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4xIB318" vs two ported monsters

post #1 of 94
4/2/12 at 8:28am
Thread Starter 
Hey All,

I started a thread in the speaker section to discuss the new system I am putting together. The cliffs note on that thread are that ED is building me lcr, surround, rear and height speakers using variations of their ED(c)12 upgraded cinema line. The mains will have dual 12" drivrs, all other will have single 12s. This is for an aggressive family room/multi purpose room.

I initially thought I would go with either dual cpativators or A7-450 from ED, but I am reconsidering the idea of an IB setup. I can place the mouths of 2-4 manifolds at the front of the room in the floor. I have about 1500cu ft to work with for rear wav area and it will be sealed off from anywhere else in the house, so no concern for back wave effects. Based on these measurements, between 4 and 18 18" Fi IB subs is easily doable. I would place the manifolds below the floor inside and/or outisde each main. Due to the low cost of power, I could accomidate whatever power requirements come with the setup(assuming about 500watts per sub is more than enough).

My concerns are as follows:

1. If this placement turns out to be problematic, I cannot just seal up the holes and try another location. The home is new construction and I don't want to hack into it more than once. The new construction aspect also limits the pre build testing and measuring abilities, so I would kind of have to pick a spot and hopes it is optimum.

2. The claims from many that IB setups don't "kick you in the chest" the same way. I have read enough to see that most people that have rsearched this fact attribute that to a flatter response..is that the case?? The only IB I have had was a single 12" cerwin vega in my car and though I liked it, it was 15 years ago, so I can't remember much else about it.

My goal is to have low extension without much compromise to the overall output and "thump" in the chest feeling. I am also waiting on Brett from ED to give me a little more info on otimal xovr points for the mains.

Thanks in advance. Thoughts??
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post #2 of 94
4/2/12 at 9:41am
Heard an IB with four nice 18"s. The cut-out was in the floor directly between the mains a foot or two towards the LP. It was quite smooth and had very nice extension. It did have a hole around 50Hz that the owner had a tough time eliminating. All in all it was a great and unique sound. Biggest advantage was no huge enclosures filling up the room.
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post #3 of 94
4/2/12 at 10:22am
There are different ways to achieve what you want.

An IB is a superb way to achieve bottom octave extension, with low power/low compression, with a nice subjective sense of bass quickness or "speed". Due to four 18's covering the duties, the driver movement is minimized to a significant degree, thus minimizing overshoot and other non-linearities and distortions that may accompany a small diameter sealed approach, pushed to it's excursion limits.

Excursion headroom, and low power levels,....these are significant benefits of the IB approach.

The subjective kick in the chest feeling, can be related to many things, chiefly a large amount of SPL@a given chest resonance. Oftentimes this is elicited from highly capable and well optimized mains, that are ideally mated to a subwoofer system of any type. Phase coherence/integrity in the octave spanning 60-180hz (both sub and mains) will contribute strongly to this "punch" effect. Additionally, and perhaps most importantly, ample bass trapping, and a well optimized time domain absent of significant ringing contributes highly to a clean punchy response, with a high degree of perceived "speed", due to cleaning up the room overhang energy.

------

*Four IB3-18's, w/an EP4000 (total around $1200-$1300) will yield ~125db@1m@20hz.

That's with only about 200 watts per driver, so compression effects are virtually non-existent.

-----

Yes, you should attempt to assure no big issues with your IB placement, however if your location isn't perfect you do have some tools at your disposal. I just posted such a thing a short time ago at The Cult;

"However being as limited to manifold location options, I'm approaching it with my eye on the end goal. The primary IB manifold location, is but one step in the entire process. Mitigating potential response issues with another manifold, and/or small sealed balancing subs, all combined with significant bass trapping etc., will be my approach.

I'm so limited with attic height, my manifold(s) must reside along a general line running front to back through the room. Knowing the need to psycho-acoustically link the upper octaves of a subwoofer system's coverage to the bottom octaves of the mains coverage, manifold placement and proximity to the mains is key. All said, everything points to a manifold in the display/mains end of the space,...front left corner, regardless what issues arise. Clearly, a multiple location subwoofer system solves many issues encountered in small room acoustics. So whether I build a four 18" driver manifold, a mammoth 8 driver IB, or two 4 driver IB's, all indications point to my room needing additional subs, perhaps small sealed, to further smooth the response and lower distortion at any given operating level. So build the front left manifold, measure the resultant outcome, and proceed from there with filling in the nulls, and smoothing the peaks with the tools and approach I listed above."


