Just wondering what the thoughts were on using cement board for sound isolation. I had never thought about using it until I read in the Winter 2009 issue of Electronic Lifestlyes, for those of you who receive it from CEDIA, that a theater had used it. If you care to read the article it was used for isolation in the gold technical design winner 115k to 150k theater.

I would think adding that much mass would account for something. However, if using it on a ceiling the weight would be a concern to me.

any thoughts?

It's a great product for our needs, especially if you're considering a damping compound in between layers. The high-mass, high density and high-flex qualities are great.

The practicality is still the issue. Exterior 1/4" cement siding is excellent. It's super dense, etc, and a 4x8 sheet... however you can't recess a screw into the dace easily. Also produces nasty silica dust.

Smaller 3x5 sheets are easier to install, but more panels to hang and the seams can't be butted as tight. Also, these tile backer-type boards are not as dense as the exterior material but some are denser than drywall.

These types of panels would likely be the first layer on wall or ceiling. Then a second layer using drywall to finish. A 3x5 would be a bit tougher to hang evenly on studs.

Ted, would you put a GG sandwich between the cement board and the drywall?

Yes. However we have to keep in mind that optimal damping will occur when each half of the sandwich have a similar flexibility. If the cement board were significantly floppier than the drywall, some damping potential would be lost.

Although it might be overkill (and therefore a great idea) how about 2 layers of the 1/4" cement board, topped with a layer of 1/2" or 5/8"DW, each layer separated by GG? Should be a lot more mass and less thickness than 2 layers of DW

It's actually an idea that I thought about, too....but the theory behind it was far outweighed by the practicality of using it.

Just cutting it is both messy, dangerous, and time-consuming (especially since the specialty blades used for cutting it don't last very long). I would imagine cutting all of the corners, electrical, etc. into it would be prohibitively time-consuming. It is annoying enough just to cut it for flooring.

Agree.

In theory it is a good idea. More dense material gives more mass per inch of thickness. Also you get a second damping layer.

But the practicality is the issue.

I wonder just what the difference would be? Cement board is really heavy stuff and while the cutting and fitting of it would be a pain the overall benefit might just be worth it. It was the first time I had ever seen it used for a HT type application.

Simpler, cheaper, etc would be three sheets of 5/8" drywall. That's done everyday.

Keep in mind that there are a couple types of cemetitious tile backer board. You have Durock, which is a more typical cement between layers of mesh, it cuts pretty easily with a razor knife using the snap and score method, just like drywall (though you will go through lots of blades). Then you have the Hardie board and others that are similar and are the more difficult to cut with lots of nasty dust, but thinner than Durock usually.

Okay, so we know drywall is easier to work with, because cutting cement board is more of a pain (though as Lee suggests, some brands can be scored with a blade and snapped, which is not so bad, but dry wall is still much easier).

But why not hang 4x8 sheets of cement board where you can, and then fill in the leftover spots with regular dry wall? That way you avoid cutting the cement board, but still get more mass in that layer, for most of the room. Let's say you only wanted to do two layers of dry-wall (because of space issues, or just time), why not do the first layer this way?

But why not hang 4x8 sheets of cement board where you can, and then fill in the leftover spots with regular dry wall? That way you avoid cutting the cement board, but still get more mass in that layer, for most of the room. Let's say you only wanted to do two layers of dry-wall (because of space issues, or just time), why not do the first layer this way?


Your soundproofing is only as good as your weakest link, so in this case the sections with DD.

To take an extreme example - image a double cementboard room that is perfectly installed. Now add an open door to that room. No matter how perfect those walls and ceiling are, the open door defeats any sound proofing efforts.

Standard Durock, etc isn't that much different in density to drywall. For this to work, you'd have to get the ultra-dense stuff from Joe Hardy Co.

$35-$45 a 4x8 sheet for 1/4" as I recall. Drywall is $7

Your soundproofing is only as good as your weakest link, so in this case the sections with DD.

To take an extreme example - image a double cementboard room that is perfectly installed. Now add an open door to that room. No matter how perfect those walls and ceiling are, the open door defeats any sound proofing efforts.

The chain analogy doesn't really fit here. A chain will break and be useless, and thus is as only good as the weakest link. But 100% of a theater's sound doesn't escape out of the weakest point of a room. Every theater compromises in certain spots on sound isolation and other concerns (that is why we have doors to our theater :)). My point is that the less weak spots you have, the better. If cement board improves sound isolation, and reduces the amount of weak spots, that can't be a bad thing in my opinion.

Standard Durock, etc isn't that much different in density to drywall. For this to work, you'd have to get the ultra-dense stuff from Joe Hardy Co.

$35-$45 a 4x8 sheet for 1/4" as I recall. Drywall is $7

Now we are talking numbers and making sense. 1/2 inch Durock weighs cement board is 3 pounds per square foot, while standard 1/2 dry wall is 2 pounds. 5/8 dry wall is 2.6 pounds per square foot. Joe Hardie 1/4 cement board is 1.9 pounds sq. ft, They make a .42 inch think board that is 2.6 pounds per foot. So the Durock, at least according to the websites, is actually heavier. An extra pound per square foot in increased density has to mean something, doesn't it?

