Homasote Test

[AVare]

Thank you Steve and great stuff!

Andre

[DanDan]

Just reading in NRC 4068 where Bradley references the "stiffness of the contained air".....definitely pondering that.

Hi Steve. I would guess that refers to the amount of damping in the void. I have some Isover test data here which shows maximum benefit with a full fill. It also shows no difference in performance over a very wide range of densities.
I usually recommend such, with the fibre actually touching both leaves. I am assuming a light touch there will dampen bending resonances. I have never seen any test or even theory confirming this, nor any attempt to suggest how much touching.
We know of course that fibre stuffed in there will cause LF transmission. So for now for me, full fill, lightly touching.
Many seem to miss the Isover point that a full fill is best, and that even a small airgap in there is unhelpful.
Indeed a so called Professional Consultant here insisted to me that loose batts in there are best, to avoid touching.

Thoughts?
DD

[Soundman2020]

Many seem to miss the Isover point that a full fill is best, and that even a small airgap in there is unhelpful.

Very true! I have no idea why people would not want to fill the cavity entirely. It's just logical, and also supported by research.

Indeed a so called Professional Consultant here insisted to me that loose batts in there are best, to avoid touching.

Strange advice indeed...

- Stuart -

[SGleason]

We know of course that fibre stuffed in there will cause LF transmission. So for now for me, full fill, lightly touching.

Hey DanDan.....

I definitely think that a firm fill with a light touch in a wall cavity is the best way to go. Occasionally someone will ask me about stuffing 6" of fluffy in a 4" cavity and I advise against it, of course, as I believe that those compressed fibers can become a physical transmission path at that point. I do have to wonder, though, if that light touch is enough to damp the resonance of the plane.

no difference in performance over a very wide range of densities.

In the data that I posted early you can see an overall STC 1 difference between mineral wool and fiberglass.

We deal a lot with floor/ceiling assemblies where it may be a different dynamic. It has been suggested that with a 12" cavity half filled with pink fluffy there could be a subtle impedance mismatch that is created between the air and the insulation that causes some shear. It'll be tough to get any real data to back that up, though.

Steve

[AVare]

In the data that I posted early you can see an overall STC 1 difference between mineral wool and fiberglass.

Incomplete reporting confuses issues. People misconstrue IR-761 data as showing that glass fibre is better. What it does show is that insulation that is 2 to 8 times denser is not as good. See fig 2 on pdf page 17 for insulation densities. We have known for decades that lighter is better for filling cavities.

Andre

[Soundman2020]

We deal a lot with floor/ceiling assemblies where it may be a different dynamic. It has been suggested that with a 12" cavity half filled with pink fluffy there could be a subtle impedance mismatch that is created between the air and the insulation that causes some shear. It'll be tough to get any real data to back that up, though.

As you say, I'd like to see the data that shows such an effect!

Many people have the wrong idea about what insulation in the cavity does: some people think it just "stops sound" from getting through. That's not really true. If you took the same thickness of insulation and placed it directly in front of a speaker playing typical contemporary music, you'd hear the music fairly well. Muffled highs, yes, and muddy mids, it is true, but you'd still hear it with just a small reduction. Only slight TL drop like that.

Insulation by itself is pretty bad at "stopping the flow of sound" on its own. But what it DOES do very well, is to damp resonance.

I often use the analogy of a kitchen sponge: If you spilled some water in your kitchen, and need to mop it up, then a sponge is a good way of doing that: it picks up the water very well, then you can squeeze out the water in the sink (or wait for the sponge to dry), and mop up more spills. You can carry on mopping up and squeezing out all day, with no problem! Sponges are good at that. But if you then turn on the tap and hold the sponge over the end of it, the water comes pouring through, as though the sponge wasn't even there. It does nothing to "stop the flow".

That's sort of like trying to use porous insulation to "stop the flow" of sound: it does nothing much useful. But when you make it part of a SYSTEM, then it does what it is really good at: mopping up resonance, or "damping". Inside an empty wall cavity, there are multiple resonances going on at different frequencies. There's the MSM resonance, of course, between the two sides of the wall, plus probably some modal resonances at very different frequencies, plus several others. Insulation can damp all of those, very effectively.

The insulation does several things at once to help with that. Firstly, it slows down the speed of sound: sound waves travel quite a bit slower through insulation than they do through empty air, so from one point of view, the sound "sees" a longer distance between the leaves than is really there (it takes a longer time than it would have to cross the gap), secondly the presence of the insulation changes the way the air deals with heat from adiabatic to isothermal, and third, it provides impedance for the movement of sound, which isn't the same as "stopping the flow" of sound. All of those are "good things" for damping.

Now, think about your 12" cavity: if you fill it only 6" deep, then you only have half the cavity with all those "good things" happening in it. The other half is just empty air, where the resonances are all still happening... and robbing the wall of isolation. Filling the cavity entirely means that the above "good things" are happening in 100% of the cavity, not just 50% of it.

