stiff walls, TL

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stiff walls, TL

Postby Bob » Sat Jul 17, 2004 4:57 pm

I was chatting with someone the other day and I realized I didn't know if stiff walls were good or bad for TL.

I seem to recall that flimsy flexible walls lower the mass-spring-mass dip (e.g. 80hz) to a lower frequency.

But below mass-spring-mass there's the 'stiffness controlled region', which I'd assumed meant that stiffer was more TL.

Is this one of those, 'you have to consider the entire wall as a system' things, or is there a simple rule that more stiff is best?

Eric once wrote something like: Raise the coincidence dip (near 2khz) as high as one can, and lower the mass-spring-mass dip (near 80hz), gives a good wall. E.g. a wall with a coincidence dip near 4khz and a mass-spring-mass dip near 20hz is likely to be good at TL too.

In this case what I'm asking is not how many layers of gypsum to put on each side, nor how thick each different layer should be.
Assuming the layers are kept constant, and the spacing between leafs is kept constant, and the insulation is constant, what should be done with the studs?

For example, given a double stud wall as follows:
{3 layers of 5/8" gypsum, wood studs with insulation with 2x4 top and bottom plate, 1" air space, wood studs with insulation with 2x4 top and bottom plate, 2 layers of 5/8" gypsum)

What would be the approximate in-general theoretical effect of
a) wood studs as 2x4's on 24" centers
b) wood studs as 2x4's on 16" centers
c) wood studs as 2x4's on 16" centers, with horizontal stifferners mid hight
d) wood studs as 2x6's on 24" centers
e) wood studs as 2x6's on 16" centers
f) wood studs as 2x6's on 16" centers, with horizontal stifferners mid hight

(I would have written 36" centers, but I'm not altogether sure I can build walls like that. I might be able to build walls on 48" centers)

In the case of the 2x6's they would have to be offset, that is the studs of one wall could not line up with the studs of the 2nd wall, because they would touch. Actually they'd do more than touch, they'd occupy the same space.

Also, the 2x6's would need to be notched top and bottom so that they don't touch the top and bottom plate of the oppostite wall. i.e. the 2x6's are sitting on the 2x4 bottom plate, with a 1.5"x1.5" notch cut out at the top and bottom.
Regards
Bob Golds
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Postby Brian Dayton » Sat Jul 17, 2004 6:45 pm

one thing to consider when aiming to stiffen something is dimensions. if you have a simple bar - like a stud in your wall - it's bending stiffness (imagine you trying to push the wall) will vary by (i believe the following to be correct:)

the cube of stud depth (a 12" deep stud will be, in theory, 8* stiffer than a 6" deep stud) (that's why it's so much easier to bend a 2x4 the thin way than the thick way)

inversly by the cube of stud length (a 20' span will be 8* easier to bend than a 10' span) (easier to bend a 10' metal rod than a 1')

and linearly to stud width (a 3" wide stud will be 2* more rigid than a 1.5" wide stud)

if you have access to a library, you should be able to find a reference that would allow you to calculate the composite flexural stiffness and fundamental resonance points of any structure that you wish to ponder.

Stiffness will cause TL to rise under the primary resonance of the wall in question. for a 2x4 wall that is not, i don't believe, the 80-200hz resonance (that's the panels between the studs), but a different resonance at some lower frequency.

pondering: the immense dimensions of rooms leave minimal hope for stiffening improving TL at anything but very low frequencies. (length cubed)
Last edited by Brian Dayton on Tue Aug 03, 2004 9:34 pm, edited 1 time in total.
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Postby Scott R. Foster » Sun Jul 18, 2004 11:14 am

If you want to frame a stiff timber wall the first thing you often need to do is beef up the top plate... most framing systems in the real world are weakest at the top... just doubling the top plate has an enormous effect on most timber frames. If you ever need to build a frame with no top connection such as for a island bar [the kind you drink adult beverages on - set up in the middle of the room - free from connection to the walls] you will be amazed at how much the system tightens up by the simple expedient of screwing down a second top plate.

The next common approach to additional stiffness is to make the top and bottom plates wider and run double studs... just as in acoustic double stud designs, except n this case you are looking to give the wall a wider footprint as opposed to achieve mechanical disconnection of the two sides.

Another trick is turn your framing members into beams... two studs screwed together into "T" cross-section are not merely stronger in compression and shear strength [both back and forth in the wall and side to side], but they are much more resistant to torsional deflection [twisting], and all these factors come into play with both practical stability [does the bar wiggle when 6 Vikings pound the top with their mead horns, or is it solid as a rock?].

