My interior partition walls are steel stud cavity walls under construction. In the bedrooms I would like to employ an extra measure to make them quieter and isolate them from the noise of the rest of the house. Just one relatively inexpensive step would be nice, and it's not imperative that I create a totally noise free room.

Do you think an extra layer of drywall would go farther toward soundproofing than putting fiberglass or rockwool batts inside the cavities? How about in the ceiling in the joist space between the ground floor and second floor?

use a layer of "soundboard" -1/2" compressed paper- available at drywall suppliers, under the drywall on the walls & ceilings, then fill the cavities with fiberglass batts. The soundboard will prevent sound transmission through the solid surface of the stud. A second layer of drywall would have a much smaller effect.

.....also make sure to seal the bottom and top tracks with acoustical sealant and no back-to-back outlets.

Bruce

acoustical sealant - would this be spray foam?

soundboard - is this similar to masonite?

Acoustical sealants are usually a rubbery caulking material.

GreenGlue (tm) to apply a double layer of wallboard also does quite a bit.

Jelly,

At a trade show I attended one booth had a tunnel built with 2x4's and covered with drywall only on the outside. The spaces between the wood studs were packed with dry cellulose.  I was amazed how quiet it was inside the tunnel without doors.  As you probably know, trade shows with a lot of booths in the same room tend to be very noisy.   

By the way, in no way should my observations be taken to minimize what the other posters have recommended.

Using cellulose for the cavity fill buys you a couple of STC points over standard density batts but cellulose in walls is a bit more involved for a DIY project, not the requested "...one relatively inexpensive step...", and unless there's more insulation to blow, it'll be expensive to have just one small area done, whereas almost any handyman can do a reasonable job of sound abatement with batts (even if a professional would do 1-2 STC points better.)

Batts + acoustic-caulk is the easy 1-step, buying quite a bit of STC level.

Doubling the drywall using vibe-isolating adhesive would be a second step, but might still be "worth it"

Using 24" oc 2x3" staggered stud double studwall helps too, but that's making it complicated, probably not worth it.

It all depends on just how much sound you're trying to suppress.

Putting batts or blowing the ceiling joist bays full with low-density cellulose also helps. (Doable with cellulose as a DIY with a box-store rental blower.)

Posted By Jelly on 14 Sep 2010 11:02 AM
acoustical sealant - would this be spray foam?

Like this http://www.tremcosealants.com/commercial/products/product_detail.asp?id=32

This might also be an interesting air seal material since it does not harden.

Bruce

Thanks all for your input.

Bruce, forgive me if the answer is obvious, but exactly where would I put the sealant? I downloaded the Tremco installation instructions and they say basically, and I'm paraphrasing here, to make sure surface is clean then put it on. But where exactly? Do you mean under the bottom track, and over the top track? It's too late for that. Or do you mean at the edge of the track where it meets the slab, before the drywall goes on?

Dana, I assume if one were to double the drywall on only one side then it would be on the side that you want to be quiet? I read that some choose a layer of 5/8ths drywall and a layer of 1/2 inch drywall, to block two different sound frequencies. Sounds far fetched - two layers is probably enough whatever thickness.

Green glue is kind of pricey. Can you replace it with butyl caulk or something else?

GreenGlue makes their money by proving their products work- not sure if other products ever get tested for sound abatement in the same way.

I suspect it matters little which side of the assembly the double-layering is on.

Harmonically tuning the attenuation by using different thicknesses seems completely bogus to me too- the fastener & stud spacing will likely have a much bigger effect on the frequency response than multiple thicknesses in the double-layering. In general thicker buys you a bit more attenuation than thinner, across the audio band.

I second the use of greenglue. That stuff works great. I have used it several times.

Both Masonite and soundboard are made of compresed paper, but Masonite (aka hardboard) is very hard, whereas soundboard is soft & sound absorbant.

I'm not the OP but I've found this info very helpful too, thanks to all.

