gypsum/drywall: glued vs screwed

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Postby Bob » Wed Jun 23, 2004 4:54 pm

Eric:

The vertical axis, ranging 15 to 50, I believe is "db TL"

What I think you said in my own words.

First graph
We start with 'series-1' which is like a 2" thick little wall {masonite/rockwool/masonite}, and it has a wall like (mass-spring-mass) TL curve, namely lower on the left/LF with a mass-spring dip, and much higher on the right/HF.
Then you stiffened the heck out of it and the TL curve flattened out.
I'm thinking about why the HF flattened due to stiffness. Stiff materials like a steel table spoon ring at high frequencies when tapped on a table, soft materials like a rubber tire ring at low frequencies when tapped on a table. So when you stiffened the wall, I guess it acts more like a table spoon than the tire it was before and lets more HF through.
For the LF gain is sort of like making the wall more massive - stiffer means it takes more energy to move it (at least until it snaps). The 6db per doubling of mass rule.
The 'series-3' removal of the rockwool insulation shows the mass-spring-mass is around 125hz for 'series-3' and probably 'series-2' which is nicely damped by the rockwool.
So stiffinging moved that resonance down to 125hz from the 200hz it was before.
The 'series-2' dip around 630hz to 800hz I have no guess about, nor the improvement around 1250hz.

Second graph.
The point of this is to show that for the same kinds of materials, in different proportion, mass-spring-mass takes over again and we end up with a typical mass-spring-mass wall TL curve, namely lower on the left/LF with a mass-spring dip, and much higher on the right/HF.
And the resonance, even though the stiffiners are still there, is back up to the 200hz range.

OK, the purpose of these two graphs is to explain your quote in this thread
I once told about stiffness increasing low frequent behavior. Such things seem to get their own live (out of context), because I ALSO said that was valid for lightweight SINGLE leaf systems (often applied with steel). But there stiffness must be interpreted in function of real acoustic stiffness (strongly lowering the coincidence).
Namely that stiffness helps the LF in thin panels, but doesn't do much in gypsum wall construction, possibly including double stud walls.

'perfectly decoupled drywall' is a bad thing in LF TL, because the mass-spring can drop to 0db at resonance. So we actually want it to be a little coupled or at least damped, we have to do something to reduce that that resonance.

I have this feeling I'm still missing something important.
Regards
Bob Golds
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Postby Eric.Desart » Thu Jun 24, 2004 7:50 am

Bob wrote:Eric:
The vertical axis, ranging 15 to 50, I believe is "db TL"


Correct, forgot to enter labels, made those for the group.

What I think you said in my own words.
First graph
We start with 'series-1' which is like a 2" thick little wall {masonite/rockwool/masonite}, and it has a wall like (mass-spring-mass) TL curve, namely lower on the left/LF with a mass-spring dip, and much higher on the right/HF.
Then you stiffened the heck out of it and the TL curve flattened out.

Correct, but it's mainly or very important related to dampening the mass-spring. At mass-spring the wall leafs want to move in anti phase, by gluing them together (keeping distance constant) I mechanically (more or less) supress this opposed movement (which is easier with relative thin walls). In fact one creates a situation, as the one at the far left side of the mass-spring resonance were the wall leafs naturally move in phase, thereby acting as a single leaf wall.
So you can see it as a spring which is blocked from functioning which also causes the decoupling by the same spring in the higher frequencies to diminish.

I'm thinking about why the HF flattened due to stiffness. Stiff materials like a steel table spoon ring at high frequencies when tapped on a table, soft materials like a rubber tire ring at low frequencies when tapped on a table. So when you stiffened the wall, I guess it acts more like a table spoon than the tire it was before and lets more HF through.


The coincidence frequency is defined by the stiffness (simplified). While in standard tables you see only the coincidence in function of thickness of specific materials, one should go to the basics. By stiffening this panel this coincidence lowers causing those strange unhealthy looking dips (series 2 and 3), beyond the graph to the right the curve should rise again with a very steep angle.
Those profiled (corrugated) roof steel panels used in the industry have a very strange looking TL curve, caused by strongly deviating coincidences in the different angles of the panels.