Filling the cavity complete with SUITABLE insulation, is actually one of the best things you can do to a wall, to increase TL.

If my words don't convince you, then maybe actual data will...

effect-of-various-wall-cavity-insulation-fill-percentages.png

I also know that I have a report somewhere titled "The Acoustic Role Of Glass Wool In Double-Leaf Walls" someplace, but I can't find it right now. I'll dig some more, and see if it turns up... All I have is this extract:

insulation-in-MSM-walls-cavity-fill-isolation-study-awg_paper_CH_1-extract.jpg

- Stuart -

[DanDan]

Yup. There are points worth exploring though IMO.
Bending waves and their resultant dip in TL at resonance.
How much pressure of fibre to damp those but not decrease LF TL.

DD

[SGleason]

I can't tell you guys how much I appreciate this discussion!

I also know that I have a report somewhere titled "The Acoustic Role Of Glass Wool In Double-Leaf Walls" someplace, but I can't find it right now. I'll dig some more, and see if it turns up...

I'd love to see that report if you can find it. I appreciate your above post.

Yup. There are points worth exploring though IMO.

Indeed. Theory is part of the learning process but it only becomes a productive exercise when it gets backed up (or not ) by data, right?

EDIT:
Would any of you like to speak of the specific relationship between RC and insulation? It is my understanding that the benefit of insulation is maximized when it is deployed in conjunction with resilient channel. I am curious about those dynamics.

[AVare]

EDIT:
Would any of you like to speak of the specific relationship between RC and insulation? It is my understanding that the benefit of insulation is maximized when it is deployed in conjunction with resilient channel. I am curious about those dynamics.

There is no maximizing. Insulation absorbs sound that travels through the cavity.

Resilient channel was created to give acoustic isolation to wood stud walls as metal studs. Below the STC frequencies it is actually worse. It works reducing the strength of the mechanical connection between the leafs. Used on both leafs it makes things worse. On a double stud wall I makes TL worse. Designers have frustrations using RC because installers either miss the channel, screw into the studs and/or install the channel upside down.

Andre

[Soundman2020]

I'm gonna pull an Andre trick here: Take a look at the paper "NRCC-44692 : A Simple model of the sound insulation of gypsum board on resilient supports"

Recently uploaded to the reference area... http://www.johnlsayers.com/phpBB2/viewt ... 4&p=149463 It's the last one at the bottom of the list...

As you can see, and as Andre already said: RC isn't very exciting, even when tested under perfect lab conditions... Put it in the hands of a typical careless framer or drywaller, and all bets are off...

- Stuart -

[DanDan]

I'd love to see that report if you can find it. I appreciate your above post.

Stuart already showed a screenshot. But there is a lot more in the Powerpoint Doc. All stuff we know but very nicely presented IMO.
The file is too big to post here, so if anyone wants it, ask me via real email soundsound@eircom.net
DD

[SGleason]

Thanks for that link, Stuart.......I'll look it over today.

Regarding any insulation performance enhanced effect from RC, I brought that into the discussion because of a minor statement in an NRC-CNRC document that I've had kicking around for quite a while that has always nagged at me. I have highlighted that passage in this link:

NRCCNRC RC Ins.pdf

Steve

[AVare]

Regarding any insulation performance enhanced effect from RC, I brought that into the discussion because of a minor statement in an NRC-CNRC document that I've had kicking around for quite a while that has always nagged at me. I have highlighted that passage in this link:

NRCCNRC RC Ins.pdf

Uh, I am confused. On page 1 of this thread I inked a thread from 15 years ago regarding sub 100 Hz TL performance of walls from IR 761. In the table I put in there it shows an 11 to 16 point improvement in STC from the addition of RC. Assuming you read that, what is your question?

Andre

[SGleason]

I definitely read all of that data, Andre, and I do apologize for beleaguering this point.

It's just that one line in that NRC-CNRC report that has always nagged at me....."Adding resilient channels to one face of a single row of studs improves sound reduction considerably, allowing the sound absorbing material in the cavity to be effective." That seems to imply that the insulation will not be as effective without the RC.

So the specific question is.........is the optimal performance on the insulation in the cavity in any way dependent upon the presence of resilient channel?

Steve

[AVare]

It's just that one line in that NRC-CNRC report that has always nagged at me....."Adding resilient channels to one face of a single row of studs improves sound reduction considerably, allowing the sound absorbing material in the cavity to be effective." That seems to imply that the insulation will not be as effective without the RC.

So the specific question is.........is the optimal performance on the insulation in the cavity in any way dependent upon the presence of resilient channel?

Thanks for the clarification. Expanding/adding/re-enforcing my first post on this sub topic, as shown in fig 5 of NRC 44692 there are 2 parallel paths for sound through such partitions. Each type of sound isolation address different paths.

As a sadly humorous related isolation issue is floating floors (AKA why doesn't my floating plywood floor with 20 Hz springs and 2" air space not do anything for lows?}.

Andre