Panel flexing even with cross bracing in a sealed sub - Page 2 - AVS Forum
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post #31 of 40 Old 09-27-2012, 11:10 AM
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Not getting at anything. I just think there is a better way to do it. It's really simple, not complicated. Wide braces. I just need some figures/diagrams to explain why it works better. Maybe your braces already do what I'm thinking about.

The image is just a diagram I was able to find on the internet about slender column design.

Here is a better one I managed to find: http://www.ejsong.com/mdme/modules/fea/Buckling.GIF

Because the panel is "fixed" at the corners, we have the second from the left case.

Now look at this: http://1.bp.blogspot.com/_721bPXg5FDs/TVA9Q1RzCQI/AAAAAAAAAME/BN0Zp1e11oM/s1600/fig.jpg

By adding a cross brace you add 1 or 2 or nth nodes to the panel thus reducing the effective L that can vibrate. That's because a dowel or a 1" wide brace acts like a node. If you use braces that are say 3 or 4" wide (even if you want it light, just cut out the middle portion) then you aren't adding a node, you add a fixed brace which reduces L again by 0.5. I just can't find the last diagram I need to explain that part.

It's why holey braces are so effective. But I'm proposing something smaller than holey braces, yet close to the same effect. Nothing new, many have done it I'm sure. Possibly everyone does it. I just don't see many pictures of it. I usually only see 1" strips, dowels, or holey window braces. The best brace for the weight is a 4" wide stip, imo.
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post #32 of 40 Old 09-27-2012, 02:48 PM
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I'd be more worried about your room flex, you lose more SPL there because of the greater surface area. Go Go Gadget Green Glue tongue.gif
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post #33 of 40 Old 09-27-2012, 03:19 PM
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I have an idea to visualize flex/vibration if anyone wants to try it. Lemme know if it sounds like a sane thing to try.

Put some sticky dots on a surface of the sub box in a grid pattern or whatever, turn the lights out and play some test tones. Use an adjustable strobe light to look at the dots using the aliasing effect (x < f < 2x the frequency of the vibration/flexing).

My theory is it will appear in slow motion and you can see the dots moving up and down and back and forth and be able to tell the difference between flexing and vibration. It's the same idea as using a strobe light on a fan or timing belt.

Similar to this video:
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post #34 of 40 Old 09-27-2012, 05:00 PM
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Quote:
Originally Posted by Stereodude View Post

SPL = 20 Log(p/p ref)
p ref = reference pressure in pascals = 2 x 10^-5 Pa (pascals)
1 Pa (Pascal) = 0.000145037738 Psi (Pound Per Square Inch)
I made the assumption his sub's output was under 130dB and did the math.

I believe the SPL inside the enclosure will be much higher.

Or just do the simple gas law calculation: % volume change of box from cone excursion x 14.7 psi.

I just did for my abovementioned sub and got .25 psi.
Quote:
Originally Posted by tuxedocivic View Post

There is an even more effective bracing method than that Bill. I can't find a picture of it online because no one is doing it, so I'll have to take one. I'll take some scans out of my structural engineering text books sometime as well.
http://ars.els-cdn.com/content/image/1-s2.0-S0141029600000365-gr5.gif

Not sure what you're getting at either, but maybe it's stress stiffening, like tightening a guitar string to increase its resonant freq.

You could do that by using crossbracing slightly too long, or too short and screwing into it from outside to pull the panels up to it.

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post #35 of 40 Old 09-27-2012, 08:20 PM
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Using simple beam theory, deflection is related to span ^ 4. So 1/2 span gives 1/16 deflection. Very approximate for 2D bending only.

Cheers,
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post #36 of 40 Old 09-27-2012, 08:55 PM
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Quote:
Originally Posted by Peter M View Post

Using simple beam theory, deflection is related to span ^ 4. So 1/2 span gives 1/16 deflection. Very approximate for 2D bending only.Cheers,

Bending stiffness varies with the cube (^3) of length, same for section height.

It would be ^4 if increasing the dimension in 2 dimensions, as for a round or square section.

Noah
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post #37 of 40 Old 09-27-2012, 09:41 PM - Thread Starter
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so i take it that nobody else is having panel flexing and/or vibration with their build?



my second pair which is also built with 18mm MDF also exhibit the same vibration albeit at a much reduced manner. probably because of the less capable 15" driver used (dayton dvc15).
i doubt the vibrations/flexing are caused by panel resonance as the side panels are of difference length (slanted build) and they still exhibit vibration when only one driver are mounted (the other woofer mount sealed with MDF).

i'm not able to mount the ixl15 in this enclosure for testing as the drivers won't clear the side panel. too bad.

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post #38 of 40 Old 09-28-2012, 08:59 AM
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Quote:
Originally Posted by noah katz View Post

maybe it's stress stiffening,

No that wasn't it either, but once I drew up the diagrams I was thinking about, I realized where I went wrong. The concept of buckling doesn't apply. Well, it does, but not how I was thinking. Major disconnect. Oh well.
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post #39 of 40 Old 09-28-2012, 02:06 PM
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Quote:
Originally Posted by tuxedocivic View Post

The concept of buckling doesn't apply. Well, it does, but not how I was thinking.

How so?

Buckling, also called elastic instability, only occurs in compression loading of thin members, which is not the situation here.

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post #40 of 40 Old 09-28-2012, 03:02 PM
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It applies to the brace if it's suffeciently slender (a dowel), but as I already said, what I originally was thinking doesn't work how I thought.

I've never heard it called elastic instability. That phrase sounds like it can be applied to much more than just column buckling.
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