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Old 7th July 2009, 03:32 PM   #11
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Default Stiffness

I have been thinking about this stiffness issue and just how stiff a sand filled panel enclosure needs to be....
Someone PLEASE correct me if Im wrong here,
For a loudspeaker enclosure we require stiffness for only 2 reasons that I can think of. One, so the enclosure doesn't expand and contract with the pressure changes from the woofer and two, to raise the sympathetic panel resonance frequencies so they are less audible.

Even if I completely remove the steel tubes from my enclosure above and simply used two 1mm sheet metal walls and fill the cavity with sand it would still out perform a well braced timber enclosure. Is this so?
I don't intend to actually do this as it's easier to include the frame, it's simply the notion of a truly dead panel not requiring stiffness Im questioning.
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Old 7th July 2009, 05:46 PM   #12
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I agree. In that case you don`t need the diagonal braces at all.
If internal reflections worries you, you could cover the inside with heavy carpet tiles or something.
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Old 7th July 2009, 07:17 PM   #13
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Quote:
Originally posted by Peter M.
Why diagonal? If you make them straight across it will be stiffer.
Diagonal braces increase the shear rigidity.
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Old 7th July 2009, 07:48 PM   #14
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So why not building a geoidal structure composed of many polyedres ?
C'mon ! The force of the pressure of a moving membrane can be easily handled by the normal materials usually found in loudspeakers . Eventually the entire surface could be covered by thick steel ...or put in an electro galvanycal bath to get that silver finish.
Just joking
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Old 7th July 2009, 11:43 PM   #15
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Diagonal braces aren't needed, the steel sheet will provide more than enough shear bracing. The sheets might bow outwards slightly under the pressure of the sand, so you may need cross braces attached to the sheets to prevent this.

I'd be more concerned with the cosmetics. Unless you're deliberately trying for a steampunk look, they won't look pretty. You could consider having stainless steel cosmetic covers bent up to slip neatly over the cabinets, similar to the covers available for covering ducting for extractor fans in kitchens. They are also available ready-made in a variety of sizes and lengths, if your cabinet dimensions are compatible. They have a lip on the open edges ideal for neatly fitting between the cabinet body and the baffle. Just add a neat wooden top cap for Spousal Acceptance Factor.

Since many of the ready-made duct covers have a "U" profile, you could roll some steel sheets to make a half-round profile back for your cabinet to match the cover. Even higher SAF.
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Old 7th July 2009, 11:50 PM   #16
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Ahhhh ...hA! Now I understand why the tall and narrow boxes are becoming so popular !
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Old 7th July 2009, 11:56 PM   #17
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... and since you're using 1 mm steel sheet, you aren't constrained to a flat sided cabinet. You could roll the sheets into an ovoid profile, with round or square section tube verticals at intervals to maintain the inner and outer wall spacings. If you fasten (weld / rivet) the spacers to the inner wall and make the outer wall a snug fit over them, you wouldn't need to fasten the outer wall to the spacers (no welds or rivets to hide). The outer wall could then be stainless steel for a ready-made finish.
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Old 8th July 2009, 12:14 AM   #18
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OK , they're done ,and well ,too .
I'd suggest black galvanization not to interfere with tv when watching movies.
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Old 8th July 2009, 02:59 AM   #19
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Rigid, stiff, dead... whatever the cabinet structure is, there is an inevitably weak link in the whole thing -- driver's diaphragm.

The pressure change, internal reflections, or standing waves... etc inside the box would escape through the weak point -- in this case, the driver. (And, outside the box, there'd be room acoustic issues which play an even bigger part of the game.... )

I know it'll probably bring up some arguments, but this is my own first hand experience.

I have built some big, heavy, dead boxes with heavily braced and damped internal structures, they still boomed like hell with little to no vibrations on the box panels. So it's obvious not the boxes, but other things I did wrong....

Sorry for spoiling the pleasure, but I really don't think it's just so important to build an invincible box. Good enough is good enough. There are many other things to worry about before or besides this.

A creative and original building technique is excellent, though.

If I can handle metals, I'd probably build some space frame with transparent skin or just leave it OB -- I mean, metal skeleton alone is spectacular enough! Why cover it?
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Old 8th July 2009, 08:53 AM   #20
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Don,
great idea's mate, and easy too. I'll be giving that some thought. Ovoid provides the strength to hold it's self. No uprights required just spacers. Nice.

I wonder how expanding foam would go instead of sand......


CLS,
I've arrived at these methods because Im trying to avoid using timber for several reasons. If my method turns out to be an 'invincible box' it's something of a bonus, not a requirement. You already have the invincible box ...but OB has it's own set off issues that Im not prepared to deal with and pay for.
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