I think I finally found my marbles! And a question

[cdwitmer]

Somebody wants to give me more than three gallons of epoxy cement. Somebody else wants to give me 20,000 11mm chrome-plated steel ball bearings. I'm thinking I'd like to use these materials to make a pair of composite subwoofer enclosures. I should think that a network of steel ball bearings epoxied together (two or three layers deep), in combination with a strong outer shell of 3/4-inch plywood and some judicious internal bracing, ought to make an extremely strong, vibration-resistant enclosure. I also have a few sacks of fine lead shot leftover from my days as a shotgun shell reloader that I could use as "filler" in the gaps between the steel ball bearings. (That would also reduce the amount of epoxy required.)

Here is my question: If I wanted to create little "pyramids" of ball bearings on the inside of the enclosure in order to prevent the formation of standing waves, how large should they be in order to be effective? Or will the inherent "bumpiness" of the flat layers of ball bearings on the inside walls of the enclosures be adequate in that regard? (I'm inclined to think I could still get standing waves with just flat layers of ball bearings.)

Any other comments, suggestions, ideas, or even analyses of my mental health would be greatly appreciated.

Christopher Witmer
Tokyo

[ThomasW]

The wave lengths of subs are so long that standing waves inside the box aren't an issue.

As long as the Fs of the cabinet is out of the passband of the woofer; it doesn't matter what the cabinet is made from, as long as there's no wall flex.

[cdwitmer]

Great, that means my approach to building can be greatly simplified: just go for maximum rigidity and mass.

[ThomasW]

Actually all you need is the rigidity. An Fs above the passband is fine too.

Baltic/apple/birch ply ~15 layers per 3/4" is great stuff. If you want absolute overkill laminate up 1.5" for exterior walls and use bracing every 8"-10" or so. (note that 1.5" will be HEAVY!)

[Brandon B]

Man are you outside the box. Er, thinking of a weird box.

You must have a big place for Tokyo for all those ball bearings. You could do some pretty cool stuff with them with a bunch of huge magnets.

BB

[David Meek]

three gallons of epoxy cement. Somebody else wants to give me 20,000 11mm chrome-plated steel ball bearings. I'm thinking

Too much TV for me. . . I was thinking CD was going to build a giant anti-personnel mine (Swordfish flashback)! :

[cdwitmer]

Actually, it did occur to me that if the sub flops, stuffed full of ANFO it might make it into the Guiness Book of Records as the world's largest claymore mine . . . and a way to blow away all rivals at the next SPL competition.

[Brandon B]

Take them all to Shinjuku ekki in a couple of big duffle bags at rush hour and let 'em go!

BB

[David Meek]

Shinjuku eki? Google wasn't very helpful.

[ThomasW]

David,

Shinjuku eki

[cdwitmer]

I think I'll stick to "detonating" it in my living room . . . once I get the sucker built, it isn't going anywhere without a block and tackle. It better not bomb!

[Ten 99]

Are these ball bearings going to be visible on the outside of the sub in some way? If it's not more than an asthetic thing, then I guess I'm having a hard time following the purpose.

It isn't just to add mass is it? I would think that it would be mass loaded enough with thick walls of MDF and some plywood bracing. I don't see the ball bearings adding any strength. If they do add strength, could someone explain how? I can't figure it out. Sounds neat and all, but I'm perplexed.

[cdwitmer]

The amount of additional stiffness from ball bearings depends on how they are used. When you join the centers of closely packed spheres, you get tetrahedral or octahedral cells, which, if epoxied into place, ought to have a certain amount of resistance to displacement. Frankly, I have no idea if the resulting increase in stiffness is worth writing home about. More importantly, having significant quantities of dissimilar materials in the enclosure will certainly tend to deaden vibrations, as each of the different materials will want to vibrate at different frequencies, so they "fight" with each other and resist each others' tendencies to vibrate. Even so, I'm assuming that the number one cause of wall movement in a sealed subwoofer enclosure is air pressure, and traditional cross-bracing should be highly effective in resisting that. Thus the main rationale for using these materials is "because I can." The other benefit I anticipate is magnetic shielding, although one hardly needs to go to such extremes to accomplish that . . .

[David Meek]

I think I'll stick to "detonating" it in my living room . . . once I get the sucker built, it isn't going anywhere without a block and tackle. It better not bomb!

CD, do you have any SAF to deal with?

[cdwitmer]

I have not one, but eight shes who must be obeyed. This sub is part of a "package deal," a much grander scheme whereby I shall be divesting about 20 objects of scorn (which you and I usually refer to as "vintage audio components"), and thus there shall be a net "slimming down" of the unintentional audio museum. If all goes according to plan, this will make all of the shes happy. Even so, I shall have to exercise what is for me considerable self-restraint and careful planning, lest I resuscitate unpleasant memories of audio projects gone awry. (I was only kidding about the need for a block and tackle, by the way.)

[cdwitmer]

BTW I am not a polygamist but a father of seven girls who think just like their mother.

[David Meek]

I have not one, but eight shes who must be obeyed.

Eight!?!?!? Jumping jackrabbits Batman! How'd you survive all this time? :

(I was only kidding about the need for a block and tackle, by the way.)

No, you weren't - think about it. :yesnod: I put 22 pint-sized boxes of steel BB's in my Lovan Classic II stand's multiple levels of legs and the box that I carried out of the Walmart I bought them at weighed 75-80 pounds. You are going well over that from what I've gathered from your comments. Okay, maybe not a B&T but some serious back and leg muscles. . .