The effect on TL by adding additional layers (Graphs)

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The effect on TL by adding additional layers (Graphs)

Postby Eric Desart » Mon Feb 27, 2006 8:20 pm

Hi all,

I made some graphs in order to get a better feel about what happens infunction of either increasing the cavity, or increasing mass, or a combination of both.
This is a stylized version related to the MSM (mass-spring-mass) resonance, not meant to analyze every possible other influence, mounting method etc.
It assumes a Pink Noise and doesn't bother about coincidence frequencies, which only in rare cases will influence weighted TL values, certainly not in the studio world.

Those graphs and related calcs were really time consuming.
Please (is in fact not a question) respect Copyright and leave those pictures and included text here in the Forum.
Just link to the thread if you think they can be helpful for others.

Image

Image

Image

Image

Image

Image

Mainly that picture TL-005 is interesting to see in function of the ratio of the effect of increasing mass versus cavity depth.

While making this it reminded me about several suppliers who include real estate space cost in their comparison versus alternative methods.
I always felt this as bean counting.

In fact it seems simple:  If the thickness of a gypsum board becomes that important, that it represents real estate value to be taken into account, then I should say redesign the wall, and use mass rather than space (just make it a bit thinner).
Saving the thickness of a gypsum board by decreasing the cavity width correspondingly will hardly influence the TL value in function of this space (decreased cavity width to compensate for panel thickness often will prove only fraction of a dB), and the gain by the added board mass will remain the dominant contributing factor.

Warm regards
Eric
.
PS:
I know there are still some cosmetic errors (copying and forgetting to adjust and so on), but I will check them once more and correct them.
.
Last edited by Eric Desart on Wed Mar 01, 2006 2:04 pm, edited 5 times in total.
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Postby J.F.Oros » Mon Feb 27, 2006 9:12 pm

Interesting info, thanks Eric (and glad you came back, we started to miss you again)  :D

I'll have to study it carefully, but right now, after seing picture TL-004, one thing pops up in my mind :

- in a 2 leaf window or wall, is it better to use equal thickness on the leafs, because the loss at the (equal) coincidence will not be the defining factor in the overall TL ?

(I know that the usual recommandation is to use different thicknesses ...)
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Postby Eric Desart » Mon Feb 27, 2006 9:44 pm

J.F.Oros wrote: - in a 2 leaf window or wall, is it better to use equal thickness on the leafs, because the loss at the (equal) coincidence will not be the defining factor in the overall TL ?

(I know that the usual recommandation is to use different thicknesses ...)


Flaviu,

Before saying anything final to this, I first need more time.
Several years back I made a measurement database for the Belgium establishment of one of the largest European glass producers.
I'm going to analyse this.

What you can be sure is that this standard recommendation is a typical axiom (right or not, within or outside context), repeated over and over without knowing why.
Anyone tells so, hence why not me?

Fact is that the in the graphs shown negative effect with moderate pane thickness differences is small.
Fact is that different pane thicknesses will show a shifted coincidence frequency, broadening the coincidence dip area and making it less deep.
Fact is that laminated glass will damp this area more than enough to make this a theoretical discussion.
Fact is that nowadays better damping foils exist as the "Stadip Silence" from the Saint Roche group, which keep the original coincidence of the individual panes, and damps rather well what's left.
Basically you can only tell if one pane is better than the other when you calculate it as a weighted pane.  I can as easily imagine that broadening this coincidence dip rather than having a higher deeper one can as well be a disadvantage.
For me it all depends.

But as you can see with this relative moderate shift in MSM when using logical pane ratios, the negative effect of the MSM is relative insignificant.  Hence this argument seems not important enough not to do it (within reasonable ratios).

If I find time, I'm going to try to get more statistical information from my database.
Mostly when one advices 2 different panes it relates to the coincidence.  Hence that can go either way.

With the exception of the fact that non-laminated glass is less damped (+ no absorption between panes), hence more sensitive, what is the difference with drywalls?

But I'll check later more in-depth.

Warm regards
Eric

:twisted:  Have you noticed already that it's easy to play the clever guy by repeating axioms. Nobody ever asks for proof or the why.  
:mrgreen:   Except Bert , ..... he starts with questioning the whys even before one told him something. ( :oops:  sorry Bert couldn't resist)
Try to question that or put it in a correct context, than one gives oneselves a huge amount of work.
.

