IB installation question on wall bracing - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 16 Old 11-08-16, 09:48 PM Thread Starter
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IB installation question on wall bracing

Hello all. I have a 15'x20' theater room with a vaulted ceiling and dormer windows. Due to the way the interior was finished, I have two fully enclosed (drywalled) areas in the front of the theater room, one on the left wall, and one on the right wall (symmetric) that I want to use for an IB setup. I have some questions at the bottom (after all of the background info below).

Both of the enclosed spaces are have the following characteristics:
4 feet in length
2 feet in depth
5 feet in height
Total volume is 40 cu ft.
The UM18-22 driver has a Vas of ~7.5 cu ft.

Two of the walls (4 foot and 2 foot lengths) are exterior insulated walls
The 'ceiling' is insulated
The floor is OSB flooring on top of composite wood I-beams, the ones typically used in home construction.
The 2"x4" wall studs are no farther apart than 16". Most are closer together.
I have run several 2x4 horizontal braces between the vertical studs (screwed in), and attached them to the drywall with construction adhesive. The idea was to reduce the area of unsupported drywall.

I want to put a single Dayton UM18-22, baffle mounted, on the 4' length wall (5' high), near the floor.

I am going to mount the speaker on a thick baffle of plywood/MDF screwed into the studs.

When I hit the baffle wall hard with my fist, I hear a drum response at (guessing) at around 50-100 Hz. So I want to to brace the wall to fix that. The studs in the region of the speaker baffle will be braced with a horizontal 2"x12" board, tied back to another 2"x12" board on the opposite wall (two feet away) with horizontal 2"x4" boards.

I am not going to go crazy with power, so I will run each with about 300-400 watts RMS. There will be two drivers, one on each wall, baffle mounted, facing inward toward the centerline of the room. I am going for SQ over SPL.

I have run sealed box simulations in WinISD looking at the differences in response, speaker excursion, SPL, etc. comparing a 40 cu ft enclosure to a 7 ft standard sealed enclosure (which I could build two of, and have in the room). They are close enough in performance such that I can live with either configuration. But I would prefer the IB.

I know using opposed speakers in a manifold is better, but even though I know they are a good value, these Dayton UM18-22 speakers are not cheap by my budget. So buying two more (four total) drivers, just for cancellation in a manifold, that I do not really need to reach my desired SPL, is not really what I want to do unless I absolutely have to.

OK, now the questions:
1. I want to do the IB instead of the sealed sub. But, I want to get a general idea if the space and bracing described above will be sufficient for the driver I am using (I already have both). If it is not, I may have to abandon the IB and build the sealed instead.
2. I saw a previous post where someone suggested filling the IB space with stacked pieces of insulation, floor to ceiling. I suppose the intent is to act like a bass trap for the back wave of the speaker. Would this be a good idea in this application as well? I can go wall to wall, floor to ceiling, but only have the space to go one batten deep.
3. If I can raise the 1st mode response of the wall, using the bracing described above, to say one octave over my crossover frequency, will I be OK? I don't expect to cross the IBs over much higher than 100-150 Hz. The main speakers are Def Tech BP6 towers that can go that low without too much trouble.

Thanks in advance for the help.
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post #2 of 16 Old 11-09-16, 09:32 AM
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Re: IB installation question on wall bracing

IB is just that, and you have a sealed room if I understand. I think you would have to open the ceiling for it to be a true IB. I have not seen extra bracing with IB builds because the amount of movement is absorbed with the "infinite" air space. It sounds like what you really want is a closed box which would be sealed or ported. With the box size that you have to work with, I would recommend using it as a tapped horn design. I think you would have more bass than you could handle with that design. Even more with 2!
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post #3 of 16 Old 11-09-16, 10:51 AM Thread Starter
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Re: IB installation question on wall bracing

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Tonto wrote: View Post
IB is just that, and you have a sealed room if I understand. I think you would have to open the ceiling for it to be a true IB. I have not seen extra bracing with IB builds because the amount of movement is absorbed with the "infinite" air space. It sounds like what you really want is a closed box which would be sealed or ported. With the box size that you have to work with, I would recommend using it as a tapped horn design. I think you would have more bass than you could handle with that design. Even more with 2!
As I understand it, the typical rule of thumb is 5X Vas and you are very close to IB response. Based on running 40 cu ft sealed versus 10000 cubic feet sealed (essentially infinite) in WinISP the response is very, very close. So the main concern I have is whether the wall bracing will be OK.

