips are usually audible. Now we get room-induced peaks and dips all up and down the spectrum, but only in the bass region are they typically far enough apart (due to the wavelengths involved) that the ear cannot smooth them out. So in the bass region the problem is not too many peaks and dips – the problem is that they are too few and far between! Another factor is that it takes the ear a fair amount of time to hear bass frequencies. The ear cannot even detect the presence of bass energy from less than one full cycle, and it takes several cycles to detect the pitch. So considering the wavelengths and room dimensions, by the time we can hear bass tones the room’s effect is in full swing. Perceptually, in our home listening rooms there is no such thing as “direct sound” in the bass region; by the time we even begin to hear it, it’s all reverberant sound.
The Swarm consists of four fairly small subs and a single kilowatt shelf-mounted external amplifier. The subs are “voiced” to have a gentle rolloff over most of the bass region that is the approximate inverse of anticipated room gain (the vented version comes closer to this ideal than the low-Q sealed version does). The amplifier has a steep 24 dB per octave lowpass filter so that the subs can be scattered without betraying their locations by leaking lower midrange energy, along with a single band of parametric EQ in case the scattering alone doesn’t do the trick. To the best of my knowledge, none of my customers are using the parametric EQ because the in-room bass is sufficiently smoothed as it is. Not only does the Swarm result in a smoothing of the in-room peaks and dips, but the peaks and dips that remain are more numerous and closer together, so that the ear’s smoothing mechanism can be effective. Now at first glance it might seem that multiple bass sources with multiple arrival times results in loss of impact and/or mud, but that is not the case in practice. As explained above, we cannot hear the bass wavefront before the room has its say. The ear responds primarily to frequency response (rather than to time-domain behavior) in the bass region, so when we smooth the frequency response we are solving the biggest problem. Because the low fundamentals and first few overtones are present in proper proportion, the pitch definition is very good. The argument for a single large equalized sub can of course be made, and there are some exceptional examples on the market, but equalization is a local rather than a global solution. In other words, the room-induced peak-and-dip pattern varies so much from one location to another within a room that fixing the frequency response at one location via equalization will almost inevitably make it worse elsewhere. In contrast, the multi-sub approach smoothes the bass throughout the room (decreases the spatial variance), ironically making equalization (if needed) even more effective."

     I thought his statement, "So considering the wavelengths and room dimensions, by the time we can hear bass tones the room’s effect is in full swing. Perceptually, in our home listening rooms there is no such thing as “direct sound” in the bass region; by the time we even begin to hear it, it’s all reverberant sound.", was especially relevant to this thread discussion.       He's basically pointing out that the very long and omnidirectional bass sound waves behave very differently in  domestic sized rooms than the much shorter and highly directional midrange and treble soundwaves.           Which means that sound wave time coherence, the timing of sound waves actually reaching a listener's ears, is very important, necessary and doable for the shorter midrange and treble frequency sound waves for good stereo imaging.  But sound wave time coherence is not very important, doable or even necessary for the much longer bass frequency sound waves since there is no such thing as "direct sound" in the bass region, it's all reverberant, or reflected, sound.  The fact that humans are poor at locating the source of sounds below 80 Hz and progressively better at locating the source of sounds above about 80 Hz, the fact that we require the detection of a long and full cycle bass sound wave to even perceive bass sound present in the room and several full cycle bass sound waves in succession to perceive pitch, also distinguishes the differences in bass versus mid/treble sound tone perceptions in our rooms.  
     Another interesting fact about bass sounds is that, perceptually, we're much better and more naturally able to perceive them outside or in a very large concert hall because the very long, full cycle bass sound waves, due to the much larger space, have  the capability to arrive at our ears as "direct sound" rather than arrive as reverberant sound that has reflected off one or more room boundaries on its path.

Just some food for thought,
               Tim
That is a great quote and relevant Tim, and thanks for saving me the time of reading the whole interview. Your last comment is one of the clues that helped convince me DBA was the way to go.   
The only times we experience truly low bass in normal life is in very large to wide open spaces. For me it was always one of the coolest parts of going to a rock concert, that vast space feeling of bottomless bass. This was always missing, particularly in orchestral works, because even if it is on the recording the system simply cannot reproduce it at home. Even if the system response measurements look good still there is always the room that ruins this.   

I'm sure you know where I'm going with this. One of the more surprising things with a DBA is the enhanced feeling of envelopment. People have commented on it with my system. It is very deceptive. First, because no sound whatsoever appears to be coming from the subs at all. But also because the low bass that triggers this feeling isn't even heard or sensed as bass at all. It has no pitch, nor location. It has no attack or slam, no harmonic, no tone at all. Yet somehow it creates this ineffable feeling of being in a huge space.  

Nothing else does this, at least not to this extent, and I have heard some pretty awesome gear in some damn fine rooms. Yours as I recall are one per corner. Another one has this plus one way up high near the ceiling. Mine are along 3 of the 4 walls. It seems to work well almost regardless of how the subs are laid out.

Please correct me if any of my comments here are incorrect as I don't claim to have any expertise. I do believe you are misinterpreting the  "term" - "stereo subwoofers". It is a "term" not a sonic phenomena or an implementation in any recording process . I do believe it was originally  meant as the description of using 2 subs on receiver/pre amp as 2 separate channels so not to not to be confused with 2 sub outputs running parallel. Although this is a 2 channel discussion listening subject  I do believe the whole concept began with surround sound and then was deemed profitable for 2 channel listening. So, when a surround sound mix is configured for a movie the "LFE" channel is single and mono ( as is every channel / speaker ), it does not have a left sub mix and a right sub mix, much less a 4 sub mix, it is a single signal. Then the receiver through it's processor separates it to a left channel and a right channel so the mic calibration processing then allows for different volume levels to be "matched" at the listener position. Now recent implementations of Audyssey and definitely Dirac LIve may incorporate separate phase shift and delay but it all comes from a single mono signal being manipulated to create a "means to an end".
Again correct me but isn't "stereo" in itself just taking a recording from multiple "mono" microphones and then an engineer manipulating all the separate mic channels into a "stereo mix" in an effort to create a "perceivable" soundstage again as a means to an end ? I think we should use the term multiple.
I listen to Paul McGowen on Youtube ( maybe the meaning of stereo subs is a good question to send in ) and consider him quite informative but don't take all he says as undeniable truth. Actually, I believe he has mentioned that the owner of Audioquest is a friend of his and I heard that guy the other day on YouTube say that power regenerators are useless ( and a great amount of Paul's expertise and companies monetary value is based on that very component ) so I guess even his friend and also an "expert" doesn't agree with him. I might mention that his company is often criticized as not legit in the premises that their products are based on . So who are the experts? 
But let me conclude in complete agreeance that we can't discern the location of the lowest frequencies, it's just a fact. Our ears/brain don't work that way for a reason. In real world circumstances there is no practical use for "hearing" those frequencies. One main component of hearing and discerning placement is to realize where danger may be coming from and if your in danger from something creating sub harmonic frequencies it's probably an earthquake and you ain't getting away from that.
My B&W older 803's, as well as my stacked DM24's sat on top of my stereo subs.  I measured the woofers on all speakers and phase aligned them.  I know that it is taught that sub-wooferage is non-directional, but tell that to my ex-laundry room bass trap, electronics room when the door is left open.
Any Sub Woofer set @ 80 HZ Crossover will interact with every Good Hi Fi Speaker's Bass Output.  This should be avoided.

Setting @ 60 Hz or so should minimize interference & blend in better.

With Sub Bass, a Crossover of 40 Hz or so would be appropriate.