Test Data For Wall Assemblies With 3x 5/8" drywall per leaf?

[Elusive Sounds]

Hello everyone. I'm in eastern Canada and studying the feasibility of a multi use (mix and drum tracking) room build out in a loft area above my garage, and adjacent to a bedroom. I'm on my second read of Rod Gervais' book. I'm currently drawing a very detailed 3d model of the existing construction of the space which I will share here once it is complete.

Edited April 2020:
Sketchup File: https://drive.google.com/file/d/1ALMR0N ... sp=sharing
Design Thread: http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=22353

Rod's book as well as other books, the Green Glue website, NRC's IR761, and USG's Gypsum Construction Handbook all list different variations of assemblies, but never with 3 layers of 5/8 in each leaf. Searching this forum also did not yield results.

This type of wall construction seems common among the builds I've seen on this and other sites, and with my specific needs, this is likely the type of wall I will need to build. I would just love to be able to get my hands on published test data to help in my STC, materials and budget calculations.

Question:

Can anyone please point me towards any test data documenting a double wall mass-air-mass system using 2x4 and/or 2x6 wood framing and 3 layers of 5/8" drywall in each leaf?

Thank you kindly,
Marc.

[Soundman2020]

...studying the feasibility of a multi use (mix and drum tracking) room build out in a loft area above my garage, and adjacent to a bedroom.

From what you say in your post, it seems like you are looking for extreme isolation. Trying to do that on an upper floor is really hard, and expensive. I'm not trying to discourage you: just pointing out the facts:

Building on an upper level means that your floor is not damped, acoustically. It is not resting on Mother Earth, and therefore does not have the entire planet as the damping system. In fact, the damping system consists of "nothing at all"! Just the empty air under the floor. So basically your floor is a huge drum: it is a membrane stretched across a resonant space (the room below), and it will act exactly like a drum does. Impact noise, and even loud airborne sound, will cause that drum-head to vibrate, and resonate, and transmit energy into the room below, and also into the rest of the structure of the building.

For this reason, we do not recommend building rooms that need high levels of isolation, on upper floors. It is possible to isolate a control room reasonably well, but for a live room where there will be acoustic drums, the sound intensity levels are about one thousand times higher (115 dB vs. 85 dB is one thousand time increase. Three orders of magnitude). It's really, really hard to isolate that. You would need to float your floor, which is a major undertaking: complex, and expensive. I'm not sure if you have already read this thread, but now would be a good time to do so: http://www.johnlsayers.com/phpBB2/viewt ... f=2&t=8173

The second problem with building a well-isolated studio on an upper level, is structural. You are talking about adding many thousands of pounds (kg) of weight to the existing structure, in order to achieve the level of mass you need. Three layers of drywall on each side of studs, plus a floated concrete slab, plus a similar ceiling, plus massively heavy doors and windows, plus the massively heavy silencer boxes for your HVAC system, plus everything else, add up to a huge amount of weight. It is VERY unlikely that your existing garage structure would be able to support that, so you will need to beef it up considerably. We are talking things like LVLs, RSJs, or other very substantial structural support. To put this in perspective, every single square meter of the wall you are asking about, will weigh about 100 kg. You didn't say how big the room is, but assuming a typical 5m x7m studio, 2.5m high, the ceiling structure alone will weigh 3,500 kg, and the walls will add another 6,000 kg, for a grand total of nearly 10,000 kg. And that's without the floor or beefed-up roof, or the gear, furniture, HVAC, electrical, ... Count on around 12,000 to 15,000 kg in total. An average medium car or small SUV weighs about 1,500 kg, so you'll be adding mass equivalent to parking ten SUV's on your garage roof.... Do you think it can handle that? ...

Conclusion: You will need to hire a structural engineer to tell you how to beef up the structure of your garage, such that it can handle the load of more than a dozen cars parked upstairs...

the Green Glue website, NRC's IR761, and USG's Gypsum Construction Handbook all list different variations of assemblies, but never with 3 layers of 5/8 in each leaf.

The performance can be extrapolated from the results of two-leaf tests, using the normal equations for MSM resonant systems. The mass will be about 50% higher, so you would be getting around 7 to 9 dB of additional isolation, and starting at a frequency about a quarter to half an octave lower.