Good luck
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post #4 of 94
4/2/12 at 11:35am
Thread Starter 
Thanks FOH...how did you come up with those output results??

Also, If one ep on 4 drivers yields 125db, then two ep's would yield 128, two eps and 8 drivers would yield 131db and four eps with 8 drivers would yield 134db correct?? I used the rule, which I think I am correct about, that each doubling of drivers or power will result in a 3db gain.
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post #5 of 94
4/2/12 at 11:44am
Quote:
Originally Posted by ChopShop1 View Post

Thanks FOH...how did you come up with those output results??

Also, If one ep on 4 drivers yields 125db, then two ep's would yield 128, two eps and 8 drivers would yield 131db and four eps with 8 drivers would yield 134db correct?? I used the rule, which I think I am correct about, that each doubling of drivers or power will result in a 3db gain.


That only works if the original 125 number WASNT at the excursion limits. It probably was or he would have upped the power himself and given you that number.

Just sayin'
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post #6 of 94
4/2/12 at 12:17pm
Thread Starter 
Quote:
Originally Posted by MoFinWiley View Post

That only works if the original 125 number WASNT at the excursion limits. It probably was or he would have upped the power himself and given you that number.

Just sayin'

Then basically what I would get is 3db by doubling drivers and then another three by doubling power once there are eight drivers instead of 4??
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post #7 of 94
4/2/12 at 12:32pm
Quote:
Originally Posted by ChopShop1 View Post

Hey All,

I started a thread in the speaker section to discuss the new system I am putting together. The cliffs note on that thread are that ED is building me lcr, surround, rear and height speakers using variations of their ED(c)12 upgraded cinema line. The mains will have dual 12" drivrs, all other will have single 12s. This is for an aggressive family room/multi purpose room.

I initially thought I would go with either dual cpativators or A7-450 from ED, but I am reconsidering the idea of an IB setup. I can place the mouths of 2-4 manifolds at the front of the room in the floor. I have about 1500cu ft to work with for rear wav area and it will be sealed off from anywhere else in the house, so no concern for back wave effects. Based on these measurements, between 4 and 18 18" Fi IB subs is easily doable. I would place the manifolds below the floor inside and/or outisde each main. Due to the low cost of power, I could accomidate whatever power requirements come with the setup(assuming about 500watts per sub is more than enough).

My concerns are as follows:

1. If this placement turns out to be problematic, I cannot just seal up the holes and try another location. The home is new construction and I don't want to hack into it more than once. The new construction aspect also limits the pre build testing and measuring abilities, so I would kind of have to pick a spot and hopes it is optimum.

2. The claims from many that IB setups don't "kick you in the chest" the same way. I have read enough to see that most people that have rsearched this fact attribute that to a flatter response..is that the case?? The only IB I have had was a single 12" cerwin vega in my car and though I liked it, it was 15 years ago, so I can't remember much else about it.

My goal is to have low extension without much compromise to the overall output and "thump" in the chest feeling. I am also waiting on Brett from ED to give me a little more info on otimal xovr points for the mains.

Thanks in advance. Thoughts??

You have mains with dual 12"s. Cross to the IB at 60hz and I don't think you'll have a problem with mid bass impact.

I imagine a lot of people with "impact" problems comes from mains that cant keep up, and/or cross low enough for good integration.
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post #8 of 94
4/2/12 at 12:47pm
Thread Starter 
Quote:
Originally Posted by Jay1 View Post

You have mains with dual 12"s. Cross to the IB at 60hz and I don't think you'll have a problem with mid bass impact.

I imagine a lot of people with "impact" problems comes from mains that cant keep up, and/or cross low enough for good integration.

Yup, had the guys at ED mondify the cinema series and make them each dual 12" with the DE250 CD. I will have to run that 60hz xover by Brett and be sure it would be optimal for the mains. Thanks for the insight
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post #9 of 94
4/2/12 at 2:10pm
Too many variables to predetermine a crossover frequency prior to in room measurements and optimization.

In general, more capability in the LF section of the mains, and specifically dual 12's, would allow you the latitude to pursue a lower crossover point. However, the actual implementation choice of which frequency would depend on the acoustic interaction with the room.

So it all depends on phase, SBIR, boundary proximity, listening position, etc.

Good luck
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post #10 of 94
4/2/12 at 3:17pm
Quote:
Originally Posted by ChopShop1 View Post

Thanks FOH...how did you come up with those output results??

