Thanks to you both for your additional, certainly most interesting, comments.
I have had another look at the Geddes material on bass, "Why Multiple Subs?" and "Optimal Bass Playback in Small Rooms", and I cannot find any reference to dipole (open baffle) subwoofers. It was a rewarding refresher for me, though.
The idea, that monopole subs are best suited to multiple deployment and dipole subs are best suited to stereo deployment, is intriguing.
What I am keen to see, though, is if anyone has done some modelling of dipole subs, so we can see the effect as multiple units are deployed in various locations.
Measurements from someone using dipole subs --multiple dipole subs -- would be fascinating and very helpful too.
I am mainly keen to find the best way to arrange multiple dipole subs. Discussion of which is the better type of subwoofer was not my intention.
thanks again
Arg,
My colleague has GR Research NX Otica's along with a pair of GR/Rythmik Triple 12 inch OB subwoofers, placed as recommended here in a stereo setup in a 15 foot wide and over 25 foot deep room with tall ceilings as well as a good number of treatments. We performed NUMEROUS measurements. The FR was choppy below 300Hz to say the least. No amount of changes to the DSP (i.e. EQ, gain, slope filters, etc...) helped to completely address the issue. The addition of (2) monopole sealed subwoofers very much flattened the frequency response curve. This was something he did on his own after he listened to my setup. Sadly I do not have any of those measurements, as I was just assisting.
Prior to my multiple monopole setup I also experimented with a pair of H-frame dual 12 inch OB subwoofers from GR Research that I had picked up used. Of course it sounded good and detailed but the measurements in the frequency response domain was also choppy (though the waterfall measurements were very clean and quick). It was most certainly
subjectively better than any SINGLE boxed sub I had used. Although I thought of adding additional GR Research/Rythmik OB subs
I went through the same question/answer session that you did. That was when I had discovered some of the reasons for why OB was sounding cleaner. They are directional and as such try to engage less modes, and this was evident in the waterfall measurements with their quicker decay times (less than 450msec in the region from 40Hz to 300Hz). I also found that it became inefficient to be using MORE OB subwoofers than just a stereo pair because at that point you just entered the multisub paradigm. Boxed subs have 360 degree radiation patterns which when used in multiples will engage MORE of the room modes and as long as they are not colocated (they should be distributed around the room), they will render a flatter response along with better/quicker waterfalls as well - you don't have that advantage with OB subs due to their directional radiation patterns. This was why I didn't add additional OB subwoofers. The other advantage I found was that multiple monopole subs were more dynamic at higher SPL's and sounded even cleaner. So as to not show favoritism, I stuck with 12 inch Rythmik drivers in sealed boxes for all my experiments.
The research that Geddes had done with OB subs is not available online. It was part of his thesis which he may have copies of. So you will have to contact him directly to be honest. He mentions it here and there in some whitepapers and on forums but you will have to dig for that information I suspect.
BTW, my previous post is credited to my readings of articles and comments (it wasn't to sound interesting as you said!) published by Earl Geddes, Floyd Toole, Sean Olive, Matthew Poes, David Griesinger, etc...
https://www.audioholics.com/room-acoustics/stereo-bassI think what you will find is that most people haven't tried multiple OB subs primarily because it is 1) Expensive since you can't just use 1 driver in each OB sub, the EQ requirements are too high, you must have at least 2 drivers per OB 'sub' module or the driver itself has to have a large piston area and large Xmax or both, and 2) There are space constraints with each OB sub, i.e. they must be "X" feet away from a wall boundary, so suddenly you have the OB subs sitting on your lap (!) and 3) If you are going to try multiple OB subs, it seems so wasteful when the advantages in engaging more room modes in an efficient manner to render a cleaner and flatter response may not be there due to narrower radiation patterns.
But if you do build multiple OB subs, please report back! And please ask your question on diyaudio specifically to see if anyone there has implemented and measured multiples of OB subs in a room.
Best,
Anand.