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Discussion Starter · #1 ·
I thought I had it all figured out ;) Now this!


Here is a quote from the JM SoundSHEILD documentation:

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The Influence of Insulation Density on Sound Transmission Loss


Several tests (ASTM E 90/E 413) performed by independent laboratories have shown that variation in the density of insulation has little to no effect on overall STC rating of a total construction. Insulation thickness (or volume), not density, is the primary factor.


Comparative testing has also shown that low-density fiber glass is equal to many higher-density insulation products in reducing sound transmission through walls.


Fiber glass is particularly effective in sound absorption because of the many small air pockets between the glass fibers that trap and dissipate sound energy. This discontinuity of sound paths, combined with thickness rather than density, is the most important contributor to the performance of cavity-fill insulation in the reduction of sound transmission.


The chart below shows the effectiveness of fiber glass and the lack of correlation between insulation density and sound transmission loss. Tests were performed using specimens similar to Johns Manville fiber glass insulation.


STC Value of Equal Thickness of Mineral Fibers

(35⁄8" [92 mm] Steel Studs; Single Layer 1⁄2" [13 mm] Gypsum Board Each Side)

Material (in) (pcf) STC*

Fiber Glass 2 0.75 46

Fiber Glass 2 1.5 46

Fiber Glass 2 3.0 46

Rockwool 2 2.0 45

Rockwool 2 2.4 46

* Source of data: Loney, W.; “Effect of Cavity Absorption and Multiple Layers of Wallboard on the Sound Transmission Loss of Steel-Stud Gypsum Wallboard Partitions;â€

The Journal of the Acoustical Society of America: Vol. 53, No. 6, 1973.

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This seems to be at odds with some of the other information I have gleaned from this forum.


Now everyone knows that a little information in the wrong hands can be very dangerous! :D Is this the case here? Have they taken some results and presented them out of context? Or have I taken things I've read in this forum out of context?


I will have a 12" cavity between the walls of my HT. I was worried that I had to "stuff" them with fiberglass (or rockwool) insulation to get the density up. Now it seems I can just fill them with insulation. Is this correct?
 

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Bump...


I'd like to know this as well...


Sure would be easier to just lay some R-13 or R-19 batts in the stud cavities.
 

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Yes. Be careful here.


1. They are comparing fiberglass (or fiberglass like) products to each other (rules change if they were to compare 1/8 of lead to fiberglass, for example).


2. They are close to shooting themselves in the foot since they failed to give you the STC of the wall w/o any fiberglass...ie, how much of that STC is coming from the gypsum board?


3. The STC is a nice weighted average and does not reflect the gains/losses by 1/3 (or even 1/6) octaves. (This test has no bearing on the use of the material IN the room for RT60 purposes. You'd find the coefficients of absorption different between each of these BUT the tests used for determining Sabines is different than that used for TL (transmission loss).


4. The test results shown did not do a comparison with different material thicknesses (these are all 2" products .... where's the 3.5" and 5.5" inch products?).


This does not mean the information presented is bogus, it is acurate; but, you must know what you're looking at as contrasted with what you're attempting to achieve with the material. The Acoustical Society of America is a first class, reputable organization of the caliber of the AES and IEEE.
 

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Discussion Starter · #4 ·
I was aware that the sabines (NRC) would be vastly different, I'm concerned mostly with the sound transmission (STC). I will be using InsulShield on the wall for sound absorption.


I believe the different densities would be the equivalent of stuffing the 4 inch and 6 inch products into a 2 inch cavity. I realize a lot is coming from the gypsum (in their example, a empty wall, steel studs, 1 layer of 1/2" on each side is about STC40)


There was more information (I just took an excerpt) at JM Soundshield , where they show the values of different thicknesses with the same density.
 

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Equating Sabines and NRC is not precisely correct. The absorption coefficients are measured at 1/3 octaves. The NRC (a generally useless number), is a weighted average of the individual 1/3 octave values.


I say "useless" because if we're attempting to get to a flat response in the room, we cannot use NRC. If we're not attempting to get to a flat response, then we don't need NRC. :)


If you are interested sound transmission, then you ARE interested in density. Perhaps not the density of fiberglass (how dense can it get?), but, the density and mass required to obtain transmission loss you need at all frequencies to achieve your goal.
 

