super thick wood vs. crossbracing.

Discussion in 'Archived Threads 2001-2004' started by Ron D Core, May 1, 2002.

  1. Ron D Core

    Ron D Core Stunt Coordinator

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    What do you guys think would be a better idea if cost and weight are not an issue, tons of cross bracing or really thick sandwiched MDF and ply. I was thinking of doing something With maybe doubled up 1" MDF with 3/4" Birch on the outside. The only thing on the inside would be egg crate and Polyfill. Maybe even some sort of array of ridges on the walls to break up as many free standing waves as possible without having to fill the inside with cross-bracing.
     
  2. Patrick Sun

    Patrick Sun Studio Mogul

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    2 major choices:

    1. 1 layer of 3/4" wood/MDF with plenty of cross bracing.

    2. 2 layers of 3/4" wood/MDF with minimal cross bracing. If you can handle the weight, the double-layer approach yields a more dead enclosure.
     
  3. Juan Castillo

    Juan Castillo Second Unit

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    don't remember the website, but one guy built an enclosure with a resevoir around the box, and filled it with leaded sand, or something like that. Of course, he still added plenty of bracing, so it did not eliminate that factor. Will look for the link and post it.
     
  4. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    I like the North Creek Music construction method. Their $5.00 18 page North Creek Cabinet Handbookbooklet is more than worth the price: http://www.northcreekmusic.com/Publications.html
    Basically, they use a laminate of 3/4" baltic birch ply and 3/4" MDF (MDF is cabinet exterior).
     
  5. Ron D Core

    Ron D Core Stunt Coordinator

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    What about concrete? Anyone try that? Its about as dead as you could ever hope to get. As long as it could remain under 300 pounds I should be able to get some guys to help me move it around.
     
  6. Dan Hine

    Dan Hine Screenwriter

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    Seems to me like concrete would actually be very porous and not a good choice. Especially when adding in the weight factor. Double layers and minimal bracing sounds like the best idea so far.

    Dan Hine
     
  7. Greg Monfort

    Greg Monfort Supporting Actor

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    Ideally you want the cab to resonate well outside the intended BW. Unless

    you're prepared to make them out of lead and/or concrete or braced titanium,

    this isn't practical, so the next best option is to make it rigid enough

    that its resonant BW is high enough that it's easily damped to below the

    noise floor since acoustic energy falls with increasing frequency.

    Rigidity can be achieved through using a high Fs material, increased

    thickness, bracing, or a combination of them all. As material thickness

    increases so does damping.

    WRT your choices, either way works equally well as long as you wind up with

    an apples n' apples comparison.

    I prefer well braced 3/4" Baltic Birch plywood and minimal damping since

    it's easy, relatively light, and you rapidly reach a point of diminishing

    returns where some fiberglass on the walls is all that's needed. The main

    thing with this construction is don't use the 'knuckle' test, 'cause if it

    passes it means it resonates right down in the LF BW where you don't want it

    to. You want it to ring like a bell when struck with a hammer before any

    damping is done.

    If you go massive, then with the knuckle test all you should hear is your

    bones cracking (ouch!), like when you rap them against concrete. Also, be

    sure and heavily chamfer out the rear of the driver holes so the drivers can

    'breathe'.

    WRT concrete, it works well if you want to mess with it. Early on I used

    concrete and tile drain pipe for cabs and I've auditioned a sub built with

    preformed A/C slabs epoxied together, but it didn't perform any better IMO

    than a good plywood version, though of course the builder disagreed. At

    150lbs/ft^3 for Portland cement I don't think it will meet your weight limit

    though and be thick enough to be 100% effective and stay together.

    FWIW, MDF is much less, and regular birch ply is somewhat less rigid (lower

    MOE) than BB ply, so layers of BB will be thinner/lighter than an

    equivalently stiff panel of MDF/birch. For instance, 2" thick MDF is

    ~equivalent to 1.32" thick BB ply assuming the same glue is used to laminate

    it, and weight/ft^3 is much less at ~50lbs Vs ~41lbs.

    GM
     
  8. Jeff Rosz

    Jeff Rosz Second Unit

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    so where does marine ply stack up compared to BB and mdf?
     
  9. Greg Monfort

    Greg Monfort Supporting Actor

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    >so where does marine ply stack up compared to BB and mdf?

    ====

    It used to be ~equivalent to BB and my preference for decades, but it's been downgraded to allow voids, so you pays your money and takes your chances.....(not me)

    GM
     
  10. Jeff Rosz

    Jeff Rosz Second Unit

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    gotcha, thanks
     
  11. Robert_Gaither

    Robert_Gaither Screenwriter

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    Ron, if cost and weight is not an issue, why not do both?
     
  12. Ron D Core

    Ron D Core Stunt Coordinator

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    I just dont like the idea of cluttering the inside of the box.
     
  13. Brian Bunge

    Brian Bunge Producer

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    Ron,
    Have you ever seen the quad Seismic 18's that a fellow forum member built? He used 1.25" thick marine grade ply for the enclosure and all of the internal bracing. He doubled up the ply for a 2.5" thick baffle.
    The point is, he used extremely rigid material, AND added bracing.
    Brian
     
  14. Jeff Rosz

    Jeff Rosz Second Unit

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    wasnt that "the black hole" subwoofer?
    edit...ya it was heres the link. the pics are still there.
     
  15. Greg Monfort

    Greg Monfort Supporting Actor

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    Yeah, Chris seems to be one of those folks that figures anything worth doing is worth doing to excess, but there's a point of diminishing returns and the physics is that with no void marine (if you can find it now), BB, or apple ply didn't need to >3/4" thick with bracing.

    I have a slightly smaller sized pair of cabs with dual 15"s using no void 3/4" marine ply and braced with 2x4s. They resonate at ~800Hz, and with fiberglass lining the top, back, and one side are audibly 'dead' WRT the 16-500Hz/4th order BW they reproduce.

    WRT 'cluttering' up the inside of a sub, until the dims become long enough for standing waves/reflections to develop the particle density field is ~uniform so any bracing is 'invisible'.

    GM
     
  16. Jack Gilvey

    Jack Gilvey Producer

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    My 3/4" marine ply did have a few small voids, so I'll use BB next time. No noticeable problem with my 2X2 braced sub cab, though, rings nicely on the knuckle test. [​IMG]
     
  17. Jeff Rosz

    Jeff Rosz Second Unit

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    ok i have another.....

    appleply, how does it fare?

    where do you guys get your BB, cant find it anywhere locally, in fact they never heard of it. (i gotta move outta the 3rd world area i live in). i found alot on the web but i dont need the large pallet loads they sell. any ideas?
     
  18. Greg Monfort

    Greg Monfort Supporting Actor

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    Appleply is considered to be as good, or better, than BB, but I've been told by folks who build high power basshorns for a living that it's not worth the extra cost for sub building.

    I order BB from Home Depot, who in turn orders it from a distributor in a town north of Hotlanta.

    GM
     
  19. Brian Bunge

    Brian Bunge Producer

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    Greg,

    Do you know if any HD can order the BB ply? If so, I might have to check it out sometime. They're building a new HD in Loganville right now! Should be open by the summer I'd think.

    Brian
     
  20. Greg_R

    Greg_R Screenwriter

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    Appleply is a brand name for Baltic Birch (BB) plywood and has grade levels (just like BB). Find a wood distributor that caters to cabinetmakers & other professional woodworkers. The prices of wood will be MUCH cheaper than HD.
     

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