A follow-up question: With the emphasis on the wall and ceiling, I'm wondering if the type of door is important. Maybe everyone goes with solid wood doors (?) - in my current entry-level builder house the doors are hollow and made of a masonite type stuff. Very cheap. I hope to do better in the next house - but not sure what it will be yet. Does door choice play a big part in this, or are the walls and ceiling/floor the 80 in the 80/20 game?

Thanks

A solid door will definitely help the soundproofing, but be sure it is weatherstripped well! You need to close up the openings through which the sound will travel and there are always opening around doors, so good pliable weatherstripping carefully applied is a must.

http://www.diy-home-theater-design.com/sound-proofing-ideas.html

Posted By Jelly on 13 Sep 2010 08:47 PM 
inexpensive step would be nice,

Earplugs ..................... oh but seriously an addtional layer of drywall over RC-1 channel is effective or the sound attenuation batts work

Posted By Jelly on 14 Sep 2010 05:26 PM
Thanks all for your input.

Bruce, forgive me if the answer is obvious, but exactly where would I put the sealant? I downloaded the Tremco installation instructions and they say basically, and I'm paraphrasing here, to make sure surface is clean then put it on. But where exactly? Do you mean under the bottom track, and over the top track? It's too late for that. Or do you mean at the edge of the track where it meets the slab, before the drywall goes on?

Put a big fillet on either side of the track and bed the drywall in it.  The old USG manual I have says to do it on both sides and outlet boxes coated with the sealant, but the important thing is to eliminate sound paths.

30 or so years ago, we made an acoustical mock-up in a (commercial office building) marketing office to give tenants the opportunity to judge what level of speech privacy could be achieved with different wall and ceiling configutations for their buildout.  I think we had 4 different offices and we used different construction between the offices and between the offices and corridor.

The best "bang for the buck" was providing good acoustical seals and Thermafiber batts to a normal (floor to ceiling) wall.  Eliminating sound paths with sealant, door gaskets, etc. is really important.  

After that, the ceiling path (acoustical tile) with the abovementioned partition was always the weak link unless tenants opted for "slab to slab" partitions with return air baffles or used a drywall ceiling with insulation above.  Only after solving the ceiling problem did adding drywall layers, staggered studs, RC channels, etc. make sense.  I do not recall if we tested anything with layer of Homosote or similar, but it sounds useful.

Acoustics are like total wall R value.....detailing is everything.

Bruce

Great thread and it offers an opportunity to address a few common myths as well as reinforce some best practices already mentioned here. There are many good points being asserted and fortunately there are independent lab reports that help straighten out some of this.

First of all there are 4 elements involved with soundproofing:

Decoupling- For the sake of this thread, this is essentially detaching the drywall from the studs and joists. The drywall layer can move independent of the studs.

Absorption- This is a fluffy material that interacts with the sound wave in the joist or stud cavity. Generally an insulation.

Mass- The wall or ceiling will isolate sound better if it weighs more. Mass is a key factor in sound isolation.

Damping- The drywall panels are very resonant, meaning they are not internally damped and will conduct a vibration quite well. Damping the drywall (or subfloor) panels significantly increases sound isolation.

These are the only 4 elements of sound isolation we have to deploy to stop sound. Generally a system that deploys all 4 elements will have the best results.

So how do some of the processes / products mentioned in this thread fit with these 4 elements?

#1 Soundboard. This product, sold under many different names, is an attempt to decouple the drywall from the studs or joists. The problem is it is very inefficient at this, as it is too dense to accomplish the task. Better to use the staggered stud system mentioned, resilient channel or resilient clips. Soundboard is too dense and has too much surface area in contact with both the drywall and the studs to decouple. Soundboard is also thought of as an absorptive material, since it’s a bit compressible. Unfortunately it is too dense to offer significant absorption (compared to insulation which works well). Soundboard is too dense to offer competent absorption. Soundboard is very lightweight, and this is distinctly not in our favor. Very little mass in that ½” thickness. Compare to much more massive drywall. Soundboard does not effectively damp the resonant drywall. Independent lab data clearly shows the limitations of a sound board. Interestingly, you don’t find formal lab data on these products. Soundboard isn’t recommended for walls and ceilings.