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Discussion Starter · #6 ·
You are, of course, correct, overall density will stop sound. Actually it's mass that stops the sound, but density is just more mass in less space!


If I fill a cavity with lead, it will be better than fiberglass.


However, what they are saying is that for this product (fiberglass), thickeness is better than density. Since most of us are using fiberglass in our walls, I thought it interesting. As you well know, there are three things that increase STC's:


1) Mass

2) Decoupling

3) Sound absorption


I guess their point is that for the same amount of fiberglass, the mass is the same, but if you compress it, the sound absorption characteristics are reduced.


In my case, with a 12" cavity, I'm going to loosely fill the whole cavity with 3 R13 batts , instead of compressing them for density.


I also apologize for oversimplifiying the NRC to sabines comparison. I know the NRC is an average and is basically useless for HT purposes.
 

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And here I thought Sabine was the character that Rebecca de Mornay played from the Three Musketeers. Geez am I an idiot :p Sorry I can't help myself sometimes. They really need to balance my meds:D
 

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In a previous post, I had asked if mineral fiber or rock wool is worth the extra money over plain fiberglass bats, and I don't think I got a specific answer. After reading this thread, it sounds like fiberglass batts are just as good as mineral fiber. Does that seem to be the consensus?


Art
 

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Please add me to the list of confused. Is rock wool better or not? My understanding is that it IS better - but that seems to differ with JM's conclusions on the test.
 

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Rockwool is slightly better. In a 2x4 wall you can expect an improvement in STC of about 6 (approximately a 75% improvement over a non-insulated wall). From NAIMA's publication (North American Insulation Manufacturer's Assoc.) a non-insulated 2x4 wall with 1/2" gypsum board will have an STC of 35. Adding 3-1/2" of fiberglass batt, your STC is 38.


By the way, Rockwool is far, far more commonly used outside of North America. There are still some local U.S. jurisdictions that mandate Rockwool over fiberglass products. At the end of this post I have listed some informational links. The link to the Rockwool manufacturer in Pakistan is for current events interest.


Again, from NAIMA, a wall from 3-5/8" metal studs, 24" O.C. has an STC of 39 without batts and 47 with. (J-M's published numbers are 46 but with only 2" of material rather than 3.) I do not have the comparable numbers for Rockwool.


Owens-Corning has also published results of studies with respect to thickness vs density. They conclude that density does have an effect; but, much less an effect than thickness (they are not talking about mass ... just density). One side notation from this study is that an 1" of airspace between two layers of fiberglass (or an 1" air space between the fiberglass and a concrete wall) will have a greater positive impact on sound absorption than by adding an equal thickness of fiberglass batt). I have the density and thickness plots by 1/3 octaves in my office from this study. You'll also find reproductions of these charts on page 221 in Sound Studio Construction on a Budget by F. Alton Everest.


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http://www.rockwool.ltd.uk/web/produ...stic/index.cfm

http://www.bradfordinsulation.com.au...SND/RCKSND.pdf

http://www.rockwool.web.pk/
 

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Discussion Starter · #12 ·
Quote:
Originally posted by Dennis Erskine
... One side notation from this study is that an 1" of airspace between two layers of fiberglass (or an 1" air space between the fiberglass and a concrete wall) will have a greater positive impact on sound absorption than by adding an equal thickness of fiberglass batt). I have the density and thickness plots by 1/3 octaves in my office from this study. You'll also find reproductions of these charts on page 221 in Sound Studio Construction on a Budget by F. Alton Everest....
So, in my 12" cavity (2 2x4's with 4" in between) it would seem like it might be better to put a 4" bat (Fiberglass or better yet, RockWool) between each set of 2 x 4 studs, and leave the 4" as an air space.


It will come down to comparative pricing. Unfortunately, my budget is not unlimited (like anyone's really is!). I am also seriously considering blown in cellulose (12"). i have used it in the past and it is very easy to use. I just need to compare pricing as well.


I'll read more in the links you sent me.


Thanks again,
 
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