I cannot open up the 'ceiling' of the enclosed space, because it is the underside of the roof.

The selection process is IB first. If I cannot make it work, then I'll go with sealed. For various reasons these are my two choices in order of preference. For example I used to have a great sounding DIY sonotube transmission line in the same room but had to remove it due to size. I can make the sealed enclosures work space wise, but still prefer the IB because it will essentially take up no space in the room, and the SAF is better for the IB.

Basically, I want to build the IB, but just want to be reasonably sure the baffle wall with the bracing will be up to the task, since the UM18-22 is a monster of a driver. So I am looking for feedback from others who have built baffled (non-opposing driver) IBs under similar circumstances.

Last edited by ZX-Tex; 11-09-16 at 10:57 AM.
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post #4 of 16 Old 11-11-16, 11:32 PM
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Re: IB installation question on wall bracing

Volume for an IB can be anywhere from 4-4.5 to 10+ times the Vas parameter. If your IB chambers are ~40ft^3, you're in range but only just. I think you'll be good there. Adding insulation batting into the space increases the effective volume of the chamber by slowing the propagation rate of the sound waves, and therefore lowering its apparent volumetric resonance. In your case, I'd go ahead and do it... it doesn't really hurt anything, and it's cheap and easy. So even if the net effect is small you haven't really wasted much in terms of money and effort.

I've got a 4x12 linear array IB, and I can tell you I did not brace it enough. If you can, I'd recommend framing all the way around the driver with 2x4s, and attach that frame to the back walls of the chamber in several places. The down-side to this approach is that by bracing the baffle wall to the rest of the chamber, you're mechanically coupling the drivers to the rest of the structure. That means vibration in strange places all over the house. That can be fun and exciting... but probably not good. The other thing I'd recommend is to dampen the baffle as much as possible by adding mass and rigidity. MLV, extra sheet rock, homasote board, etc. More is better. Anything you can do to reduce the vibration in that panel is going to increase efficiency and fidelity. There's really no such thing as "too far", but there's a diminishing return.

I think the biggest question is how much access you'll have to the interior of those chambers... it might be difficult trying to get all that done through a 17" diameter hole... (OSHA requirements for enclosed spaces and restricted access don't apply because you live there, but be careful with aerosols/paints/coatings/adhesives/solvents anyway). You best bet is likely going to be removing the entire section of drywall and adding the framing you need, and then replace existing drywall with a multi-layer baffle.

While the UM18-22 is formidable, you might be better off with a pair of UM12's in each chamber. The total price of 4 12's will be higher, but you won't have to work anywhere near as hard to make them work in that space since you can build an opposed manifold. You'll end up with slightly less displacement than 2 18's, but I'd bet you'll still get acceptable results.

Do you have pictures of the room and the corners you're talking about?
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post #5 of 16 Old 11-12-16, 09:01 AM Thread Starter
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Re: IB installation question on wall bracing

Yes I have some pictures to help, was just about to post them.

The first picture shows the 2x4 frame that the speaker baffle (plywood) will be attached to. The dimensions of the inside of the stud box is about 18" x 18"

The second picture is the inside of the 2 foot x 5 foot interior wall.

The third picture is a shot of the two exterior walls.

The fourth picture shows the position of the enclosure in the room, and the location of the baffle opening.
Attached Thumbnails
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IB installation question on wall bracing-20161111_180602.jpg  

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Last edited by ZX-Tex; 11-12-16 at 09:10 AM.
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post #6 of 16 Old 11-12-16, 09:06 AM Thread Starter
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Re: IB installation question on wall bracing

I have a picture of the bracing I have added to tie the baffle wall to the exterior wall but am having trouble uploading it.

Last edited by ZX-Tex; 11-12-16 at 09:12 AM.
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post #7 of 16 Old 11-12-16, 09:16 AM Thread Starter
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Re: IB installation question on wall bracing

OK well here is a completely other thought, per your suggestion DqMcClain...

