Ted- National Fiber specified the wall construction, as 2x4, and while they might have put their thumb on the scale with R11 instead of R13 batts or something, I have serious doubts that they'd be so bogus as to compare it to less than a full cavity R8 or R3 econo-batts as you suggest. (They have a much better reputation than radiant barrier vendors, after all.) They are specifying 3.5lb density on the cellulose too, which is a dense-pack density, not low density. If they're cherry-picking in their favor it's most likely that it's by studwall type & density, not cheating the batts. But it's a studwall that's common in residential partions (2x4 with a single layer of wallboard on each side), if a less favored construction when building in serious sound abatement. And note in their table for adding/subtracting STCs by wall component type that they reference NRC data as their basis.

While I believe you that higher density will increase the lowest frequency stuff, sifting through a 300 page document (which you haven't given a link for) to pull the relevant bits together would take some time- can you connect the dots for us with page/chapter numbers? In the 83 page NRCdocument you linked to the numbers for cellulose & fiberglass are all fairly comparable, with most (but not all) configurations favoring fiberglass batts by a few STC points.

In nearly all non staggered-stud configurations where two different densities of cellulose were compared, the higher density goods outperformed the lower density, your narrative interpretation notwithstanding. Lowest density would not be favored in Jelly's studwall, if the wood studwall density comparisons have any relevance, but they didn't test different densities in single-steel-stud walls. See p.28 (p29 if using the pdf pagination) and p.14 (p15 in the pdf) http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/ir/ir693/ir693.pdf

In real-world applications blown goods tend to perform closer to spec than batts. A test lab with uniform stud spacing and no wiring/plumbing in the walls presents issues with batts that aren't there with blown goods. But if you can dig up the real data for different densites in simple steel studwall like Jelly's (no absorptive furring etc) I'd be curious to see it.

I appreciate the dialog Dana. Thank you for that.

Again, it's hard to say what the fiber group tested and how it was tested without the formal lab report. Certainly not inferring any cheating, as the lab (if NVLAP certified) should be completely objective and forthcoming as I'm sure the fiber group is as well.

While they have a table to add / subtract STC values depending on components, it's perhaps good to know that STC is a logarithmic calculation, and values cannot be summed. I realize they are not advocating this, but someone might think this is what they are suggesting.

Another important distinction is that STC is a less than prctical system of measurement, in that it does not take into consideration the problematic frequencies below 125Hz. So a "high STC" wall design might be completely intolerable in the low frequencies. A lot of architects and designers do not factor this into their designs.

QUOTE: " In the 83 page NRCdocument you linked to the numbers for cellulose & fiberglass are all fairly comparable, with most (but not all) configurations favoring fiberglass batts by a few STC points."  That's my only point. They are quite comparable with only a slight edge for fiberglass in the low frequencies. Again, however, insulation is the least contributive component in the overall sound isolation solution. An additional sheet of drywall would bring more to the table than insulation of any kind. Mass is a much bigger driver than absorption.

As I've said all along, the choice of insulation should be based on the overall picture that includes cost, thermal needs (if any) and practicality. While a valuable component, insulation selection alone doesn't really make a impactful difference.

I certainly appreciate your interest.

I think we agree- some insulation is better than none, and the differences are of lower consquence that other factors.

I was surprised when flipping through the document how in some configurations adding a layer of gypsum to one side made the fiber component flip as well. There are no simple-rules that make the fiber choice a no-brainer- it's assembly specific, but in no cases (in either the Nat'l Fiber sheet or the NRC summary) does one simply blow the other out of the water on performance.

Clearly as a DIY batts are easier for small jobs.

The stud-spacing affect on where low frequency attenuation minimum occurs (Figure 7 p.80) is consistent with physical intuition, and likely makes a bigger difference than fiber type & density for the low-end. The 12" oc studs attenuate about 10dB better than the 24" oc studs at 100hz, but pass a lot more than the wider spacing in the mid-range frequencies.

Your right. Stud spacing and decoupling are a much bigger deal. Moving stud spacing from 12" to 16" to 24" will progressively improve low frequency isolation. Actual decoupling really moves us into a better position.

When a partition is decoupled the added mass as well as absorption both provide a better return (bang for the buck)

This is turning into a great thread!

Don't know if it matters much, but some of the interior walls are 6 inch steel studs spaced 16 inches O.C., and others are 3 and 5/8ths inch steel studs spaced 16 inches O.C.

