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Argument about the air gap between isolation walls
Old 28th August 2018
  #1
Gear Head
 

Argument about the air gap between isolation walls

After days and days of research the following argument never seems to end and it's impossible for me to find a clear answer.

I'm builing an isolated studio room. A double leaf system.
So basically a stand alone wall, not touching any other walls, an air gap and a second wall.

Pretty much every site out there tells you you need mass, air, mass. More specifically: double gypsum board (with green glue in between), insulation, air gap, insulation, double gypsum board. Or something similar.

BUT then if you start reading about this on forums you have this group of people who claim you NEED to fill the air gap with a light type of fiberglass.
Other people then show up saying you NEED to have an air gap and then they start fighting over it.

In the meantime nowhere i have been able to find the answer to this.

To me it seems odd to fill the air gap as i already have 2 layers on insulation on both sides of the air gap. So it will simply add a 3rd layer. Might as well use a super thick insultion then instead of 3 different layers.

Now what also bothers me, air gap or no air gap, everyone has a different opinion about these layers of insulation. 1 group says it needs to be as thick and compressed as possible, so basically a really compressed type of rockwool that blocks air and 1 group says no, it needs to be fluffy fiberglass so air can move through it.

Now i certainly don't want to start another fight here. I just hope someone has a clear answer to this.
Old 28th August 2018
  #2
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Jens Eklund's Avatar
The purpose of the wool is to absorb the resonance in the system, thus lowering the Q (and thus increasing TL at this frequency range). The properties we therefor look for are the same as when absorbing low frequencies; usually low GFR (more often than not: low density):

Low density fibre in 4" thick panels - worthwhile or worthless?

And as long as the wool is loos enough so that it doesn’t transfer vibrational energy between the two partitions; you can fill the gap completely, but safer is to leave a few cm of gap between the two sections just in case.
Old 28th August 2018
  #3
Gear Head
 

Quote:
Originally Posted by Jens Eklund View Post
The purpose of the wool is to absorb the resonance in the system, thus lowering the Q (and thus increasing TL at this frequency range). The properties we therefor look for are the same as when absorbing low frequencies; usually low GFR (more often than not: low density):

Low density fibre in 4" thick panels - worthwhile or worthless?

And as long as the wool is loos enough so that it doesn’t transfer vibrational energy between the two partitions; you can fill the gap completely, but safer is to leave a few cm of gap between the two sections just in case.
So basically what you are saying is sure, keep the air gap and on both sides of the air gap use a not too dense wool. Correct?

Seems pretty straight forward and i'll take your word for it.
It's just odd that when you read about these type of walls you are told you need as much mass as possible. isn't dense rockwool considered mass in this case? Are they strictly talking about the stuff you put after the insulation? Gypsum board in this case? And the insulation is simply not part of the mass? Thanks.
Old 28th August 2018
  #4
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Jens Eklund's Avatar
We want mass, yes. But we also need a wool that does what is should. Dense wool (often high GFR) will not absorb the resonance as well, and a lower TL would be the result of that (at frequencies near resonant frequency):

Low density fibre in 4" thick panels - worthwhile or worthless?

Also, denser wool could directly reduce the TL if the two sections have contact thus transferring energy from one partition to the other via direct coupling.

So never ever use dense wool. Mass is increased by adding layers of gypsum (or whatever), not by increasing the density of the wool.

Low resonant frequency (high TL at this range) of the system is achieved by adding mass and/or increasing the distance (volume) between the partitions (and to some extent by adding the absorber in the cavity).
Old 28th August 2018
  #5
Gear Head
 

Quote:
Originally Posted by Jens Eklund View Post
We want mass, yes. But we also need a wool that does what is should. Dense wool (often high GFR) will not absorb the resonance as well, and a lower TL would be the result of that (at frequencies near resonant frequency):

Low density fibre in 4" thick panels - worthwhile or worthless?

Also, denser wool could directly reduce the TL if the two sections have contact thus transferring energy from one partition to the other via direct coupling.

So never ever use dense wool. Mass is increased by adding layers of gypsum (or whatever), not by increasing the density of the wool.

Low resonant frequency (high TL at this range) of the system is achieved by adding mass and/or increasing the distance (volume) between the partitions (and to some extent by adding the absorber in the cavity).

Great info! Just the kind that i needed.

Out of curiosity. The companies advertising with really dense rockwool specifically for audio insulation. Who are the buyers? If you don't want this for studio isolation what would one want it for then? Any idea?
Old 28th August 2018
  #6
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Jens Eklund's Avatar
Not really.
Old 28th August 2018
  #7
Lives for gear
Quote:
Originally Posted by NerdyUsername View Post
After days and days of research the following argument never seems to end and it's impossible for me to find a clear answer.

