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PostPosted: Thu Oct 08, 2009 9:07 am 
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I don't believe inserting a third leaf makes LF worse than before

Fortunately for acoustics and studio builders, what you "believe" is irrelevant: all that matters is the science. Science works despite what you believe, not because of it. I can't understand how you can possibly even make such a statement, when scientific proof to the contrary is staring you in the face, in the above graphs and publications. Inserting a third leaf ALWAYS makes LF isolation worse, all other factors being equal, for the simple reason that it raises the resonant frequency of the entire MSM system due to there now being two thin air gaps, instead of one thick air gap. Raising the resonant frequency of a wall will bring the resonance dip into the low frequency part of the human audible spectrum, thus REDUCING isolation in that area. There simply is no question about that. There are several comments on this forum and elsewhere by people who have experienced exactly that in real life while building studios, despite what they might have "believed" beforehand.


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MSM-walls.gif



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PostPosted: Thu Oct 08, 2009 9:14 am 
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Stuart. Maybe you should read my last question more carefully

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PostPosted: Thu Oct 08, 2009 9:30 am 
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I read it, and I gave you the data (perhaps you didn't notice, since I updated my post to add more). You state quite clearly that you do not believe that adding a third leaf to an existing two-leaf wall will reduce the low frequency isolation of that wall, ie, make it transmit sounds better in the low frequency range. The data that proves your belief wrong is right there, in the graphs and links I posted, most notably the final diagram of several walls. Move from right to left across that, and the second step will get you to exactly that situation: A two-leaf wall on double studs with one layer of drywall on the outside of each stud frame, and STC 57. One step to the left gets you to the exact same double-stud wall, same insulation same two leaves of drywall, but now with a third leaf added on one side of one of the stud frames, hence making it a three-leaf wall with 33% GREATER mass that its predecessor, yet the STC rating DROPS by 7 points. OK, STC means practically nothing in the LF area, but if you check the graphs on the Green Glue web site for the exact same situation, you will note that practically all of this drop is on the low frequency end of the spectrum, and is practically all due to the resonance dip now occurring at a high frequency.

Like I said before: there simply is no question about this, despite your beliefs. Eric's Desart's carefully prepared tables (above in this thread and elsewhere) show the exact same effect, as do many other sources. Adding a third leaf to an existing two-leaf MSM wall will drive up the resonant frequency, and will reduce LF isolation, unless the third leaf is hugely massive enough to compensate.


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PostPosted: Thu Oct 08, 2009 9:53 am 
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Ted White wrote:
Does anyone have data that shows a triple leaf makes the LF worse?

Ted White wrote:
And thanks once again! We all have heard the ills of triple leaves and the advantage of double. No question on the data. I don't believe inserting a third leaf makes LF worse than before

Ted White wrote:
Stuart. Maybe you should read my last question more carefully


Ted,
Maybe you should formulate your remark (was that a question?) more clearly? You ignore data entered bij Stuart (Stuart can you please enter sources of your pictures?)

"I don't believe inserting a third leaf makes LF worse than before". Can you please substantiate that sentence?
Just a question: Are you the former colleague of Brian Ravnaas, of am I mistaken here?

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PostPosted: Thu Oct 08, 2009 9:59 am 
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From your images, looking at the STC 36 Double Leaf. Insert a single leaf in front and you have the STC 50 wall, no? I should qualify that I realize STC Does not measure below 125Hz.

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Last edited by Ted White on Thu Oct 08, 2009 10:10 am, edited 1 time in total.

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PostPosted: Thu Oct 08, 2009 10:10 am 
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Ted White wrote:
From your images, looking at the STC 36 Double Leaf. Insert a single leaf in front and you have the STC 50 wall, no?


What does that have to do with LF?
Further, what does it have to do with this: "double leaf effect cost more than triple in most situations."

As you, or anyone with good eyes can see, the walls you compare are completely different in build. One is coupled the other is not.

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PostPosted: Thu Oct 08, 2009 10:33 am 
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Ted White wrote:
From your images, looking at the STC 36 Double Leaf. Insert a single leaf in front and you have the STC 50 wall, no?


You choose an extreme. Inserting means putting in between.
But more important you compare STC which isn't representative for the Low frequencies which you explicit mentioned. STC ignores these Low frequencies. The problem of triple leaf systems manifests itself specific in the lows which are defining for the single number rating of the spectral distribution of studio music. Kick drums and other other low frequent instruments fall outside the STC boundaries.

The difference for double leaf between studio music and STC can be between 12 to 15/18 dB, for a triple leaf system this difference (can) becomes larger. This is all caused by these low frequencies dominant defining for the STC values.

And further: I do have practical AND measured experience. Years ago I was confronted with the traditional triple leaf windows. I started with demounting the stuff and removing the center window (hence at the expense of mass). The result : increase in TL for studio use.

It's all a matter of ratios, but that no argument for : "I don't believe .... " as some slogan without whatever boundary or specification are just words until you substantiate them in a better manner.
In fact we should check the ones Brian published from which we know where and how they were measured (official reports).

xSpace wrote:
As you, or anyone with good eyes can see, the walls you compare are completely different in build. One is coupled the other is not.
Another additional good comment.

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PostPosted: Thu Oct 08, 2009 10:40 am 
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Eric,

It is important that you know, someone has purchased the domain name you use in your sig.

It is not being used with the spirit you had begun that statement with so you are in effect advertising for a venture that you most likely would not want to be associated with.


Brien

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PostPosted: Thu Oct 08, 2009 10:59 am 
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Eric_Desart wrote:
Inserting means putting in between.


You are absolutely right, I can see that was misleading on my part. Of course inserting a leaf in between two existing leaves would worsen things. My apologies. My question was more exactly:
Ted White wrote:
Has anyone seen data where an existing double leaf was made triple and LF was worse than before?