You're looking at a strange combination of phenomena were the coincidence behavior is the dominant one.
NOTE: this comment relates to series 2 and 3 (it's difficult to keep things organized in this small editing/reading/writing window, so I think some of my answers aren't positioned correctly)

For the LF gain is sort of like making the wall more massive - stiffer means it takes more energy to move it (at least until it snaps). The 6db per doubling of mass rule.


No it isn't. Explained above.
True is that when you force something to act as a single leave system the stiffness is an additional factor to overcome the amplitudes needed for the low frequencies. But this is also related to the wavelenght in the material itself (velocity dependent)
However the mass-law ONLY refers to mass, assuming a wall to move as an infinite stiff piston excited by a PLAIN wave in function of the angle of incidence.
So don't mix different things because it's convenient. You're speaking of different phenomena with their own physics.

The 'series-3' removal of the rockwool insulation shows the mass-spring-mass is around 125hz for 'series-3' and probably 'series-2' which is nicely damped by the rockwool.
So stiffinging moved that resonance down to 125hz from the 200hz it was before.


Stop trying to find rules based on every individual thing you see. This can be related to a changing apparent mass, but can be also something else.
Bob you have the tendency to draw explicite conclusions and translate all this in simple rules. DON'T.
Just to theoretically model those special measurements you can make a PhD thesis.
Concentrate on the main basics.

The 'series-2' dip around 630hz to 800hz I have no guess about, nor the improvement around 1250hz.


That's mainly the coincidence.

Bob this becomes too extensive. You want to become an acoustician via the net activated by individual messages.
I strongly suggest you should follow a course somewhere or read educational books.

I hope you can do something with the things I said already.

<snip>

I have this feeling I'm still missing something important.

Yes, learning acoustics in a systematic way if you really want to go in-depth.
That's not critisism, I do appreciate your curiosity and urge to know and understand.
Only this is walking in circels.


Warm regards AND respect
Eric
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Postby Eric.Desart » Thu Jun 24, 2004 5:22 pm

For info,

I added a 3rd picture with some single number TL ratings (in previous message with pictures).

Notice the significant difference between STC and Studio music.

In fact the difference should be MORE significant if the measurements were executed down to 50 Hz.
However my data only starts from 100 Hz, so only those bands and up are taken into account.

What those TL values mean (simplified, normally the calculation is more extensive, but the relative comparison is ca valid) is, that when you have an emission level X in the source room, that in the receiving room you have an immission level equaling X minus the single number rating.
This then represents the total level of the bands covered by the TL ratings.

Notice that the high STC values drop very strong, when weighted taking the low frequencies into account. (and in fact the below 100 Hz bands are NOT covered by STC and Rw, nor yet integrated in the other TL ratings.

Both STC and Rw are close to a dB(A) weighting. Statistically a dB(A) weighting is ca 1 to 2 dB lower than both STC and Rw (more exact study on my site).
A studio music number taking all frequencies into account will be between 12 to 18 dB lower than STC or Rw values (for double leaf systems).

While the Rw seems less stringent than STC based on this picture, this is statistically not true. The Rw is a bit more stringent than the STC.
The Rw covers 100 Hz to 3150 Hz and the STC 125 Hz to 4000 Hz.
The reason for this are the deep dips, which limit the STC here in those measurements by the -8 dB rule, which is skipped in the Rw in 1996 and substituted by a more accurate (pseudo) dB(A) weighting. This causes the current Rw itself to be somewhat less accurate.

Kind regards
Eric
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Postby Bob » Thu Jun 24, 2004 6:33 pm

Eric:

is ca valid

What does 'ca' mean in english? You use it a lot.
I've been interpreting 'ca' to mean 'sort of', 'approximately', 'near to'.
Regards
Bob Golds
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Postby Eric.Desart » Thu Jun 24, 2004 6:55 pm

Hello Bob,

Sorry I thought that ca was international.
It stands for circa which means about, plus/minus, approximately, roughly.
I often use it to avoid the need to tell about all (side) ifs and buts scenarios for which it shouldn't be true.

It's good that you tell me that this notion is not international as I wrongly assumed.

Hé, I checked my dictionary and circa is noted as correct English. You maybe don't know the abbreviation.

General: If I say things wrong, you help me by telling me. It's the only way I can learn.

