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Thread: ideal bracing for sub

  1. #1
    Senior Member lgvenable's Avatar
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    ideal bracing for sub

    I've read many relevant threads, including the one on monster 2242's (350 lb cabs!)

    What is the take on the best internal bracing materials? In th thread that I mentioned, the authors mentioned 12 ply finply, however I was going to use 1" MDF with 3" wide x 1 " thick bracing.

    Is the best choice a laminate plywood vs a hardwood stringer (many ply laminate = much stiffer).

    Where do you find 12 ply finply?? Anyone have a supplier?

    Anyone have any comments on spraying the box internally with something like the Rhino coating used to help dampen resonance in the cab underneath the insulation??
    Larry
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

  2. #2
    Senior Member Steve Schell's Avatar
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    I built the sub using the 18" 2245J driver that was described in the Timbers/Kramer article in Audio magazine in the early 1980s. I believe that scans of this article are on this site somewhere. I built the enclosure from unbraced 1" MDF and it seemed to work fine, meeting my needs at the time. This sub left my possession about 1990.

    In recent years I have built really large, low flare rate exponential horn subwoofers, the optimal solution IMO when their size can be accomodated. Large unbraced panel areas are a concern, as the resonant frequencies of these large areas can drop into the passband of the sub (20Hz. to 80Hz.). If the panels get moving with applied signal then there is loss of output, time smearing, all sorts of nasty stuff. My strategy for the horn subs has been to brace the structure in such a way as to raise the panel resonant frequencies above the passband. One then has a relatively light, stiff structure that behaves almost perfectly in the intended operating range, though it will sing like a canary if fed a full range signal. I don't see why this design strategy should not apply equally well to box type subwoofers.

    I would think that Baltic birch plywood would be a better choice than MDF due to its greater stiffness. Not sure where you are located, but Anderson International Trading in Anaheim, CA is a resource for us southern CA speaker geeks and is probably the largest importer of Baltic birch plywood in North America: http://www.aitwood.com/default.asp

    One hip idea is to tie opposite sides of a box together with cross bracing. These sides would normally move opposite one another, but motion will be killed effectively with one or two braces that tie the sides together at the points of greatest movement. Cutting the braces a tiny bit short and drawing the sides together with screws places the panels under tension, further raising their resonant frequencies. I learned this trick from Dr. Bruce Edgar.

  3. #3
    Senior Member lgvenable's Avatar
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    I'm operating off the same article, and was just going to update the interior slightly (by using the baltic plywood as bracing), and veneer the outside w/walnut, and style it to look like a L100T styled exterior, complete w/push on grill.

    FYI We were going to use cross tied bracing, the idea of drawing everything together when its assembled by cutting the braces slightly short is very good, we'll do that as well. I have a professional cabinet builder doing the work for me; as for a project like this you need all the correct tools.

    One question, has anyone done anything other than butt joints at the cabinet edges, to increase the cabinet strength? (I'm sure today they also use biscuits too)
    Larry
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

  4. #4
    Senior Member John W's Avatar
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    Here are a couple profiles I've used to increase strength and make glue-up easier. The second one uses splines that run the whole length of the joint rather than biscuits.
    Of course these will both be visible from the front and back, depending on how you mount your baffle.
    Attached Images Attached Images  

  5. #5
    Senior Member lgvenable's Avatar
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    joints for sub

    John
    The one you posted on the left is the one I was asking my cabinet maker to use. It seemed the most straight forward, without a lot of tooling. I guess you could spline it or dowel it as well as well to help increase the joint rigidity. The idea is to make the last sub box I'll use, as it is to be made to match the look of L100T's (right down to the offset grill). So no joint will be visible, as the entire box will be veneered with walnut veneer to match my L100t's.

    I found that compressor seals (rubber end seals) are the right size to fit in the missing holes of my L100T's, and so I'll make my grill, dowel it, and then use these rubber plugs to help hold the grill like the ones on the L100T's.
    I'll attach the photo later.
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

  6. #6
    Senior Member SMKSoundPro's Avatar
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    For your entertainment are picture of the 2 x 2240 sub cabs that Lisa and I built. We used cross bracing out 2x4 fir stock , the same as the JBL SR4718a. We also used a plywood call "asanti" which had many plys, and resembled a mahogany species when cutting and shaping.

    When building a pro-sound sub, there are three rulles to follow:bracing, bracing and finally, bracing. I always use the finest quality plywood I can afford for these projects, as the whole is a sum of the parts. I am building SR4733 style of cabinets for the club, now. The woodworking is complete, and am awaiting the components to come in. I used 13 ply Baltic Birch, which is a dream to work with! The results of the joinery are as good as it gets in my shop.

    Good luck with your project, and as always... don't cut your finger off.

    9-fingered Scott.
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    One step above: "Two Tin Cans and a String!"
    Longtime Alaskan Low-Fi Guy - E=MC▓ ▒3db

  7. #7
    Senior Member lgvenable's Avatar
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    Scott
    so no MDF, just a cabinet made from 13 ply baltic birch?

    and lots of 2 x 4 fir bracing huh?

    Sounds much more straight forward than what we were going to use. I think I'm redesigning the box using your materials.
    Integra DHC80.1,3x 4636LF, 2360-2446J 2404H,12 x 8340 Surrounds, 2 x4645B, BGW 250D's,250E's,& 750B's 16 amps...7600 watts

  8. #8
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    I much prefer B&W matrix style bracing and never leave more than a 100mm x 100mm area unsupported.

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