Subwoofers and Phase Question For You Sub Experts

I should say image shift for poorly integrated systems

millercarbon:
"Exactly. Two completely different animals. This may be one of the hardest parts of the whole thing to understand. Though granted, the whole DBA approach relies on a number of hard to understand concepts. No one of them really so much hard as different. At least I feel it must be something like that. Else why so many have explained so clearly and yet so few seem to get it?"

Hello millercarbon,

     Besides Duke, I think we're both aware that we are probably the biggest believers and proponents of the 4/5-sub DBA concept on these forums.  There are several other members that are strong believers in the DBA concept but you and I are likely the most prominent and vocal advocates.  I believe this is due to us both doing a lot of research on the sound bass principles underpinning the DBA concept, being relatively early adopters of the concept and having years of practical experience actually using the concept in our own systems/ rooms and knowing first hand how consistently, reliably and exceptionally well the concept performs.
     I think I have a fairly good grasp of most of the dynamics afoot in the behavior of bass soundwaves in any given room that enables the 4-sub DBA concept to deliver near state of the art bass reproduction in any room and seamless bass integration with any pair of main speakers. 
     I understand that bass radiates in a omnidirectional pattern, the length of soundwaves are inversely proportional to their frequency, the length of deep bass soundwaves can often exceed any of a room's dimensions, humans require the entire full cycle soundwave to be present in the room to even perceive a sound, we require 3 complete soundwaves to perceive a change in pitch, we cannot localize bass soundwaves at or below about 80 Hz, soundwaves continue to reflect off of room boundaries (floor, ceiling and walls) until they are absorbed, diffused or run out of energy, the multiple soundwaves reflecting around the room will typically collide with each other causing frequency peaks, dips and nulls.  
     Utilizing multiple subs will actually increase the number of bass peaks, dips and nulls in the room but our amazing brains are able to process these multiple peaks, dips and nulls by summing the bass by frequency and averaging it out and creating a perception that the bass is smoother, faster and more detailed.  As a bonus, this excellent bass response perception is created no matter where in the room one's head is actually positioned. All quite amazing and very beneficial to music lovers desiring excellent bass reproduction throughout their entire room.
     I understand all of the above but I'm still having difficulty recalling exactly why it's not important that the timing of the bass soundwaves reaching our ears matches the timing of the midrange and treble soundwaves reaching our ears.  I know it's unimportant just by listening to my system, bass from 20-40 Hz reproduced by the 4 distributed subs and midrange/treble reproduced by a pair of carefully positioned Magnepan 3.7i main speakers.  The bass deep, powerful, smooth, fast, detailed, dynamic and natural seamlessly integrated with the main speakers.  
    I know there's a logical and reasonable explanation but I can't, for the life of me, recall it.  Hopefully, Duke will chime in again and explain it again. I'll write it down someplace this time if he does.

Tim

Wow this has turned into a great thread. I was a little concerned about it going off the rails early on, but it hasn't.

Thanks to everyone who has chimed in so far and keep the ideas coming.

My take away so far is mostly derived from an earlier post from Audiokinesis:

1 - Get the room as 'tuned' as possible using conventional bass traps and treatments.

2 - Then, utilize one or both of EQ/DSP and DSA to dial-in the best response. As Duke says, smooth bass = fast bass.

Then things get a little more subjective around the following:

1 - Time-alignment is a non issue due to low frequency wavelengths. I can see the physics of this argument yet there are others in the know who disagree. Lyngdorf for example (The TACT spinoff) factor for sub positions and have a delay factor for the mains built into their DSP or 'Room Perfect'. Perhaps that's a novelty or selling point and nothing more. I don't know.

2 - Phase control. Is it an issue or isn't it an issue? In my OP I was asking whether or not DSP factors for phase alignment and the consensus has been that it does not. Some of the more upmarket subs have 360 degree control where others simply have a 0-180 toggle. REL, known for their 2-channel prowess offer neither on their T/7i, which is aimed at the 2-channel market. So what gives? I suppose their answer would be to flip the leads on the amp binding posts. (high level)

My choice of the word 'phase' when describing a specific issue was perhaps misleading. Maybe it's a phase issue maybe it's something else.

Just to describe the problem a little more fully -

I have my mains about 75 inches from the front wall and with the mains only I get an excellent perspective, sense of depth and space, and very focused imaging. By perspective I mean that the soundscape is almost entirely behind the front plane of the speakers and extends through the front wall and beyond the outside boundaries of the speakers. The bass from the Dunlavy, in my room, is poor in two ways. Using a test CD and SPL meter, the extension from the sealed box speakers is good into the 45hz range then rolls off quickly. There are dips around 70-130Hz, again at 150-170Hz. I can clearly hear those on certain tracks.

Adding a pair of subs on the outsides of the mains, about an inch or so forward, helps to flesh out the bottom end and mid-bass, but the subs are obviously affected in the same way as the mains by the room modes and the frequency response anomalies.
So it's still far from ideal, but - the attractive imaging and soundstage properties are maintained. There's no negative effect. There's a positive effect but it isn't 'problem solved'.

So moving the subs and placing them by ear/test to get the best and smoothest response at the listening position I end up with the right channel sub about 2 feet behind the right main and the left channel about 5 feet in front of the left main, both up against side walls, both well away from corners and neither firing directly into a wall!

In this way I get a smoother response all the way through the frequency range. Not perfect but better. However, the depth and width of stage is diminished and I lose the laser-like focus of images in the soundscape. There's almost a smearing affect where the air and space around images diminishes and that's what I'm referring to as a 'phase anomaly'.

I've moved the mains to and fro ad nauseam, as we do. I can get a little better response closer to the front wall but the perspective and depth is diminished and I don't want that.
I've tried different listening seat positions, obviously. 
I've tried sub placement all over the room and messed with phase toggles (0 or 180) and right now I seem to have the best compromise. But I'm not entirely happy with it.

So if the above is something other than a phase issue I'm happy to stick a new label on it and I'm all over any ways to fix it that don't involve bringing in more gear!

Since posting this thread I've written to a couple sub manufacturers to see if they'd come up with some comments and perhaps even loan out a quad of subs so I can set up the DBA and write the process up as a review. I doubt that will go anywhere but plan B is to go out and buy two or four subs on the used market.
All of your comments are helping and my trigger finger is twitching, but I'm not quite ready to buy more subs just yet.

Used a Lyngrof DPA-1 to delay my main speakers as suggested by JL Audio 10 years ago.
It did work and I listened for several weeks after which I went back to my original set up and removed the Lyngrof processor which yielded time delay, room correction and preamp.
I found the music in my system was artificial enough that it bored me, the excitement and physical sensation was not truly present.
Perhaps after 10 years technology has leaped forward and I am willing try some other devices as suggested by JL Audio eg DEQX.

Rooze, way off topic questions.
What model phono cartridge are you using and what did the Zesto Audio Andros bring to your setup?

m