Yeah, it's a cool project but it's also lot of grunt work. I've been constantly beat by the end of the day for weeks now.
Regarding the safety issues around the airtight room:
The US Dept of Homeland Security suggests
that ten square feet of floor space per person provides sufficient air to prevent carbon dioxide build up for up to 5 hours. With all the blue fill in there, let's say the the dimensions have been reduced a bit -- to 11x9' -- which is 108'. I, alone, could stay breathing and alive in there for a couple days, according to DHS.
A more detailed scientific article entitled What would happen if you were locked in an airtight room?
at How Stuff Works says in a 12x12x8' room , there's 1,152 cubic feet or 32,621 liters of air, which would take 3 days for a person to inhale/exhale. However, the reduction in oxygen and buildup of CO2 come into play. "When oxygen falls to 19% and the carbon dioxide level reaches 2%, you're in trouble. Not only are you getting less oxygen, but you're now also taking in carbon dioxide, which is actually a poison. Therefore, in reality, you will only last a day and a half or so. Then your body will begin having problems. So, you had better start working on finding a way out of that room!"
I think it's safe enough. It might be great for naps in the middle of a noisy day or when there's a too-noisy night-time party in the neighborhood.
The roof is tar & gravel.
Additional layers of drywall or equivalent on the existing walls with resilient stuff like Green Glue in between would certainly improve the overall noise isolation, but the effect is limited -- or at least assured -- to only frequencies above 125 Hz. STC ratings, widely used to rate acoustic transmission qualities of building materials, are specific to 125~4000 Hz.
Here's what STC Ratings
has to say about low frequency noise: "The rating provides no evaluation of the barrier's ability to block low frequency noise, such as the bass in music or the noise of some mechanical equipment."
Everything I've read/studied suggests that to keep out low frequency noise, I need mass. Lots of it, everywhere. The low freq noise of planes has to be airborne and comes right through the house like a hot knife thru butter; it's not just a floor issue.
Having said that, there's no question that the floating room-in-room with perhaps duble layers of QuietRock and green Glue would help, but there's no question too, that this will not eliminate the external low freq noise interference, simply reduce it. By how much we can only guess or estimate. It's really a costs vs benefits question.
The 80Hz-12db/oct filter on the mic preamp is a really useful tool here. Turning it on essentially reduces the low freq noise interruptions by some 75% -- in other words, it makes it possible for recordings to be made through all but closest low freq noises, like a car within a block or a plane at its closest distance to the house (still several miles I'm sure, except for the pesky little ones that sometimes fly close to overhead!).
So then the question becomes -- could I hope to achieve this kind of low noise reduction from any practical construction in that room? Well, what is the reduction of the low cut filter? it's 12 dB/oct and starts at 80Hz. That means it's -12 dB at 40Hz, -24 dB at 20Hz, and -36 dB at 10 Hz. Anyone with even a modicum of acoustics knowledge will answer: NO WAY!
But a floating room-in-room might be achieve a general... say... 5-6 dB reduction in the sub-150Hz region, along with probably 10-20 dB further reduction in the frequencies above (not that this is so useful).
But take a look at the low freq curve in my last post and tell me whether that level of reduction would be meaningful. At 100Hz, the level is already >15 dB higher than at 1kHz. At 50Hz, it is >25 dB. At 20 Hz, it's >40 dB. To get these freq to midband levels, we'd have to go underground -- which takes us right back to the ridiculous acoustic bunker under the back yard and garage fantasy I've had for six years!
At what point does this project become an obsessed "How low can we go?" crusade?