Best regards
Eric
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Postby Bob » Thu Jun 24, 2004 7:34 pm

If I say things wrong, you help me by telling me. It's the only way I can learn
Been there. Done that. Image

BTW, do you see the smilies in this post ? I've had smilies in some of my posts for almost a month now, using the [ img ] command.

circa is a word that has been in my vocabulary for years. I was not aware of it's abreviations: c. and ca

Although I define it as 'approximately', I had previously used it only in the context of a 'year' or 'date'. As in Cleopatra was circa 0BC, and Plato circa 350BC, and Badari circa 4000BC.
Regards
Bob Golds
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Postby Scott R. Foster » Thu Jun 24, 2004 9:42 pm

It's Latin you vulgate tongued barbarians!

It means "about" and has the same root as the word "circle"... as in making a circle on a map and saying "somewhere about here."

Common abbreviations are c. and ca., most often seen in dates as Bob explains, as in c. 1800, or ca. 1740.

But for non-calendar numeric purposes or technical explanations perhaps the direct translation "about" would be better? Or maybe that oh so useful mathematical term: +/-
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Postby Paul Woodlock » Thu Jun 24, 2004 11:31 pm

Eric.Desart wrote:...
Hé, I checked my dictionary and circa is noted as correct English. You maybe don't know the abbreviation.
...


c. or ca. is definitely common in English UK.

However "Hé" isn't :-)

I presume it's belgian for 'hehe' or the sillier, 'heehee'

Although I do think "WOO HOO!!!" is now becoming widely known throughout most of the the Solar system. Meaning "An exciting and excellent result!"

:)


Paul
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Postby Eric.Desart » Fri Jun 25, 2004 9:01 am

Paul Woodlock wrote:I presume it's belgian......


Neigbor from just over the water ..... :):)
What language is belgian??? Does it sound as ukingdomian??

So I shouldn't know if it is belgian, since I don't know belgian either.....
And we ARE on the map too. It's that small country where the CAPITAL of Europe is.
Just a good 30 miles swim and you are here. :):)

A Belgian, NOT speaking belgian.:) (as in UKer not speaking ukian)
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Postby Paul Woodlock » Fri Jun 25, 2004 3:03 pm

Eric.Desart wrote:
Paul Woodlock wrote:I presume it's belgian......


Neigbor from just over the water ..... :):)
What language is belgian??? Does it sound as ukingdomian??
[/b]

hehe - I realise there is no "Belgian"

So I shouldn't know if it is belgian, since I don't know belgian either.....
And we ARE on the map too. It's that small country where the CAPITAL of Europe is.


Yeah, but they get fed up of Belgium every so often and move the whole lot to Strasbourg!!



Just a good 30 miles swim and you are here. :):)


One of my Ancestors, a certain Capt Webb was the first guy to swim the Channel



A Belgian, NOT speaking belgian.:) (as in UKer not speaking ukian)


:) :) :) :)

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Postby Bob » Thu Jul 01, 2004 9:29 pm

Hello

Eric gave a wonderful example of the effect of stiffining. Unfortunately for us wall builders it was one of those "I don't forget the resulting joke of my (back then) foreign colleages telling: don't ask Eric, he makes things that perfect that you can't use them anymore."

I thought I'd add this bit of data:

brianr820 wrote: "ever seen the huge drop in frequency of the cavity resonance with changing stud spacing from IRC-IR-693? it falls well over an octave by changing stud spacing (progressively) from 12-16-24-32 on center. A true mass-air-mass resonance would not change from stud spacing."

http://irc.nrc-cnrc.gc.ca/fulltext/ir693/ir693.pdf
page 80, figure 7
This figure is not a double stud wall, it's a single stud wall.
Regards
Bob Golds
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Postby Brian Dayton » Fri Jul 16, 2004 9:31 am

hello all,

i'd like to extend my thanks to this group/site, etc., it is a tremendous resource.

i fear that i cannot contribute to the room acoustics knowledge base and discussions, but if i may i would like to offer some thoughts on the TL graphs in this thread.

in the graph MasoniteDoubleWalls.GIF we see that bracing 6" on center drastically slashes transmission loss and introduces a dip in the 600hz region. this dip goes up in frequency without insulation in the cavity.

it was offered that the 600hz dip is the coincidence region, which would have moved to such a low frequency due to the stiffening effect.

again, if a newcomer to the forum may be allowed, i would like to offer several points

1. the dip at 3.15khz looks like a coindidence effect for the unbraced wall. if we could see farther up in frequency, we could be sure that this was the low-point, but even assuming that is the low-point, that would predict a coindidence dip in ~the 800hz band for a solid 2" bonded wall of the same material (4x thicker, 4x lower in frequency). so that would fit, if we had attained the same stiffening effect with our braces that we would with a solid 2" slab.