Also, If one ep on 4 drivers yields 125db, then two ep's would yield 128, two eps and 8 drivers would yield 131db and four eps with 8 drivers would yield 134db correct?? I used the rule, which I think I am correct about, that each doubling of drivers or power will result in a 3db gain.

I calculated the numbers, and yes, either doubling the drivers, or a doubling of power, typically increases SPL by 3dB. Thus, adding a like powered sub yields 6dB. This is theoretical, weird things happen in small room acoustics when boundaries interact etc. But in a manifold IB, they'd sum appropriately.


Fi, rates the Fi IB3-18 at 550 watts@20hz free air.

An EP4000 power output is as follows;

Stereo;
8 ohm stereo=450w
4 ohm stereo=635w
2 ohm stereo=815w
------------------------
Mono;
8 ohm mono=1336w
4 ohm mono=2000w

In my opinion, I'd stay away from a potentially bottoming scenario. Yes, they may handle 500 watts at 20hz, which would be great for music, but with the huge effects that populate the bottom octaves of contemporary releases on DVD/BluRay, you need to examine the handling below 20hz. If you powered the drivers with an EP4000, config'd with a 4-ohm driver on each side, that's ~635watts/driver,...that's too much. Mono, you could feed the series parallel 4-ohm group ~500 watts apiece. This would be fine until the big 10 hz effect came along and hard bottoms all four drives at once.

Ideally, you could series two 4-ohm drivers, and feed each one ~225watts,...this resides safely under the limits at 10hz, yet still retains some serious SPL fire-power@~125dB@20hz@1m. You could use the higher power approaches, and experiment with a high pass filter to assure enough power above the knee, yet some assurance from over excursion.

Hope this helps

Good luck
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post #11 of 94
4/2/12 at 4:17pm
Thread Starter 
Quote:
Originally Posted by FOH View Post

I calculated the numbers, and yes, either doubling the drivers, or a doubling of power, typically increases SPL by 3dB. Thus, adding a like powered sub yields 6dB. This is theoretical, weird things happen in small room acoustics when boundaries interact etc. But in a manifold IB, they'd sum appropriately.


Fi, rates the Fi IB3-18 at 550 watts@20hz free air.

An EP4000 power output is as follows;

Stereo;
8 ohm stereo=450w
4 ohm stereo=635w
2 ohm stereo=815w
------------------------
Mono;
8 ohm mono=1336w
4 ohm mono=2000w

In my opinion, I'd stay away from a potentially bottoming scenario. Yes, they may handle 500 watts at 20hz, which would be great for music, but with the huge effects that populate the bottom octaves of contemporary releases on DVD/BluRay, you need to examine the handling below 20hz. If you powered the drivers with an EP4000, config'd with a 4-ohm driver on each side, that's ~635watts/driver,...that's too much. Mono, you could feed the series parallel 4-ohm group ~500 watts apiece. This would be fine until the big 10 hz effect came along and hard bottoms all four drives at once.

Ideally, you could series two 4-ohm drivers, and feed each one ~225watts,...this resides safely under the limits at 10hz, yet still retains some serious SPL fire-power@~125dB@20hz@1m. You could use the higher power approaches, and experiment with a high pass filter to assure enough power above the knee, yet some assurance from over excursion.

Hope this helps

Good luck

Very informative, thank you for taking the time. I am at the point that I think the realistic appraoch, as you said, is a little less power, but adding more drivers. My thought would be 2 ep4000 and 8 18" fi's. I just hope that if I do decide to spend the time and $$ on the setup, I can get it integrated properly and it yields better results that just buying a couple of subs and placing the, at optimal locations in the room(the very big room)...20x26 with a cieling that peaks out at 14ft.
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post #12 of 94
4/3/12 at 8:08am
Eight 18's powered by two EP4000's would make for one INSANE IB setup. Please do a Build Thread if you do that. Good luck.
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post #13 of 94
4/3/12 at 8:52am
Quote:
Originally Posted by michaelddd View Post

Eight 18's powered by two EP4000's would make for one INSANE IB setup. Please do a Build Thread if you do that. Good luck.

Actually, (8)18's, in an IB alignment, in a space that big, wouldn't be extreme at all,...IMO. If you limited the bottom octaves with a high pass, yes, that would be extreme. But wide open with no excursion protection, I'd suspect (8)18's would encounter their excursion limitations at 20hz and below with HT material. With music, no problems.