Considering the need for mass loading, and the risk of coupling to the house (not good for the SAF rating), I am going to go the opposite route, and use opposed speakers in a manifold. Turns out that Parts Express just started a good sale on the UM15-22 after I started this thread, so buying 4-15" drivers (UM15-22) is not much more painful than buying two UM18-22s.

The downside is that it will just about double the Vas for the driver pair vs the 18", getting away from the IB rule of thumb. So, I'll have to brace the drywall a bit more, take it easy on the SPL, add a lot of insulation battens, and cross my fingers.

I know the 12" speakers have a much lower Vas than the 15", and I could use those to get the Vas down for the volume, but I am concerned they may not have enough output for a 20'x20' room.

I'm going back to WinISD and modeling the 12" pair versus the 15" pair and check the output differences at Xmax, as well as the comparative response profiles.

Last edited by ZX-Tex; 11-12-16 at 10:37 AM.
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post #8 of 16 Old 11-12-16, 11:02 AM Thread Starter
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Re: IB installation question on wall bracing

OK here is the comparative response of four different setups:
All four are using different sizes of the PE UM driver
All four have the same input power of 600W.
All four use the same enclosure size corresponding to the space in the wall.
The 12" and 15" simulations use double the number of drivers as the 18" (baffle versus manifold)
None of the comparisons are exceeding the Xmax of the speakers

Yellow line is 15" drivers (manifold)
Red line is 15" drivers (manifold) but in a IB enclosure that is 10X of the combined Vas
Blue line is the 18" drivers (baffle)
Orange line is 12" drivers (manifold)

Clearly the 15" manifold (yellow) has the best response, and the response of the 10X Vas enclosure (red) is very close. That tells me there is little enclosure interaction with the smaller enclosure (what I want to use). The Qtc of the two systems is .574 and .507 respectively. The Qts of the 15" driver is .474

So I am thinking I am OK if I add the batten insulation to the space, brace the wall studs, and do what I can to dampen the drywall?
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Last edited by ZX-Tex; 11-12-16 at 11:18 AM.
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post #9 of 16 Old 11-12-16, 06:27 PM
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Re: IB installation question on wall bracing

Ah... so, As I expected, the pair of 12's keeps pace with the single 18. The pair of 15's outclasses them all, which isn't surprising either, and the simulated difference is about 5dB over the 12's.

My gut reaction is that 118dB at 60Hz is pretty loud. I have a little more than 1920ft^3 in my room with the 4 12's in my array. They're GRS 12SW4's (the driver that best matched my desired specs for Vas, Qts, and power) which are VASTLY inferior to the UM12-22s... I mention this because I'm constantly turning them down. When LFE tracks start to deliver the business, they're just too loud. I'm driving them with a Crown XLS1500 with a Behringer FBQ1000 taking care of EQ. The amp sees a 4Ohm load. It's simply ridiculous.

Your setup, having a much larger air volume in the listening area, will not be as overbearing as mine... but I'd think the 12's will do the job.

If you're committed to the idea of the 15's though, I wouldn't worry about the differences between the red curve and the yellow curve. The in-room response of those will be pretty close. A quick-and-dirty look at specs tells me you're at about half the volume for an IB. You can't make up all the difference with batting, but like I said earlier I don't think going a little heavy on that is going to hurt anything. The real trick will be damping the drywall... without actually going inside those chambers, I don't have the slightest idea how you'll pull that off.
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post #10 of 16 Old 11-12-16, 08:33 PM Thread Starter
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Re: IB installation question on wall bracing

Thanks for the feedback. Your description on your setup and your results is good information. I know it is a bit silly, but since those 15" UMs while on sale are only $10 more (each) than the 12" UMs, I had to get them.

I can actually get into and move around a bit in that space in the wall, so I am able to add bracing and the like. It is difficult to get a good picture of what I have done so far without a fisheye lens. Panoramas did not work well either. At any rate, I have already added some more rigidity to the drywall and the wall structure. I used construction adhesive to glue the drywall to the wall studs to augment the drywall fasteners. That made a noticeable difference. So did the horizontal bracing between the studs in the second picture; imagine that image is actually rotated 90 degs clockwise.

I've removed the low-frequency ring from the 4x5 foot wall by tying it back to the exterior wall with some very stiff bracing. I can add some additional damping to the drywall if I have issues at higher SPLs. I plan to make the manifold removable just in case I need to crawl back in there to beef things up.
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