#2 Sealing up the wall and no back to back outlets. A VERY good point, and lab-tested to be a good practice. Many mentioned this in this thread.

#3 Damping material and double drywall is a very effective strategy. You are significantly increasing the mass of the system as well as damping the resonant drywall. This is a very good thing.

#4 Cellulose is a reasonable absorptive material that works best in a low-density application. The common error is to use too much where it becomes compressed and then conductive. As a side note, the data here: http://www.nrc-cnrc.gc.ca/obj/irc/d.../ir693.pdf    shows that simple, humble fiberglass actually does better in the most problematic low frequencies (bass). Cellulose is not better than fiberglass, however I’d defer to which ever was less expensive or most practical in any particular build.

#5 You would prefer to have the sound isolating treatment be on the noise producing side of the wall or ceiling. The reason is Flanking. #6 Doors. There are two main issues with doors. Their mass and their seals. You want the most massive slab that is practical, and sealed well. If you could have a 1 ¾” solid core slab mounted on an exterior jamb (with weatherstrip) you’d be a happy camper.

Many of these specific issues are described in greater detail here: http://www.soundproofingcompany.com/blog/

Posted By Ted White on 15 Sep 2010 09:40 AM

#4 Cellulose is a reasonable absorptive material that works best in a low-density application. The common error is to use too much where it becomes compressed and then conductive. As a side note, the data here: http://www.nrc-cnrc.gc.ca/obj/irc/d.../ir693.pdf    shows that simple, humble fiberglass actually does better in the most problematic low frequencies (bass). Cellulose is not better than fiberglass, however I’d defer to which ever was less expensive or most practical in any particular build.

National Fiber begs to differ. 

Of course they have a product to sell, but  STC 41 vs STC 38 isn't nothing, then there's the "typically installed" vs. "perfectly installed" aspects that come with batts, which are less of an issue with blown goods.  

Hollow steel studs on standard centers would have large voids when using standard batts too- another complication adding labor to a batt solution in this case. Buying them wide and cutting them down to fit would be the most likely approach. It's possible to do a decent job of low-density dry-blown cellulose with rental blowers, but getting it dense enough that it won't settle (much) over time leaving voids at the top could be an issue.  Wet sprayed goods wouldn't settle, but it's not DIY.

I hear you, however the NRC in Canada (the link in my post) is the definitive source for these matters. They have a summary in that 300+ page report. The NRC is owned by the Canadian government, and has no agenda with respect to brand. Their work is objective and thorough to say the least. They have the raw data available for all to see, so it's hard to critique the data presented by the cellulose people without their raw data. Did they use R8? R3? etc.

This has been hashed over hundreds of times on specialized acoustic forums and the consensus is that fiberglass is the best in the low frequencies. Given that our acoustic challenges generally default to the low frequency performance, acoustic systems are generally built to deal with low frequencies (bass). By practical definition if we have a wall that can withstand low frequencies, we have already dealt with the lower energy high frequencies.

The gaps you mentioned when using steel studs is not really the issue. If you look at the reams of data, you see that this is more of an issue of whether a low / medium density insulation is present or not, rather than if every cubic inch is filled. It's a bt counterintuitive, since we often think of insulation from a thermal perspective, rather than acoustic.

A 75% filled stud or joist cavity essentially fares as well as a 100% filled cavity when the data is compiled and compared.

Again, the risk associated with blown in cellulose or fiberglass is that too much is used and conduction starts taking place. I have seen this all too often in retro-fit situations where an abstacle is encountered and they keep on pumping... Creates a very problematic mass.

As I said in the earlier post, any low / medium density fluffy insulation works well. Cellulose, mineral fiber, fiberglass, recycled cotton, and polyester all work well.  Foams (open or closed cell) are definately to be avoided.

It's also good to point out that out of the 4 previously mentioned elkements of sound isolation (Decoupling, Absorption, Mass and Damping) the absorptive insulation brings the least to the table. Intuitively, most people would think that it is the largest contributor, but this is not the case. Paying more for insulation may not be the best allocation of funds in a budget-oriented project.