The ceiling is 14 inch steel joists spaced 24 inches O.C.

I would think batts would be much easier to deal with than cellulose.

Let me throw a wrench into this - one wall is a bedroom to one side and an art studio on the other. A plywood surface would be useful on the walls in the studio. I would assume a layer of plywood would function like another layer of drywall in that it adds mass? Would it matter which material, plywood or drywall, were next to the studs?

Jelly,

I guess you know about a product called Acoustiblok - a very effective sound proofing. Tampa, FL 813-980-1400 www.acoustiblok.com

Posted By Jelly on 15 Sep 2010 09:32 PM
This is turning into a great thread!

Don't know if it matters much, but some of the interior walls are 6 inch steel studs spaced 16 inches O.C., and others are 3 and 5/8ths inch steel studs spaced 16 inches O.C.

The ceiling is 14 inch steel joists spaced 24 inches O.C.

I would think batts would be much easier to deal with than cellulose.

Let me throw a wrench into this - one wall is a bedroom to one side and an art studio on the other. A plywood surface would be useful on the walls in the studio. I would assume a layer of plywood would function like another layer of drywall in that it adds mass? Would it matter which material, plywood or drywall, were next to the studs?

If the studs aren't already up you may be able to use a staggered 3" stud double-wall approach where you have the 6" wall.

Plywood has ~15% less mass of similar dimension drywall so the amount of mass it's adding isn't huge.  OSB has about the same density as wallboard, if that makes a difference to you.  Gluing it in place with GreenGlue would likely make a measurable difference. I doubt whether putting the drywall layer vs. the plywood layer next to the studs is critical.

Jelly;

24" oc framing is key for less sound transmision,  I can't figure out why your design would have joists @ 24 " studs @ 16", but since it aready is;  the quick and easy is:

caulk the track at floor and stuff batts ( any batts) in the stud cavity, this is the most common method used in homes

Many of the other suggestions will improve the STC but will be more complicated, labor intensive and more $$

Chris: 24" oc is 10db less than 16" oc in the mid-range frequencies but 10db higher in the bass, and they're about the same at the higher frequencies when looking at a single studwall. It sorta depends on which frequency ranges you're most interested in suppressing.

Posted By Dana1 on 16 Sep 2010 04:01 PM
Chris: 24" oc is 10db less than 16" oc in the mid-range frequencies but 10db higher in the bass, and they're about the same at the higher frequencies when looking at a single studwall. It sorta depends on which frequency ranges you're most interested in suppressing.

Dana1;

what is your source for that data?

Dana, the walls are in place, and the six inch ones are bearing the load of the SIP panels above them. So I'm pretty much locked in with the current configuration.

Chris, yes well, all of the joists clear span from panel wall to panel wall, so the spacing of the studs doesn't need to line up with the joists. But it would have made things easier and cheaper though if it were all 24" O.C. One lives and one learns.

Posted By cmkavala on 16 Sep 2010 05:01 PM

Posted By Dana1 on 16 Sep 2010 04:01 PM
Chris: 24" oc is 10db less than 16" oc in the mid-range frequencies but 10db higher in the bass, and they're about the same at the higher frequencies when looking at a single studwall. It sorta depends on which frequency ranges you're most interested in suppressing.

Dana1;

what is your source for that data?

I'm only as sure about it as I am of the li'l graph supplied by NRC- see  Figure 7 on p. 80. 

Which isn't as sure as if I'd measured it myself multiple times, but I tend to trust people who measure stuff.

The overall STC profile for the 24"(610mm) o.c. stuff was 36, which is four higher than the 16" (410mm) o.c. wall, but below ~150Hz the narrower stud spacing performs decidedly better. (OK, so make the differences between them 5dB, on average in the different bands, not 10dB, looking at it more carefully.)

As you debate adding insulation for sound attenuation, you are also thermally isolating the rooms. This is fine if the rooms are carefully thermally balanced, but it makes using a woodstove or other point source of heating more problematic. This may or may not be an issue in your case.

IIRC Jellys HVAC is balanced ducted air(?) running off a heat pump. But it's a good point- thermal envelopes within the primary thermal envelope may call for a micro-zoned approach.

Posted By Dana1 on 17 Sep 2010 03:38 PM

I'm only as sure about it as I am of the li'l graph supplied by NRC- see  Figure 7 on p. 80. 