Scott, I answered at SOS.
Last edited by Eric Desart on Tue Feb 28, 2006 8:56 am, edited 1 time in total.
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Postby J.F.Oros » Mon Feb 27, 2006 10:10 pm

Thaks Eric, no problem, it was more of a "phylosophical" question anyway.
I know that in practice the total mass of the panes and the distance between them is what really counts  :D

As concerning Bert, he is an axiom by himself, so it's a natural thing for him to question everything else  :mrgreen:
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Postby Scott R. Foster » Mon Feb 27, 2006 10:14 pm

"Nobody ever asks for proof or the why. "

Really... why not?


:evil: Bert made me ask that question.

PS: Eric - please email me - I have a question for you.
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Postby bert stoltenborg » Mon Feb 27, 2006 10:26 pm

I'm the knight that says: "WHY".
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Postby Scott R. Foster » Mon Feb 27, 2006 10:37 pm

Eric:  Correct me if I am reading this wrong, but the bottom line is:

1) Always add mass, even if it means a narrower cavity; and

2) Use a differentiated mass load for the two sides only if it adds mass.. if differentiation means lowering the mass [taking a layer off] - don't do it.

But what about this situation... you desire a wall to provide TL of musical noise - and you can have a double studded wall with 25mm of gypsum each side [50mm total].. or a single stud wall with 50mm of gypsum each side [100mm total].    Which do you choose?  In other words - does the step of decoupling the studs compensate for loss of mass at any stage of the decision tree - or is mass always defining?

Thanks
SRF
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Postby Eric Desart » Mon Feb 27, 2006 10:37 pm

Scott R. Foster wrote:"Nobody ever asks for proof or the why. "

Really... why not?


:evil: Bert made me ask that question.

PS: Eric - please email me - I have a question for you.


Well....  Really ... why not?
:mrgreen:  Good question .......

:wink:  With more Berts there should be less axioms. .....

Scott I answered at SOS (please read it).
Tomorrow or the day after my new CPU is reïnstalled. Then I'll email you, .... promissed.
I hestitate to download Email now in this Laptop. Then I again must merge all this email from different CPUs.
I'm dependent on others for this kind of stuff.
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Postby Scott R. Foster » Mon Feb 27, 2006 10:42 pm

lemme know if you need any help... having you offline is a tragedy.

PS to Bert:  Shame!  Get yo ass over to Eric's and fix the man's computer... you know old people can't do that stuff by themselves.  

:roll:
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Postby bert stoltenborg » Mon Feb 27, 2006 10:51 pm

Scott R. Foster wrote:lemme know if you need any help... having you offline is a tragedy.

PS to Bert:  Shame!  Get yo ass over to Eric's and fix the man's computer... you know old people can't do that stuff by themselves.  

:roll:


'Key Scotty. Trowing the welding tools in the Pinz right now.

:mrgreen:
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Postby Eric Desart » Tue Feb 28, 2006 9:46 am

Scott R. Foster wrote:Eric:  Correct me if I am reading this wrong, but the bottom line is:

1) Always add mass, even if it means a narrower cavity; and

2) Use a differentiated mass load for the two sides only if it adds mass.. if differentiation means lowering the mass [taking a layer off] - don't do it.

3) But what about this situation... you desire a wall to provide TL of musical noise - and you can have a double studded wall with 25mm of gypsum each side [50mm total].. or a single stud wall with 50mm of gypsum each side [100mm total].    Which do you choose?  In other words - does the step of decoupling the studs compensate for loss of mass at any stage of the decision tree - or is mass always defining?

Thanks


All rough rule of thumbs:

1)
a) When using a resilient skin versus a heavy wall, both are interchangeable (assuming expressed in percentage), meaning both are roughly as important (possible very slight preference towards mass, but certainly not enough to ignore cavity).  This resilient skin is any single leaf in front of a heavy wall.  Hence this applies also for a room in a room system where the inner room is often build in a brick outside boundary (you can interpret this as a resilient skin versus a heavy wall).

b) For lightweight systems as drywall. Once you respect a cavity of say >= 2", which is about a practical minimum to build studs worthy the name studs indeed your reasoning is correct, the possitive impact of adding mass is larger, to much larger, than the loss of cavity width.

Summarized: the more the assymetry between the 2 leafs of a double leaf system increases in function of mass the more the relative ratio of importance of the cavity and added mass will equalize.
But for standard drywall applications with more logical mass ratios on both sides your conclusion is 100% correct.