I'm builing an isolated studio room. A double leaf system.
So basically a stand alone wall, not touching any other walls, an air gap and a second wall.

Pretty much every site out there tells you you need mass, air, mass. More specifically: double gypsum board (with green glue in between), insulation, air gap, insulation, double gypsum board. Or something similar.

BUT then if you start reading about this on forums you have this group of people who claim you NEED to fill the air gap with a light type of fiberglass.
Other people then show up saying you NEED to have an air gap and then they start fighting over it.

In the meantime nowhere i have been able to find the answer to this.

To me it seems odd to fill the air gap as i already have 2 layers on insulation on both sides of the air gap. So it will simply add a 3rd layer. Might as well use a super thick insultion then instead of 3 different layers.

Now what also bothers me, air gap or no air gap, everyone has a different opinion about these layers of insulation. 1 group says it needs to be as thick and compressed as possible, so basically a really compressed type of rockwool that blocks air and 1 group says no, it needs to be fluffy fiberglass so air can move through it.

Now i certainly don't want to start another fight here. I just hope someone has a clear answer to this.
Hello,

A wall in concret brick with the wool glued to the wall (classic thermal insulation in France) has a lower sound attenuation than the same wall alone.
Old 28th August 2018
  #8
Lives for gear
Quote:
Originally Posted by NerdyUsername View Post
Great info! Just the kind that i needed.

Out of curiosity. The companies advertising with really dense rockwool specifically for audio insulation. Who are the buyers? If you don't want this for studio isolation what would one want it for then? Any idea?
The weight of a floor do not give the same constraint than a wall or a ceiling.
Old 28th August 2018
  #9
Gear Head
 

Quote:
Originally Posted by dinococcus View Post
Hello,

A wall in concret brick with the wool glued to the wall (classic thermal insulation in France) has a lower sound attenuation than the same wall alone.
Ok. But what does this have to do with my question?
Old 28th August 2018
  #10
Lives for gear
Quote:
Originally Posted by NerdyUsername View Post
Ok. But what does this have to do with my question?
i read

Argument about the air gap between isolation walls

and

Now what also bothers me, air gap or no air gap, everyone has a different opinion about these layers of insulation. 1 group says it needs to be as thick and compressed as possible, so basically a really compressed type of rockwool that blocks air and 1 group says no, it needs to be fluffy fiberglass so air can move through it.
Old 28th August 2018
  #11
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nosebleedaudio's Avatar
 

Finding people that will argue over or about a "Point" is not hard to find...
I would NOT worry about it...
Old 28th August 2018
  #12
Gear Head
 

Quote:
Originally Posted by nosebleedaudio View Post
Finding people that will argue over or about a "Point" is not hard to find...
I would NOT worry about it...
Hah yeah i know.
I do worry about it though because i'm spending quite some money and having people saying different things and swearing by it is confusing.
Old 28th August 2018
  #13
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nosebleedaudio's Avatar
 

Quote:
Originally Posted by NerdyUsername View Post
Hah yeah i know.
I do worry about it though because i'm spending quite some money and having people saying different thing and swearing by it is confusing.
I have had many conversations with well known acousticians and they had VERY different opinions about material ect..
BUT for me someone would have to do FAR more than argue a point...
The MAIN reason for double wall with a GOOD space is for Low frequency Isolation, adding could ONLY make things better IMHO, but IMHO the main reason for NOT adding between the space would be its simply not making enough different to justify the added expense..IMHO...

My .02 cents
Old 28th August 2018
  #14
Gear Head
 

Quote:
Originally Posted by nosebleedaudio View Post
I have had many conversations with well known acousticians and they had VERY different opinions about material ect..
BUT for me someone would have to do FAR more than argue a point...
The MAIN reason for double wall with a GOOD space is for Low frequency Isolation, adding could ONLY make things better IMHO, but IMHO the main reason for NOT adding between the space would be its simply not making enough different to justify the added expense..IMHO...

My .02 cents
Yeah, i see what you're saying!
Old 28th August 2018
  #15
Lives for gear
Quote:
Originally Posted by NerdyUsername View Post

I'm builing an isolated studio room. A double leaf system.
Maybe you already know this, but it seems like it's worth mentioning here that a 2 leaf system needs to include the ceiling and possibly the floor (depending on construction type) to fully realize it's potential.

Also if you are pulling permits for this, you likely cannot leave an entirely empty airspace as IBC requires some sort of fireblocking in most cases.
Old 28th August 2018
  #16
Gear Head
 

Some more info

To add to what Jens has said already:

A 2 leaf wall assembly is better viewed as Mass-Spring-Mass instead of Mass-Air-Mass.
The cavity between the walls acts as a spring dampening the resonance of the leaves.