2. moving stud spacing closer would stiffen the panels in a similar fashion. 6" on center studs would be "stiffer" than 6" every OC bracing. In IRC-IR-693 by Canada's NRC, we observe that moving stud spacing from 32" OC to 12" OC does not affect the coincidence frequency at all, however.

3. same reference (IRC-IR-693, page 80) shows that closer-spaced studs have a rather marked effect on the frequency of mass-spring resonance. more than an octave for halving of stud spacing.

frequency of lowest dip

studs 24" OC = 125hz
studs 16" OC = 200hz
Studs 12" OC = 315hz
the figure above = 200hz

if you look at the graph, my choice of frequencies may seem strange - i adjust for simple mass law predicting that TL rises at least 2db with each 1/3 octave. so if 315 is 1db higher than 250, that is a lower peak.

so, if we model the effect of 6" OC bracing as similar to 6" OC studs, we'd expect the mass-spring rsonance to rise.

4. and this is the most compelling point, perhaps. the frequency of the 600-800hz dip rises when insulation was removed. the inclusion of cavity-filling does not affect the critical frequency substantially. if we browse the data published by our canadian friends, we would find no case where including insulation lowers the coincidence dip. we can find myriad cases where trivial rises in coindidence frequency can be seen in insulated walls relative to no-insulation references.

but insultation does certainly lower the frequency of the mass-spring resonance. and we would certainly expect the frequency of mass-spring to rise when we remove it.

i humbly offer that i believe the dip at 600-800hz is not a coincidence artifact, but the new mass-spring resonance. much as bracing a loudspeaker cabinet drives resonances up, or using closer stud spacing drives resonances up, i think this resonance has been driven up.

anyway, i offer this in hopes that it adds to the stated premise of this forum, and again, thanks so much for the literal wealth of information that all of you have made available here. the internet age is remarkable in it's opportunity, is it not? people can learn from persons that they would never have had access to even a small number of years ago.

take care,

Brian
Last edited by Brian Dayton on Fri Jul 16, 2004 11:20 am, edited 3 times in total.
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Postby Brian Dayton » Fri Jul 16, 2004 9:38 am

if i may offer a solitary additional thought.

it was said that the mass-spring resonance was damped by rigid braces. and i feel the term was used to state that bracing eliminated the big 200hz dip, which it certainly did, and so that fits.

however, in the strictest sense, damping refers to the rate of energy dissipation in an oscillating system. rate of energy loss. and stiffening a spring doesn't increase the rate of energy loss, it moves the oscillations to a higher frequency. and with these braces, unless a viscous or damping glue was utilized, no means of energy dissipation has been introduced to the system (the wall in this case).

the poor TL across mid and high frequencies is certainly also related to the couple of the panels via the same route that a normal 2x4 wall has lower TL than a resilient channel wall or a double wall.

many compliments on remarkable TL figures for such a thin wall, by the way.

if this group is curious, i took a few hours recently and ran tests related to this topic in the lab in which i work. i used the change in flexural modulus to model predicted drop in coincidence frequency for panels laminated with various commercial adhesives (drywall adhesive, simply screwed together, sub-floor adhesives, yada yada yada). the data seems to imply that i was incorrect in presuming that fairly flexible glues would have a minimal effect on the frequency of coincidence.

i also logged the damping properties of each test panel and some broad references (laminated glass, steel, etc.)

i'd be happy to post it if anyone is curious
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Postby bert stoltenborg » Fri Jul 16, 2004 10:17 am

Hello Brian,

I'm curous!

regards,

bert
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Postby Eric.Desart » Fri Jul 16, 2004 1:35 pm

Brian
Me TOOOOOOOOOOOOOO
TIA

And welcome to the group, nice to have a chemist here (and Brian with it:)).
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Postby Brian Dayton » Fri Jul 16, 2004 1:57 pm

thanks for the welcome guys, it'll take me a bit of time to make the excel sheet presentable, and at anothers request i'm going to add another sample.

this experiment was fun, and not very time consuming, so if there is any specific materials anyone wants included that i can get at home depot, etc., post soon and i will do so.

anything that would be expensive to test, if you send the material i'll do my worst.