You can buy the Durock in 3x5 sheets for $7.88 per sheet from Lowes, maybe cheaper in bulk. And I would be willing to spend a little more for improved performance.

http://www.lowes.com/lowes/lkn?action=productDetail&productId=11729-325-172964&lpage=none

Not sure if I'll do this in my theater, but I think its a reasonable option. Admittedly a base layer of cement board + GG and a finish layer of regular dry wall isn't as good as three layers of dry wall, but it has to be better than just two layers of regular dry wall, and the expense isn't much more.

Admittedly a base layer of cement board + GG and a finish layer of regular dry wall isn't as good as three layers of dry wall, but it has to be better than just two layers of regular dry wall, and the expense isn't much more.

I am sure Ted will chine in here, but GG works best between two layers of material with similar stiffness. Its possible that 2 layers of 5/8 drywall will be better than one of drywall and one cement board.

The chain analogy doesn't really fit here. A chain will break and be useless, and thus is as only good as the weakest link. But 100% of a theater's sound doesn't escape out of the weakest point of a room. Every theater compromises in certain spots on sound isolation and other concerns (that is why we have doors to our theater :)). My point is that the less weak spots you have, the better. If cement board improves sound isolation, and reduces the amount of weak spots, that can't be a bad thing in my opinion.


Unfortunately that is exactly how sound proofing works. Sound is always going to flank through the weakest link. But again my example took this to the extreme. In all likelihood the difference between the standard DD/GG and your mix the first layer and then GG and drywall may not even be measurable and it will certainly depend on how well that first layer is installed. Also keep in mind that the more gaps between material, the weaker the performance. So it boils down to cost and effort for negligible gain.

If you really want to go hardcore with the sound proofing, just do a third layer of drywall and call it a day. Or even a first layer of MDF or OSB and then two layers of 5/8" drywall. That will make installing the drywall a whole lot easier.

Unfortunately that is exactly how sound proofing works. Sound is always going to flank through the weakest link. But again my example took this to the extreme. In all likelihood the difference between the standard DD/GG and your mix the first layer and then GG and drywall may not even be measurable and it will certainly depend on how well that first layer is installed. Also keep in mind that the more gaps between material, the weaker the performance. So it boils down to cost and effort for negligible gain.

If you really want to go hardcore with the sound proofing, just do a third layer of drywall and call it a day. Or even a first layer of MDF or OSB and then two layers of 5/8" drywall. That will make installing the drywall a whole lot easier.

My point is that if cement board can reduce the amount of weak spots, so much the better. I'm not sure if it does though, but cost and effort haven't yet dissuaded me.

a first layer of MDF would be really expensive, but OSB might not be too costly. But again OSB would be stiffer than dry wall, so we would run into the same problem if green glue only works with products that have the same flexibility, as has been suggested.

To clarify, as previously stated, Constrained Layer Damping is at optimal efficiency when the materials are of similar stiffness. However 3/8" drywall will work well next to 5/8" , just not quite as well as 1/2" + 1/2", even though both scenarios have the same 1" overall finished thickness.

For the Hardie Panels, I was referring to this: http://www.jameshardie.com/homeowner/products_siding_hardiepanelSiding.py

2.3 lbs per sq. ft and flexible. Would marry well with 5/8" drywall.

But really only an advantage if the wall thickness is critical. Money would be better spent on triple drywall

Not sure if this was covered earlier in this thread, but cement board requires specially coated screws which are darn pricey. That alone would be a deal breaker for me.

Although since I'm a few years away from building my theatre, I'll keep watching threads like these to see if any of these ideas pan out. :)

Not sure about special screws, but the Hardie Panel I references is so dense that it needs pneumatic finish nailer to secure.

Again, the overal system is really designed for standard, cheap and easy to work with drywall

Corrosion resistant screws. Called "Rock On" or something like that. $10 or so for a box of 100 at HD here in Canada.

Ahh. For use in water / humid areas no doubt

I had planned on using 19mm T&G chipboard (standard florring underlay used in OZ) and them 16mm drywall with GG in between. Given the discussions about similar flexibility would this be acceptable?

Reasons for using the T&G:
1. Its highly water resistent and will add to water proofing
2. Enable me to screw/nail anything in anywhere

If the above logic is wrong please let me know and I'll revert to something else.

Elill, that will be fine.

Ahh. For use in water / humid areas no doubt

I believe it also has to do with the corrosive nature of the cement. The heads on these screws are also larger and have some special feature under them. All I know is that they should always be used for CBU and they're pricey.

I say stick with drywall.

has anybody used QuietRock around here?


http://www.quietsolution.com/html/quietrock.html


From what i've seen, it works really good.

QuietRock comes up quite often. Great product, as are all of the pre-damped drywalls.

Any pre-fab panel is made from standard 1/4" or 3/8" building materials that you can buy yourself. Drywall and / or cement board. Squirt some damping material in there and viola.

You'd find that you can assemble your own in the field. Your result would be a much heavier wall, which will perform better in the low frequencies. Also there is considerable cost savings.