...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 -