Weight of Woods to be use for Speaker Enclosures

Discussion in 'Archived Threads 2001-2004' started by Julian Data, May 11, 2001.

  1. Julian Data

    Julian Data Second Unit

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    Guys,
    I am in the process of building a PR Sub (4 PRs, 2 drivers) and some monitors. So my question is what are the weight/cu.ft for the following.
    -MDF
    -MFO
    -Marine Ply
    -Baltic Birch
    Also, is Birch good enough to use?
    Thanks!
    JD
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  2. Greg_R

    Greg_R Screenwriter

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    Greg
    Marine grade ply and birch ply will have the least amount of flex (i.e. better enclosure performance). I'd look for marine grade, apple ply (a brand name), or a high grade cabinet plywood if you go that route. However, MDF/MDO is easier to work with and finish (veneer or paint) and is "good enough" for most speaker builders.
    Greg
     
  3. Julian Data

    Julian Data Second Unit

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    Thanks, Greg for the info.
    Is marine ply lighter than MDF?
    JD
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  4. kevin oswald

    kevin oswald Agent

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    marine ply is also about 3-4x as much as MDF. I thought about it.. then I saw the price.
    ..Kevin
     
  5. Greg Monfort

    Greg Monfort Supporting Actor

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    Birch ply that HD, etc., sells has too many voids/filler for serious sub duty IMO. I used 3/4" BB for my Contrabass kits per Tom D's recommendation, but all the other subs/mains I've built over the years have all been with 3/4" marine ply.
    According to a local distributor, Baltic Birch ply has a ft^3 density of 36lbs/ft^3, marine = 33lbs/ft^3, and MDF = 50lbs/ft^3. Never heard of MDO.
    GM
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  6. Julian Data

    Julian Data Second Unit

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    Greg,
    Thanks for the info, that's what I was looking for as I am trying to determine an "estimated" weight for my speakers. [​IMG]
    jd
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  7. kevin oswald

    kevin oswald Agent

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    Make them as heavy as possible, figure there is no way someone will come into youre house.. and actualy be able to lift them.. so they will never be stolen.
     
  8. Julian Data

    Julian Data Second Unit

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    Hehe. [​IMG]
    Given by the external dimensions for the sub, it's going to be heavy and I am thinking about adding wheels to it so I can maneuver it.
    I don't see a problem with the monitors in regarding to weight.
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  9. Greg Monfort

    Greg Monfort Supporting Actor

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    My old 20ft^3 subs, which weigh about what an equivalent sized refrigerator does, has industrial strength casters at the bottom rear with a large metal grab bar, so that I can tilt them back to roll them around.
    GM
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  10. Julian Data

    Julian Data Second Unit

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    Greg,
    Do you have a pic of the "refrigerator subs"?
    Mine will be "net", 13.25ft3.
    jd
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  11. Greg Monfort

    Greg Monfort Supporting Actor

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    Somewhere, though I know not where. [​IMG] They're out in the shed, sans drivers, but the i.d. dims per the sketch I made them from are 28.5"w x 22.5"d x 65.5"h, or ~24.3ft^3 before all the 2x4 cross/edge panel bracing, a portion of the fiberglass, and two Altec 515Bs are subtracted.
    The drivers and vent are all on individual baffles bolted to 2x4 flanges with (20) 1/4-28 countersinking machine screws, to keep rigidity up. Gasketing is some industrial strength neoprene our plant used to seal watertight rated electrical enclosures.
    I could stand a quarter on edge, and it would just stand there with down to 16Hz sustained organ notes played at up to >120dB (depending on frequency).
    GM
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    Loud is beautiful, if it's clean
     
  12. Julian Data

    Julian Data Second Unit

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    DAMN!
    That must've been one helluva tight box! [​IMG] Too bad you don't have any pics. [​IMG]
    I am finding out as this is my first "box", it's very time consuming in regarding computations for "net" volume. I do have a sneaking suspicion that the box with all contents should weigh over to at least 230lbs. Got any tips? [​IMG]
    I know I can get Marine Ply but finding Baltic Birch in a small town is "time consuming".
    The internal dimensions I have are: 28.5 X 28.5 X 36. I am also comtemplating on whether to use just acoustic foam or polyfill or a proportion of each.
    jd
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  13. Greg Monfort