Which isn't as sure as if I'd measured it myself multiple times, but I tend to trust people who measure stuff.

The overall STC profile for the 24"(610mm) o.c. stuff was 36, which is four higher than the 16" (410mm) o.c. wall, but below ~150Hz the narrower stud spacing performs decidedly better. (OK, so make the differences between them 5dB, on average in the different bands, not 10dB, looking at it more carefully.)

link did not work;  I guess I am just trying to wrap my head around the fact; if there is less surface area to transmit sound how can it be greater db ? maybe it is not a common sense issue

Chris,
The added mass of the closer oc spacing of studs actually deadens the soundwaves moving through the walls/floors. The wall/floor assemblies act the same as a drum. The bigger the spacing, the bigger the drum.

Wes;

wood studs have more mass , but worse STC ratings

1. Insulation will noticeably improve the STC rating of an assembly.

Description	Estimated STC Rating	Wall Assembly
3 5/8" metal studs, 5/8" gyp (2 layers total), No insulation	38 - 40
3 5/8" metal studs, 5/8" gyp (2 layers total), Batt insulation	43 - 44

2. Staggered or double stud walls are higher rated than single stud walls.

Description	Estimated STC Rating	Wall Assembly
2x4 stud, 5/8" gyp (2 layers total), Batt insulation	34 - 39
Staggered studs, 5/8" gyp (2 layers total), Batt insulation	46 - 47
2x4 studs, 5/8" gyp (2 layers total), Batt insulation	56 - 59

3. Metal stud walls perform better than wood stud walls.
(NOTE: This only applies to single stud assemblies. For double stud assemblies, there is virtually no difference.)

Description	Estimated STC Rating	Wall Assembly
2x4 stud, 5/8" gyp (2 layers total), Batt insulation	34 - 39
3 5/8" metal studs, 5/8" gyp (2 layers total), Batt insulation	43 - 44

4. Resilient channel can improve the STC rating of an assembly.
(NOTE: These ratings are based on laboratory tests. Because of the special care required when installing resilient channels, actual results could be substantially lower.)

Description	Estimated STC Rating	Wall Assembly
2x4 stud, 5/8" gyp (2 layers total), Batt insulation	34 - 39
2x4 stud, 5/8" gyp (2 layers total), Resilient Channel, Batt insulation	45 - 52

5. Adding additional layers of drywall can improve the STC rating of an assembly.

Description	Estimated STC Rating	Wall Assembly
2x4 stud, 5/8" gyp (2 layers total), Batt insulation	34 - 39
3 5/8" metal studs, 5/8" gyp (3 layers total), Batt insulation	39 - 40
2x4 stud, 5/8" gyp (4 layers total), Batt insulation	43 - 45

6. Drywall between double studs can dramatically reduce the STC rating of an assembly.

Description	Estimated STC Rating	Wall Assembly
2x4 studs, 5/8" gyp (4 layers total), Batt insulation	44 - 45
2x4 studs, 5/8" gyp (2 layers total), Batt insulation	56 - 59
2x4 studs,5/8" gyp (3 layers total), Batt insulation	59 - 60
2x4 studs, 5/8" gyp (4 layers total), Batt insulation	58 - 63

Wes & Dana1;

according to the Acoustcal Design Collaborative, 25 ga. steel studs have a 1 STC better rating when spaced at 24" oc instead of 16".

This makes more sense to me being there are less paths for sound to transmit thru when spaced further apart, same reason resilient channels are effective by isolatic sound thru less points of contact.

Posted By cmkavala on 17 Sep 2010 05:17 PM

Posted By Dana1 on 17 Sep 2010 03:38 PM

I'm only as sure about it as I am of the li'l graph supplied by NRC- see  Figure 7 on p. 80. 

Which isn't as sure as if I'd measured it myself multiple times, but I tend to trust people who measure stuff.

The overall STC profile for the 24"(610mm) o.c. stuff was 36, which is four higher than the 16" (410mm) o.c. wall, but below ~150Hz the narrower stud spacing performs decidedly better. (OK, so make the differences between them 5dB, on average in the different bands, not 10dB, looking at it more carefully.)

link did not work;  I guess I am just trying to wrap my head around the fact; if there is less surface area to transmit sound how can it be greater db ? maybe it is not a common sense issue

The link points to:

http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/ir/ir693/ir693.pdf

(which works reliably for me)

Dana1;

link worked that time I'll read thru it , Thanks