Thanks
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post #14 of 94
4/3/12 at 9:29am
So the limiting factor is the airspace of the "infinite" enclosure? What factor(s) would increase sub-20Hz performance?
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post #15 of 94
4/3/12 at 11:38am
Quote:
Originally Posted by FOH View Post

Actually, (8)18's, in an IB alignment, in a space that big, wouldn't be extreme at all,...IMO. If you limited the bottom octaves with a high pass, yes, that would be extreme. But wide open with no excursion protection, I'd suspect (8)18's would encounter their excursion limitations at 20hz and below with HT material. With music, no problems.


Thanks

I can see why you would "suspect" that, but many people with far LESS of an IB system, in bigger rooms than what the OP has..... have reported no bottoming issues. If things were the way you say, there would be a lot of pissed off people with blown up drivers over at the Cult of the IB.... and there are not.

I understand your theory... but it's not playing out in real life. The OP will be just fine and have plenty of headroom with 8 IB3's and two EP4000s. Plenty.
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post #16 of 94
4/3/12 at 12:16pm
Thread Starter 
I would have though so too, but I am very new to IB setups like this. My concern is will 8 ib3s and two ep4k compete with the likes of two giant ported subs in terms of output??
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post #17 of 94
4/3/12 at 12:28pm
Quote:
Originally Posted by ChopShop1 View Post

I would have though so too, but I am very new to IB setups like this. My concern is will 8 ib3s and two ep4k compete with the likes of two giant ported subs in terms of output??

Modeling the Pi-18's tuned 15hz against 8 IB's isn't a fair fight. The IB wins at every level except price tag of drivers (but you'd save on power). 12db+ at 10hz before PVG, 3db at 20hz, 7db at 40hz. Less power required and no ugly box. IB wins this race easily
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post #18 of 94
4/3/12 at 12:32pm
"My concern is will 8 ib3s and two ep4k compete with the likes of two giant ported subs in terms of output??"

yes. the ib setup should have a few more db of spl everywhere and will slaughter the ported cabs for infrasonics.
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post #19 of 94
4/3/12 at 1:15pm
Thread Starter 
Quote:
Originally Posted by LTD02 View Post

"My concern is will 8 ib3s and two ep4k compete with the likes of two giant ported subs in terms of output??"

yes. the ib setup should have a few more db of spl everywhere and will slaughter the ported cabs for infrasonics.

The last two posts certainly speak highly for the IB. I would love to go that route if it will exceed my expectations. I have also been told by another person that to get the IB to compete with large vented design, I will need an insane amount of drivers. I think I am going to experiment this weekend in my current space. Hopefully I can get my hands on some worthy equipment to perform the experiments.
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post #20 of 94
4/3/12 at 1:58pm
Quote:
Originally Posted by S_rangeBrew View Post

I can see why you would "suspect" that, but many people with far LESS of an IB system, in bigger rooms than what the OP has..... have reported no bottoming issues. If things were the way you say, there would be a lot of pissed off people with blown up drivers over at the Cult of the IB.... and there are not.

I understand your theory... but it's not playing out in real life. The OP will be just fine and have plenty of headroom with 8 IB3's and two EP4000s. Plenty.

Respectfully,...

I currently own four IB3-18's. I own two EP4000's. I've free air tested them with the EP4000, in a variety of configs. I've contacted the designer and discussed these very bottoming and power handling issues. I've discussed their bottoming characteristics with multiple IB3-18/EP4000 owners both via PM and here in public at AVS and The Cult. I've fully explored their excursion characteristics in a variety of signal and amplification environments.

My "suspect" statement encompasses the only variables here; the OP's room, and the preference for playback levels.


Thanks
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post #21 of 94
4/4/12 at 7:00am
My room is very similar to the OP's. It's 20' x 21' with a tall peaked ceiling. I have 3 door openings so the room will not pressurize. The four 18" driver IB that I have in there is PLENTY for even insane listening levels for music or HT. An 8 driver setup in a slightly larger room will simply loaf along. In addition, my mains each have dual 10" drivers fed by a 300 wpc amp and I get plenty of chest thump as well.
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post #22 of 94
4/4/12 at 1:38pm
Thread Starter 
Quote:
Originally Posted by cecaa850 View Post

My room is very similar to the OP's. It's 20' x 21' with a tall peaked ceiling. I have 3 door openings so the room will not pressurize. The four 18" driver IB that I have in there is PLENTY for even insane listening levels for music or HT. An 8 driver setup in a slightly larger room will simply loaf along. In addition, my mains each have dual 10" drivers fed by a 300 wpc amp and I get plenty of chest thump as well.

