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Thread: Box Bracing - A Question

  1. #1
    Senior Member Ducatista47's Avatar
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    Box Bracing - A Question

    This is not being posted in DIY because this box will not involve any products from the JBL extended family. I am building a pair of bass reflex boxes for 12 inch single driver (very extended range driver to the uninitiated) transducers. The speaker is mounted near the top of the baffle, seated listening height in fact, with a small tubeless port much lower down. The tube is actually the 3/4 inch thickness of the lumber.

    The idea here is a tuned pipe 13x13x46.5 inches, inside dimensions. It needs to be very well braced. This design has been built with a multitude of internal bracing schemes. The attached image is of a typical build.

    My question arises when I imagine this box with external bracing. An internal window brace below the speaker is an oft used and obvious choice, but experience shows vertical or even X bracing is an effective additional enhancement. Ignoring cosmetic considerations - and I always do, I'm hardcore - has anyone ever seen external bracing implemented?

    The interior of the enclosure will be thoroughly dampened, so no additional bracing is needed inside for that reason. Picture the ribs inside the hull of a wooden ship, or the flying butresses of a gothic cathedral. Better yet, like the internal bracing of an acoustic flattop guitar. Strips of wood (well, probably cut planks of plywood mounted perpendicularly) running vertically or in X patterns on the outside of the sides and back. Perhaps a couple on the inside of the front baffle.

    Would this be effective?

    Clark in Peoria
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    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


  2. #2
    Senior Member Ian Mackenzie's Avatar
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    I would refer to a posted review that discusses the internal construction.

    The bracing should be internal and not be anthing too clever. Use horizontal 3 x 1 inch hardwood 1/3 spacing with two front to rear struts and side to side struts. Glue with PVA and scew all joints.

    Do not attempt to over brace the box or partition the enclosure.

    This will control flexing at low frequencys.

    Use MDF for the walls.

    Follow the designers directions to the letter on damping to control standing waves.

    Use a tube port to adjust the tuning.

    Your valve amp outout impediance may dictate the final tuning

    This will make or break the end result

  3. #3
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    I wonder just how much a 3/4" mdf wall would flex on a 14" span. If you support the panels on the long side on saw horses and had a 200 lbs man stand in the centre line , ~7", how much would the panel flex? Would a 1" panel flex? The corners should be effectively locked in by the panel in perpendicular.

    Jorg

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    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by 89-300ce View Post
    I wonder just how much a 3/4" mdf wall would flex on a 14" span.
    Far too much.

    MDF has the structural integrity of al dente pasta.


    Widget

  5. #5
    Senior Member Ian Mackenzie's Avatar
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    There are numerous people who have built that box for the Super 12 on the various full range driver sites so I am sure it will work out.

    Most peope only drive them with only a few watts..they have fairly low X max.

    Ian

  6. #6
    Senior Member Ducatista47's Avatar
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    Ian, I know you are very familiar with my project. Thank you for setting me straight.

    I have read how slight changes in the tension of a brace will make the midrange sweetness come and go. This design is touchy for sure. Still, could you tell me why MDF is favored over hardwood ply? I had my sights set on plywood, but if it is a bad idea I will forget it. I am not currently set up to safely work with MDF, but I can work it out.

    Also, what are the effects of over bracing the box? Is the tone again muffled? I just want to know what to listen for.

    While I am going to build the design exactly, the reason for all my curiosity is this from James Melhuish in 2005:

    "My understanding is that John designed the Super 12 driver very carefully, and with lots and lots of listening and tuning. But I do not believe that the cabinet design is "perfected" in terms of what may be possible. Likewise, of course, the supertweeter and crossover network probably have potential for improvements. In the end, I think that John would be thrilled to have someone take his Super 12 cabinet design and experiment with it: change out the peanuts, swap bubble wrap out for something else. Listen and improve."

    How could I not rise to that bait, at least to the point of daydreaming of other ideas? This project aside, I can think of some good points of using exterior bracing in perhaps some more experimental designs and am still looking for some feedback in that area. I see no good reason other than appearance why bracing must be internal. And I have some ideas of how to make it a stunning visual statement as well. If not for this box, then for other future projects. I'm a little bored looking at boxes, but the contemporary alternative speaker designs I have seen look downright ugly and stupid to my old artist's eyes. To the point of thinking, "What miserable f***wit thought this was any good?"

    Clark
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    Too many audiophiles listen with their eyes instead of their ears


  7. #7
    Senior Member Ian Mackenzie's Avatar
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    Clark,

    As a 1st time diy speaker builder Rule No 1 is DO NOT aim at perfection on the 1st attempt. This will be a learning curve.

    Its best not to cut MDF in the dining room. You could use marine birch ply or something similar but I would not unless you are experienced in box building and know exactly how it will turn out.Maintain the dimensions etc and just do it.

    My recommendation is that you follow what someone has done and see how it goes. Talk to the Hammer Dynamics people, she is quite knowledgeable. Build a couple of test boxes to start with of you like.

    What is most important is to get the internal damping right.

    Then fiddle with ideas like sand filled partitions.

