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Rules for Swiss cheese-ing your braces?

Discussion in 'Amps and Cabs [BG]' started by V63, Feb 28, 2014.

  1. V63


    Sep 13, 2008
    Alberta Canada
    Question for my first build here:
    For bracing that will be made out of 3/4" plywood in 1 1/2" strips how big can the holes be and how close together for Swiss cheese, aviation type braces.
    When bracing a one-piece, removable, screwed-in back panel that is 18 7/16" X 24 7/16" would a vertical and horizontal "+" or a diagonal "x" bracing pattern be more effective?

    Thanks everyone!
  2. Jaco who?

    Jaco who?

    May 20, 2008
    Your using 3/4 ply, I doubt you need any bracing other than from the front to the back between the cutouts for strength. Most bracing is needed to eliminate resonance. when you rap your knuckle on each panel, can you hear a tonk, or is it more of a higher pitched tick sound? If the panel doesn't resonate, additional bracing will just add weight, and cutting holes on that 1 1/2" wide strip won't make much difference in weight.

    + pattern is best because bracing more towards the middle of a box eliminates resonance, and an x pattern bracing the corners would not.

    Most of the really strategic bracing is needed for 1/2" ply where eliminating resonance without adding too much weight is trickier.
  3. hdracer


    Feb 15, 2009
    Elk River, MN.
    Why not use fur strips?
    Cheap and much lighter than ply
  4. V63


    Sep 13, 2008
    Alberta Canada
    I have the birch on hand so that's what I was going to use. The cut-outs in the bracing is for saving volume in this case, not wieght. The back panel is removable and just screwed in with gasket to seal it, so I'd like to brace it.
    I believe it's more of a "tonk"
  5. Hi.

    One rule of thumb is 1*s (s=material thickness) for maximum weight reduction on un-stressed structures, 2*s for a happy compromise or if there are dynamic loads involved.

    Makes absolutely no sense in Your case IMO.
    Just craft curved braces if You're concerned about weight. EDIT: Or volume.

  6. The stiffness of the cheesed brace is virtually the same as uncheesed, so long as it won't break. Go to town on it.
  7. V63


    Sep 13, 2008
    Alberta Canada
    Please forgive my complete ignorance.

    So let me see if I've got this right: for 3/4" Baltic Birch the holes could be 3/4" and how far apart?

    Not sure if I know what you mean by "curved braces"

    If I take 18mm Baltic Birch cut it into 1 1/2" wide braces with a 1" circle cut in middle with a 1/4" space between circles this saves me 0.55658 cubic inches every inch and a quarter (or 0.445264 per inch). So that 1.3287 cubic inches for every inch and a quarter I use becomes 0.77212 cubic inches. Seems like a big internal volume savings to me. Looks OK when I draw it on paper but am I taking away too much material? I've got under 34.17668 cubic inches in my volume budget to brace a removable back panel.

  8. astack

    astack Supporting Member

    Nov 12, 2011
    St. Louis, MO


    You need stiff braces, not strong braces. That's mostly a function of how "deep" your stiffener "beam" is; the stuff in the middle just doesn't participate much. In other words, think about trusses - HUGE swiss cheesing.

    Speaking of trusses, check out how they drop the last triangle typically, or even taper off away from the middle. That's just like the suggestion to do curved braces (deeper/wider in the middle).

    As for max spacing between braces, I would say 20 x the panel thickness. So 10" for 1/2" and 15" for 3/4" ply.

    Your volume is like 3 1/2 cf. Calc'ing to the bajillioth of a cu. in. is over kill. Seriously. If pi can be taken as 3 for nuclear engineering, going to the tenth of a cf is plenty precise. What is the volume of the 3015 to the 10,000th of a cu. in. anyway? ;) Plus that driver is really forgiving on volume. One of the reasons it's so popular here.

    Make sure that gasket makes a very good seal. That with botch things very quickly.
  9. V63


    Sep 13, 2008
    Alberta Canada
    Thanks astack,
    I think you meant to say that I had 0.019778171290 cubic feet.
    Just joking!

    Sorry about the decimal places. I've been schooled.
    Straight lines I can cut and calculate. Curves? Not so much.

    Any suggestions for not botching up my gasket seal?
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    MESA has some cabinets that employ aircraft type bracing.

    If you look at wing rib construction where they use holes as an example, the holes are of different sizes depending on the height of the rib. They are positioned to provide strength and minimize weight. What is important to note is that the braces and other structure come together as a unit. Some braces are smaller and weaker than others but overall the structure has the strength that it needs. There isn't a single rule that applies on how to do it.

    You can see the relative positioning of the holes in these images.




  11. astack

    astack Supporting Member

    Nov 12, 2011
    St. Louis, MO
    Ha, wrote that on the train coming home from a show a bit drunk, so pardon the tone...

    Point being is it's hard to tell what's worth spending brain power on when you're starting out with something. Do the calcs, get in the ballpark, build it, see how it sounds, tweak as necessary. My guess is you won't need to tweak much.

    As for the gasket, I don't have any experience. I've only thought about it and I imagine going through the debugging of finding the cause of a buzz or a weird honk. Not impossible and maybe I'm blowing it out of proportion. Respect the gasket. :D