If you use insulation in the cavity it improves the effectiveness of isolation by lowering the resonant frequency. (Kind of like how you can dampen a bass drum by filling it partially with cloths) Here is a formula to estimate this:

Fx = c[(m1+m2)/(m1m2d)]^0.5

c is a constant: 60 if an empty cavity and 43 if insulated.
m1 and m2 are the masses of each leaf (kg/m2).
d is depth of the cavity (m).

If the insulation is very dense it might bridge the 2 leaves effectively short circuiting the spring. It's best to use a lower density insulation or leave a gap between two layers of rigid insulation in the cavity pressed up against each leaf.

Hope it helps you understand
Old 28th August 2018
  #17
Gear Head
 

Quote:
Originally Posted by nosebleedaudio View Post
The MAIN reason for double wall with a GOOD space is for Low frequency Isolation, adding could ONLY make things better IMHO, but IMHO the main reason for NOT adding between the space would be its simply not making enough different to justify the added expense..IMHO...

My .02 cents
You would be surprised just how much of an effect this can have. For example:

*****************
I can't find the correct formulas atm but a good rule of thumb advised by the guys over at John L Sayers forum is this:

Get the MSM resonant frequency of your walls down so (at least) 1.4x the frequency is lower than your lowest frequency you plan to isolate.

Example:

Two layers of 12.5mm plasterboard either side of a decoupled two leaf wall; around 17.6kg/m2 per leaf, with a 200mm cavity depth:

[(17.6 + 17.6)/(17.6 * 17.6 * 0.2)] ^0.5
[35.2/61.95] ^0.5
0.57 ^0.5
0.76

for insulated cavity:
0.76 * 43 = 32.68Hz

for empty:
0.76 * 60 = 45.6Hz

So your empty cavity starts getting good isolation at around: >64Hz
and insulated cavity starts getting good isolation at around: >46Hz

They get a lot better isolation around 2x the resonant frequency: so 91Hz for empty or 65Hz for insulated

*****************

So if you play bass guitar a lot, the lowest string is 42Hz in standard tuning (E). So the above wall might not provide good enough isolation for this frequency (depending on your requirements), you could add another layer of mass to each leaf to improve, or swap the mass there for denser material.
Eg: 3 layers of plasterboard each leaf (26.4kg/m2)

[(26.4 + 26.4)/(26.4 * 26.4 * 0.2)] ^0.5
[52.8/139.39] ^0.5
0.38 ^0.5
0.62

for insulated cavity:
0.76 * 43 = 26.66Hz

Or even cheaper if you have the space, increase the cavity! (now with 400mm cavity and original 2 layers of plasterboard)

[(17.6 + 17.6)/(17.6 * 17.6 * 0.4)] ^0.5
[35.2/123.9] ^0.5
0.28 ^0.5
0.53

for insulated cavity:
0.53 * 43 = 22.79Hz

Either of which should get good isolation for your low E string.

In the end, these are just estimates and rules of thumb though guys and gals.
Filling the cavity seems to make sense to me though, just fill it with cheap fluffy, but make sure it doesn't sag over time.
Old 29th August 2018
  #18
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nosebleedaudio's Avatar
 

Of course it would have an effect..
Plus if it was a studio I was designing and had the space I would use 2 by 6 double walls plus bat insulation thru out..
Old 30th August 2018
  #19
Gear Head
 

Quote:
Originally Posted by Dan Vernall View Post

If the insulation is very dense it might bridge the 2 leaves effectively short circuiting the spring. It's best to use a lower density insulation or leave a gap between two layers of rigid insulation in the cavity pressed up against each leaf.

Hope it helps you understand
Thanks for your input.
This is quite confusing as double leaf systems are recommended to have insulation in each leaf.

If you want a low density insulation in the air gap then what kind of insulation should be in the leafs?
Basically with a 2 leaf system if you fill the air gap as well you have 3 layers of insulation.

2 x Gypsum board - insulation - air gap filled with insulation - insulation - 2 x gypsum board.

Seems odd to me. Why call it an air gap if it's filled up (lightly) anyway?

So if people are saying you should not use dense insulation but low density fiberglass i would insulate my double stud system with 3 layers of fiberglass all next to each other. Odd. Doesn't make sense to me.

Wouldn't it make more sense then to use a dense insulation, then either an air gap or air gap filled with low density fiberglass and then a thick insulation again. Of course finishing both sides with gypsum.
Old 30th August 2018
  #20
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Jens Eklund's Avatar
Again; a high-density wool (presumably high GFR) will not absorb the resonance as effective as a wool with the appropriate GFR.