However, that is in theory: in practice there are other aspects that will limit your maximum isolation, such as the flanking limit of your structure. I suspect you will hit that before you reach the actual theoretical level predicted by the equations. In which case it does not matter how many layers or drywall you put on the walls, you would never get beyond the flanking limit. For a typical house, that would be around 70 dB.

and with my specific needs, this is likely the type of wall I will need to build.

Why? If you don't yet know how much isolation you need, why do you think that a wall with 3 layers of 5/8" drywall on each side will get you there? That's sort of like saying: "I want to go on vacation. I have no idea what my actual destination is, but I'm absolutely certain that I'll need to put 21.6 liters of fuel in my car to get me there"....

I would just love to be able to get my hands on published test data to help in my STC, materials and budget calculations

Forget STC. It is no use at all for telling you how well your studio will be isolated. STC was never meant to measure such things. Here's an excerpt from the actual ASTM test procedure (E413) that explains the use of STC.

“These single-number ratings correlate in a general way with subjective impressions of sound transmission for speech, radio, television and similar sources of noise in offices and buildings. This classification method is not appropriate for sound sources with spectra significantly different from those sources listed above. Such sources include machinery, industrial processes, bowling alleys, power transformers, musical instruments, many music systems and transportation noises such as motor vehicles, aircraft and trains. For these sources, accurate assessment of sound transmission requires a detailed analysis in frequency bands.”

It's a common misconception that you can use STC ratings to decide if a particular wall, window, door, or building material will be of any use in a studio. As you can see above, in the statement from the people who designed the STC rating system and the method for calculating it, STC is simply not applicable.

Here's how it works:

To determine the STC rating for a wall, door, window, or whatever, you start by measuring the actual transmission loss at 16 specific frequencies between 125 Hz and 4kHz. You do not measure anything above or below that range, and you do not measure anything in between those 16 points. Just those 16, and nothing else. Then you plot those 16 points on a graph, and do some fudging and nudging with the numbers and the curve, until it fits in below one of the standard STC curves. Then you read off the number of that specific curve, and that number is your STC rating. There is no relationship to real-world decibels: it is just the index number of the reference curve that is closest to your curve.

When you measure the isolation of a studio wall, you want to be sure that it is isolating ALL frequencies, across the entire spectrum from 20 Hz up to 20,000 Hz, not just 16 specific points that somebody chose 50 years ago, because he thought they were a good representation of human speech. STC does not take into account the bottom two and a half octaves of the musical spectrum (nothing below 125Hz), nor does it take into account the top two and a quarter octaves (nothing above 4k). Of the ten octaves that our hearing range covers, STC ignores five of them (or nearly five). So STC tells you nothing useful about how well a wall, door or window will work in a studio. The ONLY way to determine that, is by look at the Transmission Loss curve for it, or by estimating with a sound level meter set to "C" weighting (or even "Z"), and slow response, then measuring the levels on each side. That will give you a true indication of the number of decibels that the wall/door/window is blocking, across the full audible range.

Consider this: It is quite possible to have a door rated at STC-30 that does not provide even 20 decibels of actual isolation, and I can build you a wall rated at STC-20 that provides much better than 30 dB of isolation. There simply is no relationship between STC rating and the ability of a barrier to stop full-spectrum sound, such as music. STC was never designed for that, and cannot be used for that.

Then there's the issue of installation. You can buy a door that really does provide 40 dB of isolation, but unless you install it correctly, it will not provide that level! If you install it in a wall that provides only 20 dB, then the total isolation of that wall+door is 20 dB: isolation is only as good as the worst part. Even if you put a door rated at 90 dB in that wall, it would STILL only give you 20 dB. The total is only as good as the weakest part of the system.

So forget STC as a useful indicator, and just use the actual TL graphs to judge if a wall, door, window, floor, roof, or whatever will meet your needs.

Can anyone please point me towards any test data documenting a double wall mass-air-mass system using 2x4 and/or 2x6 wood framing and 3 layers of 5/8" drywall in each leaf?

You can estimate it from the following set of equations:

2-leaf-3-leaf-equations.jpg

Full-spectrum-TL-isoaltion-equations.jpg

If you want more details, download the paper "Accuracy of Prediction Methods for Sound Transmission Loss" by K. O. Ballagh. You should be able to find that via Google. It explains the above in detail, so you can predict the actual TL curve for any given two-leaf wall.