Brian
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Postby Eric.Desart » Fri Jul 16, 2004 3:25 pm

Hello Brian,

About this 600 to 800 Hz problem in this thin wall.
You made me think, but don't know very well how to simulate this.

What I reffer to when calling it coincidence, is that at a certain moment the coincidence of the total construction (bending waves of the wall as a unit, not as individual panels) becomes defining.

I'm searching for some measurements, but can't find them now.

Anyhow if you should be right than you are right.
Mostly guys humbly offering their opinion know what they talk about .......
They belong to a pedigree I strongly listen to ........

Thanks for your contributions.

Eric
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Postby Brian Dayton » Sat Jul 17, 2004 11:30 am

you are certainly welcome for the thought.

on a side note, the term mass-spring resonance entered my lingo via this thread, and so credit for that is due to Eric Desart.

IMO it is a far more fitting term than any of - mass-air-mass resonance, cavity resonance, or chamber resonance - (all of which i have seen utilized to describe it)
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Postby Eric.Desart » Mon Jul 19, 2004 11:18 am

Brian,

What did you main with bracing the loudspeaker cabinet, and increasing resonance?

Are you talking about stiffening the wooden panels?
By doing so you decrease the size of the panels thereby increasing/influencing the panel's modal frequencies.

Coincidence (free bending waves) is related to bending stiffness.
Higher bending stiffness causes lower coincidence not higher.

Such a panel will start acting as a unit were the bending stiffness is different in function of the angle versus the stiffeners. So one gets a rather broad coincidence range

The above panel really becomes a sandwich perfectly connected.

The comparison with the stud distance in the Canadian doc, can be true or not.
They are wittingly screwed in a manner to minimize contact with the stud. What I mean is: will this really act as a sandwich panel were the fre bending waves are defined by the wall as a single unit?
If it should be just a mass-spring then why doesn't it behave as per the mass law below the mass-spring?
Surely this isn't in the stiffness controlled area here keeping this curve flat .......
And why doesn't it rize logically behind this mass-spring. What blocks it from behaving as a double leaf system?

But I agree, I don't have the exact answer. (I said somewhere in this thread that simulating this is difficult)
What resonance do you calculate for those stiffeners (I have no idea of the springconstant valid for them)?
2*pi*f = sqrt(s*((m1+m2)/(m1*m2)))
What is "s" here????

The insul program Bob refers to is based on the mass-spring calculated on the air layer as a spring, not a structural calculation of the interaction between the studs and the air.
Possible emphirical calculations are integrated for the difference between wood and metal studs,

Summary:
While the possibility still excist that you are right, you did not convince me. The mass-spring explenation does not explain this curve.
At least there is a lot of simularity with the typical curve and broad spread coincidence of similar walls, where coincidence differs in function of angle.
Only better similution could give the outcome.
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Postby Brian Dayton » Mon Jul 19, 2004 4:12 pm

Mr. Desart,

perhaps one would find that if we took a panel, placed an accelerometer on it, and stimulated it with (knuckle rap/impact hammer/mallet tap) we would stimulate the mass-spring resonance but not a coincidence effect? i wonder...?

perhaps for the sake of discussion i could run such a test. such a test would not answer our questions here, but would perhaps allow a simple method to discern such data if curious.

i think the most compelling point is the shift upwards in frequency of the dip when insulation is removed. that is similar to expected and measured results for a mass-spring resonance, but would not be expected for a coincidence effect.

i have pondered a means of calculating the expected mass-spring resonance, but without considerable research this is beyond my academic capacity. too many variables - how much stiffening do the braces create, what is the impact of the air cavity on spring stiffness, etc.

thanks for the thoughts,

Brian
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