# Hollow sounding walls



## corock (Sep 7, 2009)

The exterior walls in my basement HT build are insulated with spray foam. Because the framing was constructed 2" from the concrete walls, and there is 2" of spray foam, the 3.5" of stud space is empty. Of course fibreglass insulation isnt necessary for heating purposes, but should I put it in regardless to "deaden" the sound of the walls? That seems logical to me but I don't want to spend the money if there is no acoustical benefit to it.


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## GranteedEV (Aug 8, 2010)

Adding insulation should deaden the wall from transmitting sound to the other side, but I can't imagine much acoustic benefit to your side of the wall, unless sounds from the opposite side are raising your noise floor.


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## corock (Sep 7, 2009)

Well, it's earth on the opposite side of the wall so I don't think it will be creating much noise, so I'm safe there. I was concerned the 3.5" space between the drywall and the spray foam would result in resonating, but I'm unsure if that is a legitimate concern.


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## kjgarrison (Nov 17, 2007)

Interested in hearing more about the room you're working on if you care to share.


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## Anthony (Oct 5, 2006)

Rather than fiberglass, I would go with Roxul. It is mineral wool instead of fiberglass. Easier to cut, still itchy, but not in the annoying persistent way fiberglass is.

They make a non-insulation product that is only for sound deadening. I'm sure it provides some thermal insulation, but they don't even offer a rating for it. It is really good at deadening sound, though. We used it at work to build a box around a noisy hydraulic pump. Worked great.


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## corock (Sep 7, 2009)

kjgarrison said:


> Interested in hearing more about the room you're working on if you care to share.


I haven't gotten around to making a thread on the room yet, but I will one of these days. It's a fairly small HT at 11' X 17' with 8' ceiling. I'm putting the focus and cash on the equipment and keeping the room simple and functional, so it won't be anything fancy. 



Anthony said:


> Rather than fiberglass, I would go with Roxul. It is mineral wool instead of fiberglass. Easier to cut, still itchy, but not in the annoying persistent way fiberglass is.
> 
> They make a non-insulation product that is only for sound deadening. I'm sure it provides some thermal insulation, but they don't even offer a rating for it. It is really good at deadening sound, though. We used it at work to build a box around a noisy hydraulic pump. Worked great.


Yes, I'm using Roxul Safe & Sound on the interior walls and ceiling already so I guess I'll just continue on with it in the exterior walls.


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## bpape (Sep 14, 2006)

Adding insulation whether Roxul or standard fiberglass will help to damp the cavity and allow the flexing drywall to provide an amount of bass absorption in the room rather than the empty cavities acting more like a drum.

Bryan


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## kjgarrison (Nov 17, 2007)

bpape said:


> Adding insulation whether Roxul or standard fiberglass will help to damp the cavity and allow the flexing drywall to provide an amount of bass absorption in the room rather than the empty cavities acting more like a drum.
> 
> Bryan


I don't mean to hijack this thread, but I'm interested, Bryan, in learning more about how sound isolation also can add acoustical treatment to a room if in fact that is what you are saying. Perhaps there is a reference or a link you can suggest to learn more about this?


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## Ile (Nov 23, 2010)

Drywall works as a panel resonator and when there is insulation in cavity absorbtion will be doubled and it also lower panels resonant frequency. Insulation lowers the Q of the trap so it will be effective over a wider frequency range.

Using different density panel materials or different cavity depth gives more even absorbtion for bass frequencies, because then resonators are tuned for different frequency.

Because corock's walls have already spray foam insulation it's also possible leave some parts without drywall. So he could fill stud cavities with mineral wool and use perforated drywall or fabric at some areas instead of drywall. So there will be empedded acoustic panels at for example at first reflection points and front wall. That will lower sounproofing, but if that is not concern...


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## bpape (Sep 14, 2006)

In many cases, isolation and in room acoustic control are 2 very different things. For the reasons well described in the post above, the case of insulating walls is a bit different. You are in fact lowering the resonance of the structure, thereby causing less energy to be transmitted as efficiently. The drywall with insulation (not foam which does nothing acoustically inside the room or to the cavity other than to limit how much fluffy you can use) does provide some membrane type absorption to the room while simultaneously reducing how much it continues to ring and add it's own information to the room.

Think of 2 kick drums - one with a damper or a pillow in it (insulated wall) vs one that is not damped. 