Having said all that, I would suspect that the amount of isolation you can get will be limited mostly by the structural aspects of your garage, which will need some heavy "beefing up" to support the types of load that you'd need. On the other end of the scale, assuming it is a typical garage with typical structural characteristics, you could probably get about 40 dB of isolation out of it, without needing to beef up too much. That assumes that you'd also be isolating the actual garage space itself (the place where you park your car), since that will be the resonant part of the "drum" created by your floor.

I'm currently drawing a very detailed 3d model of the existing construction of the space which I will share here once it is complete.

Please do post that! But first, check on your approximate weight projections, and with your structural engineer, to see what is feasible without beefing up the structure, and also what you'd need to do to beef it up enough to support a dozen large cars.


- Stuart -

[Elusive Sounds]

Hi Stuart, thanks for taking the time to reply!... Back in 2016... How time flies and life happens.

With the current state of the world, and having a lot of time to dedicate to this, I am continuing to explore the feasibility of isolating my 2nd floor space. I have been working on a very detailed post with all the specifics of my existing structure and will post that separately since it goes beyond the scope of my original question about triple layers of drywall. The new thread will detail the background, overview, use of the space, goal, budget ($15k-$20K CAD), noise measurements, and comprehensive existing construction details.

You will need to hire a structural engineer to tell you how to beef up the structure of your garage, such that it can handle the load...

I am now consulting with an architect who specializes in acoustics consultation and am reaching out to structural engineering firms to find an engineer able to first determine existing load capacities, etc. On top of that, it turns out that my wife's coworker who is a now physician used to be an acoustical engineer and specialized in recording studio design. Who would have thought... So I will be reaching out to her as well.

So basically your floor is a huge drum... ...Impact noise, and even loud airborne sound, will cause that drum-head to vibrate, and resonate, and transmit energy into the room below, and also into the rest of the structure of the building..

Figures 10.16 and 10.19 are from Rod Gervais' book. I am hoping for clarification on the application of the building methods used in these two examples to see if they might be helpful for my room.

Figure 10.16 describes "a decoupling design for a studio on the 3rd floor in a building that could not support an elevated concrete slab." Rod goes on describing the use of this system stating "The other proof (of course) is that even with full bands working in the live room, there was no transmission to the offices below." Of course we don't know any of the other structural details in that example, but that statement leads me to believe that this type of decoupling system might also reduce airborne sound as well as impact sound. Could this be the case? Rod does not specifically state that this is only useful for reduction of impact sound, but I have seen that stated on this forum.

10.16.png

Figure 10.19 describes another floor assembly where "the use of rigid insulation with plywood above creates a decoupled floor assembly." and where mass is added using 7 PSF lightweight Gypcrete. Again, here there is no specific mention on how effective the assembly in figure 10.19 is for impact noise versus airborne noise.

10.19.png

If my floor can be modified to bear all added loads, and an engineer signs off on it, could some of the techniques in 10.16 and 10.19 be used effectively in my case? Could adding even more Gypcrete mass yield even better results, if the rule of thumb is +6db of transmission loss when the mass doubles? What about adding green glue (2-3 tubes worth per 4'x8' area) in between all solid layers? Or are the flanking paths on a second floor structure such that a veritable floating floor is the only option? Also worth noting that mass will have to be added from above as the underside of the OSB deck has been spray foamed.

You would need to float your floor... ...I'm not sure if you have already read this thread: viewtopic.php?f=2&t=8173

I have read the thread describing why floating a floor is probably a bad idea, and if I can avoid it and still get "some" amount of transmission loss the project could still be worth doing. I know the math needs to be done to determine hoe much "some TL" actually is. I understand what is involved in calculating TL for MSM assemblies as described in the MSM TL calculator here:

https://docs.google.com/spreadsheets/d/ ... 1543869474

But what is the right mathematic approach for my type of "drum head" floor? I assume the resonant frequency of the existing floor first needs to be calculated, then find the appropriate amount of mass to lower the resonant frequency below the audible range? I have a feeling this might be above my math skills, and would be left to my architect / engineer...

download the paper "Accuracy of Prediction Methods for Sound Transmission Loss" by K. O. Ballagh.

I have read this document and could follow along to a degree. I understand how this shows the correlation between mathematically predicated results and lab measured results.

Why? If you don't yet know how much isolation you need, why do you think that a wall with 3 layers of 5/8" drywall on each side will get you there?....

I hear what you are saying 100%. In figure 10.17 Section Through Garage Wall, where Rod describes the process of adding drywall mass to the inside face of exterior walls he states "I would recommend that you place at least two layers of drywall or more if you are in a particularly noisy area".

