You saw the attachment?
Yes I did: that's not the way sound works. You are still not seeing the big picture. Sound doesn't go only where you want it to go: it goes everywhere, in all directions. Especially low frequency sound: it moves in the same way a balloon moves when you pump it up then deflate it rapidly: expanding and contracting equally all over. The sound from your sub (in your diagram) will not go through the floor and lower wall as a series of rays, but rather it will also go through the window, upper wall, ceiling, doors, HVAC system, etc. as pressure waves It will exit through ALL the surfaces of your room, not just the ones you'd like it to exit through. And once it is out on the other side, it does the same thing: once again expanding in all directions equally as a PRESSURE wave, not a bunch of rays. If you measure the pressure changes inside a balloon, you'll find that it is the exact same value at all points. There's no difference between the pressure just inside the neck of the balloon, or right in the middle, or towards the bottom, or the top: it's the same everywhere.
What makes you say it is not not a MSM system?
Because it isn't! There's no spring involved: just two masses. Sound that went out loudly through the low-mass upper wall, or window, or ceiling, or wherever, and is now in the cavity, has no MSM relationship with the structural floor. It is just sound waves in air. When it reaches the floor, it simply sees "here be Mass Law!", and behaves accordingly. You only get MSM between surfaces where there is a spring that couples them.
As for why there's scarce information regarding the "heavy bottom - light top" wall idea, that may have to do with the fact that most isolated rooms are equally concerned with TL through walls as well as through floors?
No. It's because TL is a system, not a bunch of parts. The isolation in a system is only as good as the worst part, or "weakest link" if you prefer. Its because isolated rooms are concerned with isolation of the room as a whole, not isolation in one or two directions only. Think in terms of my fish-tank analogy. You can't build a fish tank that only holds water on one side, because that's the only side you want to look at the fish through. You still have to make all of the other sides equally water-tight, even though you don't care if the fish can be seen, or not seen, from the other sides.
Consider the gobo - that is the very definition of "heavy bottom - light top"- You can't get any lighter than an opening!
The last time I checked, a good gobo managed to attenuate all frequencies.
You must have a very curious gobo, then! The last time I checked, gobos are not very good at all at isolating low frequencies. The last time I put a floor tom behind a rather large gobo, I could still here all of the reverberation.... the actual "smack" of the drumstick hitting the head was attenuated somewhat, yes, but all of the "boooom" was still there, on the other side of the gobo... Because the highs were attenuated reasonably well, since they travel in ray-like patterns, and can be stopped with relatively low mass any way, but the lows got through because they travel as pressure waves, not rays, and can't be stopped by low mass... Plus: diffraction...
As for being sold a pig in a poke, are you referring to the acoustician who assures me a 35kg wall will work? Or the heavy bottom wall idea? If the latter, then that's my own idea. I'm happy to be shown where it doesn't hold water (to use your analogy), apart from the fact that there's scarce evidence of its potential efficacy.
I'm referring to the fact that you are really, really, really desperately wanting to buy into any old fantasy, myth, or invented "snake-oil science" theory, because you really, really, really desperately want to make your studio do things that it can't actually do! You believe that if you can just find some magical combination of building materials, and arrange them "just so", then all will be well, you will achieve 85 dB of isolation while not overloading your structural floor, and also designing it all on your own, for under 20k$! That's the real issue here.
Yes, I get it! You need to make your place work because you already invested so much time, and money, and emotion into it. But your desperation and enthusiasm will not make the laws of physics go away. There are limits, but you reject them, and instead insist that everyone else is wrong, the theory is rong, the experts are wrong, the books are wrong, and there really is a way to o it... so you clutch at straws that you think might work: You buy the pig in the poke, in the sense of "buying" something that you totally hope will be one thing, but when you look into the bag with the light of reality shining bright, it is actually something very different.
There are very well known principles of acoustics, tried and true. Many of them are complex, but can still be expressed as simple rules of thumb, such as "isolation is only as good as the weakest link" or "you need 15dB below ambient to be inaudible" or "highs travel as rays, lows travel as balloons" or "MSM must be half the lowest needed isolation frequency" or "spring rate is governed only by gravity" or "you get 6 dB improvement for each mass doubling" or "you get 6 dB drop for each distance doubling" or "increasing 10dB sounds like twice as loud". There are solid, sound, reliable, tested, demonstrated research and equations behind all of those, and they are all absolutely correct, but they still need to be used within certain constraints. It seems you want to ignore those constraints, a
nd also ignore the rules themselves! In your hope that there just might be something that the scientists and acousticians and researchers have missed. And you'll be the one to find it....
I'm really sorry that you have this situation, where the physical building itself and the laws of physics and the budget prevent you from doing what you want to do. But there is no way around it. Either you have to accept that you cannot achieve the massive isolation that you want to achieve to avoid annoying your neighbors and thus lower your sights, or you sell the place and move to one that DOES allow you to achieve what you want to achieve.
Getting very high isolation on the 7th floor of an office building is only possible if there is sufficient structural integrity to support the hugely massive floors AND walls AND ceiling AND windows AND doors AND HVAC system. Your structural engineer has said that there isn't enough structural integrity in your building to do that. Period. That's what this all boils down to. Either except the limitations on isolation that are imposed on you by the building and the laws of physics, making do with less isolation than what you want (which can still be pretty good isolation, by the way, but won't be perfect), or find another place where you don't have those limitations.
This is why you see numerous posts here on this forum, and all the other forums about studio design and construction, recommending that a studio needing high isolation should always be built on the
ground floor, slab-on-grade, never on an upper floor. Because upper floors just do not provide the conditions that you need. Yes, there are cases of studios built on upper floors with high isolation... but with high budgets too, that extend to modifying the building structure itself to produce the needed conditions. It seems you can't do that in your case...
In a nut shell: Your studio can sound fantastic, be isolated very well, and be cheap to build. Pick any two. Or just one.
I wish I had better news for you, but reality is stark.
Short summary: You need greater than 80 dB TL to make your studio viable in the manner you want to use it (assuming rock band at 115 dB, target in the room below at less than 35 dB). I suspect that you can probably get TL into the high 60's for your place, within the limitations imposed by the building. Certainly you can get way over 50, perhaps even approaching 70, but it just isn't possible to go higher, because you can't get enough mass into the place. To go higher, you'd have to spend serious money to reinforce the floor, and switch out all the exterior glass, and modify other aspects of the building too, probably. I don't see that happening. If someone is telling you that you can get 85 dB from 35kg/m2 walls, I'd run. Laugh first, then check my wallet is still in my pocket, then run.
- Stuart -
). I'm not in denial of the laws of physics, I'm just in search of them, in this case the laws pertaining to the question of TL through a wall then floor in relation to the proximity of the wall height in relation to the floor. I'm sure there must be a BBC test somewhere. I'll try to see if my Concert Hall guy can help... 
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