    Have a look at the Zu Cable systems if you want some design ideas and their excellent user guide.

    Your focus should be to get a working box up and going. Use the recommend crossover spec and parts and wire.

    8/10 th's of what it can do is how you set it up. So play close attention to the room tuning as discussed in the Zu Cable guide.

    http://www.zucable.com/druid_2.html
    .

    If you read the review here:

    http://www.enjoythemusic.com/magazin...00/super12.htm

    ""The woofer is to be mounted in a bass-reflex vented 4.5 cubic foot enclosure. My cabinets, as built by Hammer Dynamics, are simple unbraced boxes, constructed of ¾-inch MDF, glued and screwed together. Of course, if you build your own, you have complete control over materials and construction techniques. The most unusual aspects of the enclosure are the materials used for internal damping. The inner walls are lined with bubble wrap. In addition, a square piece of felt is mounted on the rear wall behind the woofer. Finally, the cabinet volume, up to the lower lip of the vent, is filled with Styrofoam packing peanuts. Unorthodox to be sure, but Wyckoff assures me that these materials work extremely well in controlling internal standing waves, while being very cost effective. If you overfill the cabinet with peanuts, you’re in for quite a treat. It’s a cheap thrill, but highly amusing to watch bass transients grasp hold of, and then blow peanuts right through the vent."

  8. #8
    Senior Member Ducatista47's Avatar
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    Hi, Ian,

    I am in total agreement. My bottom line is that it will be faster and more sure to use MDF and build unmodified. And a lot cheaper. But...

    Actually this is not my first dance. I have built good sized cabs before, out of MDF and marine ply. I built my first pair about 1975.

    The difference is, before I table sawed the rabbets with a dado; this time I have a router. Seeing Rick demo the tool made me a believer. The MDF dust will be hell with a router. I can wear breathing protection, but the clean up... I have kids and pets. It is too snowy and cold outside just now to go outdoors.

    So I was concerned only with the sonic differences between MDF and Ply, and possible additional bracing beyond what has been suggested. I have next week off, a perfect time to build. Waiting for outdoors, it could be months, and I would be restricted to the odd free weekends.

    Truth be known, I think I've read and re-read every reference on the web concerning these speakers. Done my homework. (In picking through search results for Super 12, you can learn a lot about rugby!) The following did get me thinking.

    "Written by hitsware at 13 Oct 2003 05:43:27:
    As an answer to: What are the cabinet No-Nos for H-D Super12? written by NewbieBaby at 13 Oct 2003 04:59:38:
    >In designing a cabinet for the H-D driver, can the cabinet be made too rigid?

    According to the major school of thought, No . "

    True of false? was my bracing question, I guess.


    Clark

    EDIT: I just checked at my favorite local high quality lumber yard. (It is a great place, locally owned and in business forever. Hardwood specialists, too. The combination of the old gentleman behind the counter and his young yard man is the best I have ever seen.) I can get 3/4 MDF, Baltic Birch ply or Marine Ply (my favorite). They will precision cut it all for peanuts, so it is just rabbet, glue & screw for me this go around. On to parts Express for damping.
    Information is not Knowledge; Knowledge is not Wisdom
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  9. #9
    Senior Member Ian Mackenzie's Avatar
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    Clark,

    I think its an obvious answer ( a lot of diy post on this subject are BS) but as i said youc an to too far..simple engineering.

    Unless you plan to throw it over a cliff dont get too excited about bracng.

    Ask your self the question what is the bracing (not a B&W honeycomb) for? It makes the panels more rigid and reduces flexing. Useful at low frequencies only.

    Does it control reduce panel vibration at frequencies? No

    MDF is one of the best materials in term of what happens after is absorbs sound energy. (The JBL Everest uses MDF)

    We are not building a box for a 2245. This is a full range driver hi sensitivity driver..

    As a said earlier effort must be focused on controlling the rear wave

    I made mine out of mdf then painted it like the Zu.

    I think I laminated the front panel from 2 x 3/4 inch mdf.

  10. #10
    Senior Member Ducatista47's Avatar
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    Thanks, Ian.

    I'll shut up now and go buy some MDF. Do you mean a double thick front baffle? And did it help?

    Do you think recessing the speaker flush is a cosmetic statement or a good thing to do? It was recommended in all the other cabs I've built, but since John Wyckoff and GT (see 4345!) don't bother with it, I have to wonder.

    For what it's worth, while I have not seen them in person, I think the Everest II is one new speaker that is not at all ugly.

    Clark
    Information is not Knowledge; Knowledge is not Wisdom
    Too many audiophiles listen with their eyes instead of their ears


  11. #11
    Senior Member Ian Mackenzie's Avatar
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    It adds mass + rigidity to the front baffle....we dont want the forward facing baffle to emit any sound of its own.

    It more work to recess but looks nicer.

  12. #12
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    Not an expert at woodworking, but Altec recommendations on bracing here:
    http://www.lansingheritage.org/image...ign/page18.jpg

    Altec recommends using rectangular straight lumber, on the edge.

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