10.17.png

That is the case for me, even if "particularly noisy" is subjective. That being said, I started taking actual measurements to give me a starting point. I measured (dBc, slow) a baseline outdoor ambiance of 47dBc midway between my garage and the street, at night with no traffic. This is pretty much the quietest it ever gets here due to the COVID-19 lockdown. I then played drums and measured 73dBc at the same position, and 111 dBc in the room. This shows the absolute minimum isolation I need to keep drums from exceeding a (worst case) quiet outdoor ambience is 26db. But I know I need more isolation then that for two reasons:

1. I have used the room as a practice space for rock bands, typical 4-5 piece band with 2x electric guitars, electric bass, and amplified keys and synth. 2. The usual ambient noise during peak hours is very loud. The worst case being heavy traffic, large trucks, cars with modified exhausts systems, Harley Davidson motorcycles, air and railroad traffic all at once. I have not yet been able to measure these scenarios because of COVID 19 lockdown. Cant have musicians over to jam, and traffic almost a stand still. More testing to come.

Forget STC. It is no use at all for telling you how well your studio will be isolated.

Copy all that. I need to look at the TL graphs from which the single STC number rating is extrapolated.

Having said all that, I would suspect that the amount of isolation you can get will be limited mostly by the structural aspects of your garage, which will need some heavy "beefing up" to support the types of load that you'd need. On the other end of the scale, assuming it is a typical garage with typical structural characteristics, you could probably get about 40 dB of isolation out of it, without needing to beef up too much. That assumes that you'd also be isolating the actual garage space itself (the place where you park your car), since that will be the resonant part of the "drum" created by your floor.

Does this 40db also assume that I would be floating my floor above the garage?

assuming it is a typical garage with typical structural characteristics, you could probably get about 40 dB of isolation out of it,

Awesome! 40dB is better then my minimum of 27dB. Here is where I may be in luck. The floor is supported on 5/8" OSB atop 16" deep wooden trusses, at 16" o/c having a span of approximately 270". The previous owner used to have a wood working shop with heavy machinery above the garage. He tells me the floor was rated for 90psf, but did not specify if this was live or dead load, so I'm taking this with a grain of salt. In any case I will have my engineer confirm maximum live, dead and point loads.

3. Garage 2x4 framing and wooden webbed truss.jpg

8. Garage Truss and Spray Foam.jpg

That assumes that you'd also be isolating the actual garage space itself (the place where you park your car), since that will be the resonant part of the "drum" created by your floor

Do you mean isolating the garage as if the space within the garage itself was the de facto live room? Which is to say a complete room in a room MSM system in the garage? Or could isolating the actual garage space limited to a new decoupled ceiling?

Please do post that!

As promised, here is link to the detailed Sketchup file:


https://drive.google.com/file/d/1ALMR0N ... sp=sharing


I will update this post and link to the new thread with complete information soon.

Hope everyone is safe in these crazy times. Best of luck to all!

-Marc

[Gregwor]

Sadly Stuart no longer participates on this forum.

Once you figure out what's up in terms of load rating for the truss, hopefully you could just beef up the flooring from above because right now, that spray foam is screwing you over from beefing up from below. It seems like you have a good grasp of what's going on here so please keep us posted.

By the way, nice Sketchup! I selected everything and just moved it vertically to ground level because the "frosting" below ground level bothered me haha

Greg

[Elusive Sounds]

Sadly Stuart no longer participates on this forum.

That's too bad! He contributed a ton of useful information.

...hopefully you could just beef up the flooring from above because right now, that spray foam is screwing you

Yep. About the beef up process, do you have any experience with the decoupled techniques mentioned? Do you have an insight on how well they perform against airborne sound versus impact sound?

By the way, nice Sketchup!

Thank you! I put a ton of time into it but I think it will be worth it, especially now that site visits are unlikely to happen for a while. It should help with materials calculations as I start making design drafts.

Thanks

[Gregwor]

Yep. About the beef up process, do you have any experience with the decoupled techniques mentioned?

Personally, I've never built anything that way. I've only ever recommended the Glenn style drum riser to people.

Do you have an insight on how well they perform against airborne sound versus impact sound?

I wish I did. Ultimately, I'd suspect that if you used the second technique, you could get some decent isolation. Having both leaves of sheathing nice and beefy, separated by the insulation seems to be your best route in my opinion.

Greg