2) 100 % Correct

3) Decoupling is always important (or the Green Glue approach).
What I'm giving here is the basic theory for a perfect good wall.  Coupling or flanking can more or less destroy those principles. Perfect decoupling and maximizing internal damping will all help to obtain the maximum obtainable.
The 50 mm cavity wall as you describe, with firm coupling on wooden studs will start showing an unhealthy behavior and increase MSM (mass-spring-mass) on which those calculations are based. (Can't rate this very well).  I think Brian can simulate this based on his JASA article (forgot reference = somewhere here in forum) in function of structural resonance.

I have little or no experience with wooden studs, but I assume that even a thin stripe of felt (+/- 5 mm thick) on the wooden studs will significantly improve on that.
I saw Paul stating at SOS that point contacts by screws etc. should completely destroy such disconnection making it senseless  (:wink: the axioms).
While this indeed will be an important factor, there still are significant differences between point and line contacts with a certain width in such application. (it's part of the courses I once got very long ago, to calculate this influence).

Warm regards
Eric

PS: Scott I should link to this thread (graphs) somewhere from within your FAQ.
.
Last edited by Eric Desart on Wed Mar 01, 2006 8:02 pm, edited 4 times in total.
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Postby Scott R. Foster » Tue Feb 28, 2006 4:25 pm

Scott I should link to this thread (graphs) somewhere from within your FAQ.


Yep... please look here when you get a chance and skim for errors / overstatements.

http://forum.studiotips.com/viewtopic.php?p=157

PS: Maybe we clean this thread up later.. or repost in a clean thread in the FAQ section?
SRF
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Postby Brian Dayton » Tue Feb 28, 2006 5:07 pm

Eric,

     i'm not sure that i follow what you are presenting above.  i don't blame you, i just don't quite get my head around what the graphs are showing.  I'm not always so good at getting the jist of graphical presentations.

IR761 has alot of data down to 50hz showing the gains of adding layers to walls, and more data is available from USG circa the early 80's where on many walls they systematically added layers to the same frame/insulation set.  

the effect of additional layers has to be broken down into specific sets of rules for each type of wall

-for masonary, where adding mass would mean making it thicker (i.e., 8" of concrete instead of 4"), you have mass gains + the effects of lowering critical frequency

-for a common 2x4 wall (rigid studs, direct connection, no decoupling).  Here you have the effect of bonding (if you choose to bond) rigidly, or not-bonding, the former affecting resonance location, the latter causes minimal gains around the frequency of the resonance caused by the small air cavity between layers

-for a decoupled assembly you have mass gains and the effect of shifting location of MSM (so very large gains at the old MSM, small gains at the new MSM)

-Green Glue walls would be subject to all of the above + diminishing returns for multiple damping layers, particularily after at least one side of the wall has 2 damping layers.  

-finally, in either a lab or the real world, the flanking limit will create a situation where (at XX frequency, with more frequencies being affected as performance goes up), where no gains can be had by any means other than dealing with whatever is causing the flanking noise
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Postby Eric Desart » Tue Feb 28, 2006 6:32 pm

Brian,

This approach is not a study of real live measurements, nor all kinds of other effects as flanking, coupling, damping but a stylized theoretical relative approach based on the shift of the MSM.

The mathematics and assumptions are explained in my MSM Excel file.

The intent of those graphs is to show the relation mass/cavity  based on the mass-spring-mass theory, hence that includes masses as well as spring (cavity), with exclusion of other parameters.

Eric
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Postby J.F.Oros » Tue Feb 28, 2006 10:14 pm

Eric,

I wanted to ask you a question from some time, but kept forgeting about it, about your MSM Excell calculator :

- Am I correct in assuming that you are not using the insulation corection factor (1.4 * insulation thickness) for the cavity length when calculating the MSM frequency ?
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Postby Eric Desart » Wed Mar 01, 2006 11:13 am

Flaviu,

I've seen you use that somewhere before.

We calculate with the spring constant for the air assuming a good cavity filling. We adjust this spring constant when no cavity filling is present, or when the masses are connected with elastic materials, as e.g. resilient gypsum board pre-glued on mineral wool or fiberglass, or open cell foam.

Where did you get this 1.4 from?  Are their documents you can refer too?

Hence my MSM is calculated assuming a good mineral wool filling (no exact % defined).  I know how much a empty wall can increase about.  But we hardly ever use an empty wall when TL is the topic.