I guess I don't consider it extreme since that's a common scenario. Someone has an existing single stud wall (the STC 36 wall shown), and they contemplate leaving the drywall or plaster there and simply build a wall in font of it with a gap between old wall and new framing (STC 50 wall shown).

Again, my question (and it is, in fact, a question) is has anyone seen data where an existing double leaf was made triple and LF was worse than before? Does that STC 30 wall above have better LF performance than that STC 50 wall?

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PostPosted: Thu Oct 08, 2009 11:03 am 
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xSpace wrote:
Eric,
It is not being used with the spirit you had begun that statement with so you are in effect advertising for a venture that you most likely would not want to be associated with.
Brien
Brien,

You're right. Thanks for noticing this (at the time it was a dead link).
I adjusted it now.

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Last edited by Eric_Desart on Tue Oct 13, 2009 10:29 pm, edited 1 time in total.

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PostPosted: Thu Oct 08, 2009 1:24 pm 
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Ted White wrote:
Does anyone have data that shows a triple leaf makes the LF worse?


Avare suggested this document some time ago.

Wyle WR 73-5R.PDF (Prepared by Ben Sharp Wyle Laboratories Research Staff, circa 1973)

As much as I dislike spoiling the plot, somewhere between the pages 30 and 36 you can find the answer you seek.

If your pursuit is to ask have I seen it personally, no Sir, I have not. But the document above suggests I have no reason to look further into it especially based on my limited ability.

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PostPosted: Thu Oct 08, 2009 2:13 pm 
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Page 36, figure 18. "A Comparison of the Transmission Loss Provided by Double and Triple Panel Constructions of Equal Total Mass and Overall Thickness." Also the panels referencedhave no mechanical connections.

This is the heart of all of this confusion. I have not been describing equal mass and equal overall depth. I've mentioned adding a third leaf in front of an existing double leaf. That would mean more mass and more overal wall depth than was originally there. Your STC 30 wall compared to the STC 50 wall scenario for example.

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PostPosted: Thu Oct 08, 2009 10:22 pm 
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1) picture entered by Stuart: viewtopic.php?p=94540#p94540
2) Document linked by Brien: viewtopic.php?p=94553#p94553

Referring to Brien's link 2):
Ted White wrote:
Page 36, figure 18. "A Comparison of the Transmission Loss Provided by Double and Triple Panel Constructions of Equal Total Mass and Overall Thickness." Also the panels referenced have no mechanical connections.

This is the heart of all of this confusion. I have not been describing equal mass and equal overall depth. I've mentioned adding a third leaf in front of an existing double leaf. That would mean more mass and more overall wall depth than was originally there. Your STC 30 wall compared to the STC 50 wall scenario for example.


Ted you like slogans don't you? There is no STC 30 wall nowhere + nobody knows the original measurements of that picture 1).

Here some calculations towards the MSM.
I stylized picture 1) as being gypsum board panels of 12.5 mm (ca 1/2"). The wool thickness 100 mm ( ca 4"), and that empty cavity 75 mm (ca 3"). That's just a rough visual scaling of picture 1)
For every next wall I increased that empty 75 mm (ca 3") cavity with the same amount (doubling, tripling, ....)
Hence I compared the principle of that wall 2 (STC 36) versus that wall 4 (STC 50) as suggested previously by Ted. That wall 1 is a sic wall which should be filled with absorption if meant for sound isolation properties (even blow-in if wall exists already).

The fo of that picture 18 in link 2) relates to the MSM of a double leaf system. The f+ to the fhigh (upper) MSM of the triple leaf system.
These real fo and f+ on that picture 18 are located somewhat lower in frequency and deeper on the y-axis scale. What is noted there is where these TL curves intersect with the mass law (but start deeper at the resonance frequency).
The basic problem at such resonance frequencies is just that you don't reach the mass-law TL level (serious dip in insulation), hence these curves dive below that mass-law curve.

In these calculations you find all kinds of combinations:
Triple & Double leaf calculations

What do you conclude from these calculations (within the uncertainties of possible unknown influences)?

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Last edited by Eric_Desart on Thu Oct 08, 2009 10:53 pm, edited 1 time in total.

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PostPosted: Thu Oct 08, 2009 10:51 pm 
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I'm not sure what you're meaning by slogans, Eric. I'm sorry.

Last night at midnight I was referring to the same STC 36 wall I was referring to several times earlier. Same question, same walls.

Correct me if I'm wrong here. I'd very much like to follow your point:
Eric_Desart wrote:
I stylized picture 1) as being gypsum board panels of 12.5 mm (ca 1/2"). The wool thickness 100 mm ( ca 4"), and that empty cavity 75 mm (ca 3"). That's just a rough visual scaling of picture 1)


An insulated 2x4 wall roughly. OK

Eric_Desart wrote:
For every next wall I increased that empty 75 mm (ca 3") cavity with the same amount (doubling, tripling, ....)


You take "that empty 75mm cavity" and increased it. This would mean removing the drywall layer?

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PostPosted: Thu Oct 08, 2009 11:00 pm 
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Ted,

I was still editing my post (crossed).
I compared wall 2 (double leaf) with wall 4 (tripple leaf). I estimated that empty cavity a bit less than the thickness of that wool (visual scaling) estimating that empty part 3" and guestimated the wool at 4".
Then I increased that empty space of 3" in steps of 3". (3, 6, 9, 12, ....) and left that wool of 4" a constant.

And further I want to emphasize that this is all stylized stuff.

In my calculations you can find the buildup of the calculated walls (since that's the base on which I calculate them). I just see the cavity as a cavity with normal absorption filling.

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