    Greg Monfort Supporting Actor

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    Sorry. I have two expensive cameras, neither of which work, and all my disposable income(and then some) goes to the medical community.
    Yes, its built like a wood vault, but its worth it IMO. They're made with 3/4" marine ply, so I wouldn't bother with 14 lam BB. I only used it on the CB because Tom D. said that the motor/linkage/drivers was strong enough to 'rattle' the sides of the acoustically small cab of anything less! Of course I could have made it slightly bigger and braced it, but it's available here so why bother?
    What kind of tips? I mean I've told you all the salient facts. The way I built them was by making the 2x4 frame, bracing, and driver cradles (they weight ~25lbs each). Then I glued the panels to it and glued additional edge bracing as needed by tapping the panels with a ball peen hammer. When it rang ~like a bell, I figured that was high enough to be well above the passband of the 1kHz they were originally used for (500Hz/2nd order XO).
    These have been moved around a bunch since '69, including up/down stairs, subjected to temperature extremes, and nary a crack yet.
    I have R-19 fiberglass (no paper backing!) on one wall, top, and back only. With PRs, I stuffed them 100% with Miraflex and made the cabs 20% smaller, same as T.D. did. Worked just fine for me.
    BTW if it's not too late, rethink those dims, as the 28.5" square is large enough to be an issue even with an 80Hz/4th order XO. Acoustic or 'golden' ratios are "de rigour" with big cabs.
    GM
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  14. Julian Data

    Julian Data Second Unit

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    What's the golden ratio? I recall seeing this somewhere.
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  15. Greg Monfort

    Greg Monfort Supporting Actor

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    Golden ratio:
    1.0:1.618:2.618
    Some acoustic ratios:
    1:1.17:1.47
    1:1.45:2.10
    1:1.28:1.54
    1:1.25:1.60
    1:1.14:1.39
    1:1.26:1.41
    GM
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    Loud is beautiful, if it's clean
     
  16. kevin oswald

    kevin oswald Agent

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    I was reading through another forum earlier.. that has some insite on using Baltic Birch vs. MDF, I thought you find useful. I can not rewrite it properly.. and do it justice.. so just copying it.. but leaving the name at the bottom to give the person credit.
    "First consider the variables we can control with a fixed volume and rough dimensions pre-determined. Bracing has 2 effects on a box in that they will strengthen the walls of a box, thereby resisting flexing from high internal presures, and at the same time serves to divide the area of unbraced panels which raises the lowest resonances of the panels. Now if we double up on panels, we increase the strength, or resistance to flexing, along with increasing the mass of the panel, which actually lowers the resonance for the same panel area. If I remember correctly, the increase in thickness will benefit to lower the magnitude of the resonances, so we get into a matter of how significant and at what frequency the panel resonances end up.
    An issue I hadn't thought of before was pointed out to me recently where a box was made of 1.5" thick Baltic Birch(B.B.), and the box resonated quite severly somewhere in the lower midrange. If we examine this, though, it makes sense. B.B. is very strong, but it does resonate a good deal, albeit at a higher frequency than MDF. This higher frequency resonace is why it is rather ideal for subwoofers. If we double up the thickness of B.B., we will reduce the magnitude of the resonance some, but we will also move it lower in freq. and closer to the operating range of the subwoofer. Now Chris A., a.k.a. HealthNutt, used even thicker B.B., and I imagine his box is rigid enough where it won't be much of an issue. My personal feeling and inclination is to just brace a 3/4" B.B. box as well as possible, putting any resonances well out of the operating range of the sub. Comparatively, MDF's resonance is rather low, so for main speakers, thickening the panels makes the resonance both lower and less in amplitude, which works great as most mains have limited low frequency energy. Again, the panels get quite rigid, and at 3 layers, are pretty close to concrete type properties, so I guess we have to keep it in perspective.
    I don't claim to be an expert on this, I'm just outlining my own thoughts... I'm looking for experiences and oppinions, so let's hear 'em guys.
    Mark Seaton "
     
  17. Julian Data

    Julian Data Second Unit

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    Thanks for the info guys!
    Kevin,
    I recall reading that at the AVS Forum. [​IMG]
    I am doing Marine Ply about 2 sheets.
    Doubling on the front, rear, sides. There will be bracing inside the enclosure about every 12" or so. The way I have the braces configured it like are eight miniature 1.1 FT3 enclosures.
    I plan to add some wheels at the bottom with a skirt to hide most of the wheels so roughly a half inch would be showing underneath.
    jd
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