There is a contradicting info about the effect of the % cavity filling.  Hence I should like to see something explaining this mathematical relationship of this % filling versus MSM or spring constant (effect on dynamic stiffness). Is this an empirical relationship, or a physical one?

Best regards
Eric
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Postby bert stoltenborg » Wed Mar 01, 2006 12:08 pm

Hey Eric,

With speaker enclosures adding wool or something will increase the virtual volume of the enclosure with about 20 % and thus getting you a lower fc. When you overdo it with the wool you increase the resonant frequency, prolly due to increasing the stiffness of the internal air volume.
I (we) know this is correct by performing impedance measurements on the speaker enclosure. AFAIK these are empirical data, but I'm not shure.
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Postby J.F.Oros » Wed Mar 01, 2006 1:13 pm

Eric Desart wrote:Where did you get this 1.4 from?  Are their documents you can refer too?

Brian uses this factor in calculating the MSM frequency (I can't find now exactly in his posts where he used this formula, sorry, I'll search more), he is considering the length of spring being equal to the insulation thickness*1.4 plus the thickness of the remaining air space (if there is left).

The same constant can be deducted from the IRC IR-586 document (pag.7) if you make a ratio from the two MSM formulas, ( 60 / 43 = ~1.4 )

Bert, stop talking about speaker enclosures, these days you gotta think out-of-the-box !  :mrgreen:
(just joking, of course, dear B  :D )
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Postby Eric Desart » Wed Mar 01, 2006 1:21 pm

Flaviu,

Thanks,

Brian
Does this 1.4 comes from an empirical approach by you studying all those IRC and other measurements, or is this mentioned in this JASA article you referred sometimes (or other document)?
If so can you repeat this exact reference?

Best regards
Eric

.
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Postby Brian Dayton » Wed Mar 01, 2006 4:17 pm

Eric Desart wrote:Flaviu,

Thanks,

Brian
Does this 1.4 comes from an empirical approach by you studying all those IRC and other measurements, or is this mentioned in this JASA article you referred sometimes (or other document)?
If so can you repeat this exact reference?

Best regards
Eric

.


Eric,

    consistently throughout their literature the NRC gives formula which imply that resonance itself (a mass-air-mass resonance) would fall by the factor 1.4  (sqrt 2), implying that the inclusion of insulation effectively doubles the air space.  a readily accessible example of such discussion is found in IR-586, page 7.  the NRC has never differentiated between partially and completely full cavities.

     in the technical brief that accompanies the large volume of tests presented by USG in the mid 80's,  they show a method where the total net air volume is calculated by [empty air space] + [1.4*insulation thickness].  This is a more conservative estimate of the impact of insulation on the stiffness of the air spring.

     the reality of the situation is this:

a)  different insulation materials affect this differently, and even worse there seems to be some evidence that rigidly connected systems (like a panel trap or a common wood stud wall) resopnd a little differently than decoupled systems.  That different insulation materials have different effects on the location of MSM is made abundantly clear in IR-761, particularily in the section covering thin-gauge steel stud walls

b)  the situation has not been adequately studied for true models to be made, as far as i know

c)  all evidence points towards using lower density insulation to maximize the lowering of MSM, with progressively higher density insulations driving MSM progressively higher.  if anybody likes i can dig up some info from IR-761 that very clearly outlines this


as far as which of the two (NRC / USG) schools of thought is more correct, i cannot answer.  for a very long time it has been an intended project of mine to model this for a variety of insulation types using both a sealed-box loudspeaker and some type of decoupled wall assembly as references (either case should reflect the change in air spring stiffness very well), but to date this work has not come to fruition due to so, so many other projects standing in the way.

it seems to me that if you modleed a sealed box woofer (a woofer with very high VAS so that the air in the box could realistically define the overall stiffness), a pretty good idea of both effect on apparent air cavity volume and on damping could be attained.

if this data from above correlated to a similar experiment run on soem type of decoupled assembly, then you'd have your answers.  My thought is to stimulate the speaker and measure the T/S parameters for data #1, and to stimulate a smaller sized decoupled assembly of some type from close range with loudspeakers for data #2.

but as of today, very little work has been done on this.
Content posted by me is copyright 2004, 2005, 2006  Brian Ravnaas, but may be reproduced without permission for any non-commercial purpose so long as the intent is preserved.  NRC Canada data is copyright them and used with permission, www.nrc.ca
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