That is a very encouraging endorsement. Have you had the opportunity to take measurements?? What other sub systems di dyou have experience with? What type of placement are you utilizing in your room? Array or manifold? What are you running for power? Sorry for all the questions, just curious if this would work for me as well. Thanks
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post #23 of 94
4/4/12 at 1:44pm
Quote:


Stereo;
8 ohm stereo=450w
4 ohm stereo=635w
2 ohm stereo=815w
------------------------
Mono;
8 ohm mono=1336w
4 ohm mono=2000w

How can 2 ohm stereo be only 815w when 4 ohm mono is 2000 watts. 2 ohms stereo should 1000w if that is the case. I have always been told that 2 ohm stereo is the same as 4 ohms bridged.

Is it not now then?

Just realised that the 2000 watts is wrong as it was measured at 1khz and not 20hz. The 2 ohms stereo is 1000 watts at 1khz.

So now we all know it in fact is actually around 1600 watts bridged into 4 ohms bridged at 20hz
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post #24 of 94
4/5/12 at 1:32pm
Thread Starter 
Ok, so the framers have layed out the floor joists and the direction will allow me to accomidate more, as suggested in previous posts.Each manifold could contain 4 Fi IB 18" drivers, for a total of 8 18"s. I can pick up two ep4000s just like before. I would place one manifold beside each of the mains or in the two front corners(only options). Do those of you with IB experience think that with my limited placement options, but ability to use 8 18" drivers, this is a good way to go or is too much left up to chance due to placement restrictions?? My thoughts are that this would, in most cases, outperform 2 vented 18s like the A7-450 or Caps in my room...thoughts??

Could someone also point me in the direction to find out how to model these? I have no idea where to go or how to do it. I figured if I can model these and get the closest potential outcome it will help with my decision. Also will tell me how many "extra" drivers I will need to reach my goals
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post #25 of 94
4/5/12 at 1:39pm
Krypto (god bless his soul) had 4 IB3's in a ported alignment which essentially acted like an IB but with a port. This gave him 130db at 10hz. You will never get that with the caps. He was running two ep4000 amps. He had very low distortion indeed and much lower than any commercial sub. An to be honest the A7-450 won't go that low either as I thought their tuning was relatively high in the mid to late teens. Even 4 IB3's in a sealed will punch either of them.
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post #26 of 94
4/5/12 at 1:58pm
Quote:
Originally Posted by ChopShop1 View Post

Ok, so the framers have layed out the floor joists and the direction will allow me to accomidate more, as suggested in previous posts.Each manifold could contain 4 Fi IB 18" drivers, for a total of 8 18"s. I can pick up two ep4000s just like before. I would place one manifold beside each of the mains or in the two front corners(only options). Do those of you with IB experience think that with my limited placement options, but ability to use 8 18" drivers, this is a good way to go or is too much left up to chance due to placement restrictions?? My thoughts are that this would, in most cases, outperform 2 vented 18s like the A7-450 or Caps in my room...thoughts??

Could someone also point me in the direction to find out how to model these? I have no idea where to go or how to do it. I figured if I can model these and get the closest potential outcome it will help with my decision. Also will tell me how many "extra" drivers I will need to reach my goals

Be glad to help,...do you have any additional room details? Floorplan w/dimensions, and system layout regarding your LCR mains etc, and of course proposed IB location and backspace etc., all would be very helpful. The ability to spread your LF source out helps significantly to minimize any potential problems down the road.

Four 2 driver manifolds, spaced properly to mitigate modal issues, would likely be phenomenal in an optimized scenario. It's entirely possible that two manifolds would offer much of the same,... not sure. Bass traps, however, would be a must, but they are very easy, inexpensive, and largely very effective in a way few are truly familiar (time domain). Anyway, a properly damped time domain in the LF allows a highly delineated/high resolution bass system.

More details? ...
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post #27 of 94
4/5/12 at 3:36pm
Quote:
Originally Posted by FOH View Post

The ability to spread your LF source out helps significantly to minimize any potential problems down the road.

In particular, if your room approximates a rectangualr box, you can eliminate all odd-order modes (1st, 3rd, 5th etc) in both horizontal dimensions if you place the manifolds at opposite ends of the room at mid-wall; see http://www.aes.org/tmpFiles/elib/20120405/13680.pdf
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post #28 of 94
4/5/12 at 8:30pm
Thread Starter 
Quote:
Originally Posted by FOH View Post

Be glad to help,...do you have any additional room details? Floorplan w/dimensions, and system layout regarding your LCR mains etc, and of course proposed IB location and backspace etc., all would be very helpful. The ability to spread your LF source out helps significantly to minimize any potential problems down the road.