Bryan


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## kjgarrison (Nov 17, 2007)

bpape said:


> In many cases, isolation and in room acoustic control are 2 very different things. For the reasons well described in the post above, the case of insulating walls is a bit different. You are in fact lowering the resonance of the structure, thereby causing less energy to be transmitted as efficiently. The drywall with insulation (not foam which does nothing acoustically inside the room or to the cavity other than to limit how much fluffy you can use) does provide some membrane type absorption to the room while simultaneously reducing how much it continues to ring and add it's own information to the room.
> 
> Think of 2 kick drums - one with a damper or a pillow in it (insulated wall) vs one that is not damped.
> 
> Bryan


OK, I'm new to all of this, so if my questions seem dumb, consider the source ...

I do understand a little about how reflected sound waves result in points in a room where certain frequencies do everything from total summation to total cancellation. If I have it right, these are collectively called "modes" and "peaks" and "nulls" for summations and cancellations respectively.

I have some understanding that by absorbing the sound waves at the first reflection the amplitude of these summations and the even more bothersome cancellations are reduced.

I also understand your point about how the material behind the drywall lowers the resonant frequency of the structure, but I do not understand why the new and lower resonant frequency would be preferable to the starting point resonant frequency, unless the starting freq. is known to be problematic and the new and lower freq. is so low as to be either below audibility or be a non-problematic frequency. I mean I have seen these calculators where the locations and frequencies of modes are shown. Absorption I understand somewhat, but going to an unknown lower frequency alone, I don't.

I'm sure I'm missing something here ...


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## bpape (Sep 14, 2006)

If the resonance of a structure is higher, there is more of the range which is going to pass through easier. Essentially, the efficiency of sound passing through a structure is much lower above the resonance than below it. It's not just AT the frequency.


Also, an undamped cavity actually rings and resonates while a damped cavity actually helps to absorb by acting as a somewhat inefficient membrane absorber. Again, think of a kick drum with a pillow in it vs one without it. One is tight and controlled and stops almost immediately. The other is boomy, has overtones, and rings for a decent amount of time.

Bryan


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## kjgarrison (Nov 17, 2007)

bpape said:


> If the resonance of a structure is higher, there is more of the range which is going to pass through easier. Essentially, the efficiency of sound passing through a structure is much lower above the resonance than below it. It's not just AT the frequency.
> 
> 
> Also, an undamped cavity actually rings and resonates while a damped cavity actually helps to absorb by acting as a somewhat inefficient membrane absorber. Again, think of a kick drum with a pillow in it vs one without it. One is tight and controlled and stops almost immediately. The other is boomy, has overtones, and rings for a decent amount of time.
> ...


OK, I see. Thank you.

So if it is done right, sound isolation can also accomplish: (which apply?)


some of what is needed for acoustic treatments?
a LOT of what is needed for acoustic treatments?
possibly overdoing dampening and making the room dead?
unintended consequences, such as taking out too much of the higher frequencies, that are big problems that have to be corrected also?


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## bpape (Sep 14, 2006)

E. A tiny bit of in room acoustics by damping the walls so they don't add resonances to the space while also dropping the ambient noise floor in the room by stopping outside sounds from getting in. 

Bryan


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## kjgarrison (Nov 17, 2007)

Thanks Brian.

An apology to the OP for this little detour ...


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## corock (Sep 7, 2009)

very informative detour, thanks.

To dumb it down, I put fibreglass insulation between the drywall and spray foam in the HT room, but not in the other seperate rooms in the basement. Simply tapping on the drywall without the fibreglass resulted in a very low tone that resonated for a remarkable amount of time. The walls treated with firbeglass have a much higher tone without any length of resonance noticable to the bare ear.


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## bambino (Feb 21, 2010)

I would fill the void just for piece of mind as well as the other reasons presented by the other posters. I think it would help.:T


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## corock (Sep 7, 2009)

I did fill the HT exterior wall cavaties before drywalling. I was just pointing out that it is immediately noticable the difference between filled and not filled. Just the length of time the unfilled wall will ring after being struck is shocking.


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## bambino (Feb 21, 2010)

corock said:


> I did fill the HT exterior wall cavaties before drywalling. I was just pointing out that it is immediately noticable the difference between filled and not filled. Just the length of time the unfilled wall will ring after being struck is shocking.


Ahh, my bad i did not read through the entire post. So is there any way to get some sort of fill in the hollow areas without total destruction? Or am i still misunderstanding/:huh:


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## corock (Sep 7, 2009)

yes you're still misunderstanding. The exterior walls in the HT room have fibreglass in them. The exterior walls in the other rooms I did not bother to fibreglass because they are seperated from the HT by heavily insulated interior walls.

This is new construction. Well, sorta new. I completely demo'd the basement, except the exterior wall framing, and started over. 
Here's a pic of the walls spray foamed before I put the fibreglass into the cavity.


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