If not in a studio situation, this might not be an issue if low frequency isolation might not be needed, but if low frequency isolation is important (as it is in a studio situation), it´s important to reduce the Q of the mass-air-mass system as much as possible since it´s at fc the TL will suffer most.

Argument about the air gap between isolation walls

And again also due to the risk of coupling the two partitions together if using dense wool that might transfer energy from on side to the other if touching.
Old 30th August 2018
  #21
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avare's Avatar
 

Quote:
Originally Posted by NerdyUsername View Post
Thanks for your input.
This is quite confusing as double leaf systems are recommended to have insulation in each leaf.

If you want a low density insulation in the air gap then what kind of insulation should be in the leafs?
You are complicating things. The type of insulation to use was written in post #2 (low density). Use the same insulation in all of the wall cavity.

Andre
Old 30th August 2018
  #22
Gear Head
 

Quote:
Originally Posted by NerdyUsername View Post
Thanks for your input.
This is quite confusing as double leaf systems are recommended to have insulation in each leaf.

If you want a low density insulation in the air gap then what kind of insulation should be in the leafs?
Basically with a 2 leaf system if you fill the air gap as well you have 3 layers of insulation.

2 x Gypsum board - insulation - air gap filled with insulation - insulation - 2 x gypsum board.

Seems odd to me. Why call it an air gap if it's filled up (lightly) anyway?

So if people are saying you should not use dense insulation but low density fiberglass i would insulate my double stud system with 3 layers of fiberglass all next to each other. Odd. Doesn't make sense to me.

Wouldn't it make more sense then to use a dense insulation, then either an air gap or air gap filled with low density fiberglass and then a thick insulation again. Of course finishing both sides with gypsum.
We seem to have crossed paths somewhere. The "air-gap" refers to the entire cavity (gypsum to gypsum) you should fill this entire cavity with low density insulation, no air gaps.
If for some reason you couldn't get low enough density insulation you would need to leave a gap between two layers (one against each gypsum). But again, if you can get it, use low density to fill the entire cavity with no air gaps.
Old 1st September 2018
  #23
Gear Guru
Void

The airgap we frequently refer to is between an absorbent layer and the solid boundary. In partitions, I would call it the void. Published data by Isover UK show that there is no difference between very light insulation and dense insulation in this context. They also find it best to fully fill the void, but not compress the fibre at all. A light full fill. This prevents LF transmission which would happen if you packed it in there. Fibre lightly touching the plasterboard damps it a little.
You will need to provide periodic horizontal supports to prevent the fibre sagging. Wire or such works well.

DD
Old 1st September 2018
  #24
Gear Head
 

Quote:
Originally Posted by DanDan View Post
The airgap we frequently refer to is between an absorbent layer and the solid boundary. In partitions, I would call it the void. Published data by Isover UK show that there is no difference between very light insulation and dense insulation in this context. They also find it best to fully fill the void, but not compress the fibre at all. A light full fill. This prevents LF transmission which would happen if you packed it in there. Fibre lightly touching the plasterboard damps it a little.
You will need to provide periodic horizontal supports to prevent the fibre sagging. Wire or such works well.

DD
Hi Dan (Excellent name btw),

I have heard "air gap" used in that context when it comes to treatments such as helmholtz and panel resonators, but hadn't heard of people recommending a gap between the insulation and the plasterboard in a MAM wall. Tomato/tomato right? Do you have a link to the Isover data btw? I've noticed insulation companies tend to use STC, which (as you know) doesn't measure any values below 125Hz; and we're most interested in those values.
Old 1st September 2018
  #25
Gear Guru
Translation

Quote:
hadn't heard of people recommending a gap between the insulation and the plasterboard in a MAM wall
Sorry if my speed writing was not clear. I have never seen anyone recommend an undamped layer in there. Maybe change to MGM, Mass Gap, Mass, when talking about SoundProofing.
Then it becomes intuitive that the larger the gap the harder it is for the sound to get through. Air is not a good conductor, compared to solids. Also the heavier the M layers.....
The Isover Page vanished, but it was unequivocal. A full fill of light insulation is the best bang per buck, damping the void and the boards. Damping resonances and changing both frequency and Q. If I remember rightly, it could be worth up to 9dB.

All of the commonly published data tends to be focussed on frequencies in the speech range.
There are only a few of us!

DD
Old 2nd September 2018
  #26
Gear Head
 

Quote:
Originally Posted by DanDan View Post
Isover Page vanished, but it was unequivocal. A full fill of light insulation is the best bang per buck, damping the void and the boards. Damping resonances and changing both frequency and Q.
Thanks Dan.

You've got lots of people here now OP saying roughly the same thing, that you should fully fill the entire cavity/void/air gap between your gypsum with light insulation. Let us know how your build goes!

Dan
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