Add these three elements of design together and you can build a very stiff framed wall even when to only connection is to the floor.

But I don't see that any of this helps much in a "portable" system. I assume you will not be able using anything as heavy and mundane as conventional timber framing and keep a "portable" attribute to a wall... Aluminum alloy beams and panels with a honeycombed integral framing might be more the thing [an aeronautical approach to strength/stiffness in lightweight framing and panels?].

Also, stiffness is an element to be sure, but I would reckon that in order to get LF TL on an industrial scale from a "portable" partition the really tricky parts will be sealing the edge boundaries [and auto magically resealing same with every new placement] and creating mechanical disconnection of the two sides of the structure/partition via a "loose spring" with a very low fundamental resonance.

BTW, why does the wall section need to be "portable" - what is the application?

Good Luck
SRF
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Postby Bob » Sun Jul 18, 2004 6:27 pm

I ran my question through insul84SA.exe and it says that there's no difference in changing the stud spacing for a double stud wall.

But, when I ran the same walls shown in IR693 pg 80 through insul84SA.exe, it didn't predict what was observed either.

http://www.bobgolds.com/StudSpacing/home.htm

Is stiff better than flex for music TL in a double stud wall?
Regards
Bob Golds
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender
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Postby Brian Dayton » Sun Jul 18, 2004 7:29 pm

insuls big limitation seems to be the low frequency region. i find, at least at times, that the system predicts identical low frequency response (or very close) for any selection of wall (staggered, steel, double studs, etc.)

it nearly discounts the mass-spring resonance
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Timber Studs and TL

Postby sam » Tue Jul 27, 2004 10:47 am

Just watch out that the noggin spacing does not induce a standing wave!

I cannot put my finger on it but there has been some study regarding this effect.

Other things that can improve the TL include using different types of sheeting materials on the same side and both sides, different stud spacing (closer) and using more nails/screws to fix the sheeting that normal - this last one I think was a study in Canada. However, I imagine all these things do not add a lot.

What is important to also look out for is flanking into other parts of the structure ie there is no point having a wall with such a high TL if there is weakness in the supporting structure.

Hope this helps :-)
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Re: Timber Studs and TL

Postby Eric.Desart » Tue Jul 27, 2004 11:26 am

sam wrote:Just watch out that the noggin spacing does not induce a standing wave!


Sam, what does mean noggin spacing ?? (sorry I'm Dutch; :? restored: not sorry, I'm just Dutch speaking)
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Noggin

Postby sam » Tue Jul 27, 2004 11:49 am

Hi there Eric - Sorry I am from Oz and am not sure of who uses what terminology.

Here goes with the explaination:

The noggin is a solid piece of timber that is fixed between two studs used to brace one to the other. Usually there is one in a wall that is say 2400mm, some where around 1200mm.

Does this help? :?
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Postby Eric.Desart » Tue Jul 27, 2004 12:00 pm

Sam,

Yep understood.
Thanks

:-) And where is Oz?

Since normally absorption is placed in a wall I don't think a lateral mode will be harmfull.
But in any double leaf wall you have cavity modes (standing waves) which are related to the cavity width, and which will influence TL, absorption material or not.

And I forgot:
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Oz

Postby sam » Tue Jul 27, 2004 12:15 pm

Oz is Australia

Thanks for the welcome!
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Postby Brian Dayton » Tue Jul 27, 2004 6:02 pm

here's a thought that may be useless

but it may not...

i placed an accelerometer on a wall in my office (which is a 2x4 wall, insulation, 1 layer 5/8" drywall both sides, no glue)

and i hit it with a fist, a hammer, etc. and consistently got what appeared to be a mode at 18hz.

crudely and possibly inappropriately reverse calculating a modulus for the structure with these dimensions (96" x 156" x 120mm) and that resonance yielded 176,000 psi which is not completely bizzare. in no way is that calculation really accurate, as i just declared the whole wall a free-standing solid slab.

on the oustide wall of the office (with a mic this time) (steel/brick, 6" steel studs, 2* 5/8" drywall, 6" insulation) i found a more confusing pattern, presumably my mic is useless at low frequencies.

but anyway, Bob, that might be of some use to you in gaining an idea of how you could stiffen your walls.

theory (refer to Norton) offers that below the fundamental panel resonance (which for a 2x4 wall would presumably be the resonance of the whole drywall/stud/drywall structure acting as a large panel), stiffness adds to TL.

i think you might find that you have to make an unrealistically stiff structure to get anywhere via this train of thought. like 8" ribbed concrete or something.
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