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Discussion Starter #1 (Edited)
I see many posts stating the output is greater on ported so you need two IB for similar output. I don't see many posts stating you need two ported 18's but it seems like many state most aren't happy with just two IB 18's and recommend four.
I had a HSU VFT 3 12" not enough output and the woofer gave out after seven years.
 

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IB is really a large sealed enclosure. Knowing that fact, the larger the enclosure the less power it takes to reach Xmax. Less power means less SPL. A larger box will give more low end extention and less SPL compared to the same driver in a smaller box. A smaller box gives higher SPL but less low end extention. This applies to both sealed and ported. Ported gives more output over sealed because the back wave is being added to the front wave through the port.

I don't see many posts stating you need two ported 18's but it seems like many state most aren't happy with just two IB 18's and recommend four...
That's dependent on the size of the room and listening levels. Quite a few people have twin ported subs. Personally I have 2 ported 18's and 2 ported 15's in a 15' by 26' room.

I am upgrading to two FI IB3 18" run by EP2500 and would like to do dual 18" IB for ease factor...
Sounds like a good plan. With 1200 watts input and a subsonic filter at 15 hz you'll get over 117 db at 20 hz, plus room gain. That's a big improvement over the HSU VFT 3. Build the manifold for 2 18's and also for the addition of 2 more in the future. Or plan now for the addition of a second manifold should you decide you want more output in the future. The EP2500 will easily power 4 IB3 18's.

The area is a coat closet style of space 12.5' high that has a 25"x18" opening to crawl space under house is this area big enough?
Ideally the space required is 10 times the Vas of the combined drivers. IB's have been built in less then that with good results.

Room is open with cathedral ceilings 22x14 and graph shows a gain of over 15db @ 20Hz over the HSU I fried, is that enough to make a big difference?
That much gain will definitely open your eyes!
 

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Re: Array IB how much vibration in this setup

Any way you could mount the woofers directly to the chimeny instead of the wall?

Bracing the walls to the chimeny directly next to the woofers would be the next best bet. Mounting the two woofers 90 degrees to each other as in your second diagram would cancel vibration in the back and forth direction but not from side to side. (if that makes sense :scratch:)
 

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Re: Array IB how much vibration in this setup

Man, that looks difficult. IMO there's no real way I could even hazard a guess about performance. I do know that drywall/sheet-rock anywhere near the drivers will vibrate but I don't know how much.
My first thought was to make the entire wall facing the listening room out of 3/4 plywood. Tie all the included 2x4 studs with glue and screws and mount BOTH drivers to the right of the chimney in your drawing stacked vertically. Is that space to the right of the chimney wide enough to mount the drivers facing the listening room?
 

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Re: Array IB how much vibration in this setup

Even though you have access through the closet, it would make sense to have all your bracing in before you close up the wall. Your last bracing suggestion sounds good.
 

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I've been considering an IB (or as close as I can get) with a single 15" in the ceiling. It's a 13" x 13" room so I don't think I'll need more than that. I am worried about bracing the box. Will likely tie it to the roof trusses along with the ceiling truss.
 

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Bracing the enclosure IS a necessity. Designing an IB entails designing the bracing and using the existing house structure which includes wall panels, 2x4s, and floor/ceiling joists.
 
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