Four 2 driver manifolds, spaced properly to mitigate modal issues, would likely be phenomenal in an optimized scenario. It's entirely possible that two manifolds would offer much of the same,... not sure. Bass traps, however, would be a must, but they are very easy, inexpensive, and largely very effective in a way few are truly familiar (time domain). Anyway, a properly damped time domain in the LF allows a highly delineated/high resolution bass system.

More details? ...

I will try to attach a room diagram tomorrow, but in the meantime:
The room is rectangular in shape. The front wall is 25'8" long. The center channel will be dead center of the wall below the display. The mains will each stand so that the centers are ten feet apart from each other. The center channel is 40" wide, so there will be approx 3'4" between either end of the center and the respective main to each side. The height speakers will reside about 4 feet above each main. The side walls are 19'8" and will each hold a surround speaker at approx 14' from the front wall. The rear wall in the room is also 25'8" long and will have the rear channels placed behind the lp. Listening position will be 13'6"-14' from the front wall, meaning about 12'6" or so from the front three. The ceiling on the front wall starts at 9ft and vaults up to the rear, where it peaks out at about 14'. The only option is to run manifolds beneath the floor. I can place them inside each main, outside each main, a combo of between there and the corners, etc. I could certainly, as you suggest, use four manifolds and spread them out, My concern with this solution is how much outpput would I be sacrificing? I guess I would have to weigh the sacrifice in output vs the need for smoothing of the response.
Thank you FOH for taking the time throughout the thread. I would reallyliek to learn this aspect of the hobby better and you contiue to be a help.
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post #29 of 94
4/6/12 at 4:24am
Thread Starter 
Quote:
Originally Posted by noah katz View Post

In particular, if your room approximates a rectangualr box, you can eliminate all odd-order modes (1st, 3rd, 5th etc) in both horizontal dimensions if you place the manifolds at opposite ends of the room at mid-wall; see http://www.aes.org/tmpFiles/elib/20120405/13680.pdf

Wow, very cool read, albeit a little tough for me to understand as a scientific newb. I did love the fact that it was spelled right out what locations would potentialy work best in my room. I would much rather place the subs along the front wall in various locations and in the floor. I should have an opportunity to test different locations, even if with a weak sub. That should still show me what placement options work best.

If I place the sub at LP and then listen around the room for the best response, I pick the best position. If I use the 4 best areas of response, that would be optimal correct??
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post #30 of 94
4/6/12 at 9:21am
Quote:
Originally Posted by ChopShop1 View Post

The only option is to run manifolds beneath the floor. I can place them inside each main, outside each main, a combo of between there and the corners, etc. I could certainly, as you suggest, use four manifolds and spread them out, My concern with this solution is how much outpput would I be sacrificing?

Are you referring to co-location?

I can appreciate your concern for ultimate output, however you'll have insane output at all but the lowest frequencies. And the lowest frequencies will sum acoustically in a manner that essentially renders them co-located. Their wavelengths are so big, (56.5 feet for a 20hz wave) you'll largely enjoy the benefits of collocation into the range of concern.

The benefits gained for theoretical co-location aren't an all or nothing proposition. The manner in which these acoustic gains occur don't merely begin at zero, and jump to 6db at full co-location. It's a gradual process whereby the two distinct sources operate entirely separate from one another, and slowly sum to the total +6db due to constructive superimposition at the LP.

When two sources, IB manifolds in your situation, are operational relatively close to each other (whereby their acoustical centers are approximately within 1/4 wavelength), the gains from mutual coupling exist. This sums their acoustic output and will combine and propagate as a single wavefront over the range involved (within 1/4 wavelength). The incremental nature of the constructive superimposition is due to the sources being increasingly in phase at the LP, then the acoustic output of the manifolds essentially act as a single, much more capable IB.


I'm going to link some superb reads for you. These are outstanding, there's much I could link for you, but this is the best;

Getting the Bass Right

Loudspeakers and Rooms

Those two papers will help you determine a great deal.

The following web page, and collection of links includes a significant amount of the above material, however it's presented somewhat differently;

Here


I'm guessing you've spent some time at The Cult?


Good luck and I appreciate the kind words,...it's a pleasure to help.



Best of luck
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