Maybe I've been a bit offhanded in my last post, if so, I'm sorry for it.
There's a couple of physical realities you're facing when dealing with sound in this way. A drummer forum is highly likely a place where this topic pops up on a regular base so I'm feeling like putting together a tutorial/FAQ/best practice sheet to not repeat oneself over and over again might be a good idea, but that's future talk. At the moment we're dealing with huge water inflow in our acoustically optimized facility ourselves so not that much time left for anything else. But maybe I should start putting together a couple of thoughts and facts in this thread. So please see this as a more general approach trying to shed some light onto this topic, rather than a step-by-step advise for your specific problem.
I'm sorry but I have to stand by my claim that nearly everything you built didn't have that much effect on your sonic insulation, maybe even contradicts your efforts.
It doesn't have to cost a cow to do it properly but one has to follow the rules to make a success.
#0 General concept
What we're trying to achieve is to turn comparably fast vibrations of air and material inside the ears range (which can be physically translated to energy) into heat before it hits a bystanders ear. This can be achieved in various ways, depending on the source of the sound (e.g. a guitar amp can be easily detached from floor and walls, for drums or a piano it's much harder), the layout of the room (e.g. huge windows will make things highly expensive and small rooms don't leave enough space for insulating installations), the structure of the rooms floor/walls/ceiling and all the adjacent structures (any concrete floors, pipes or wooden timbers leading into the room from the outside?) connected to the outer world.
It's highly important to investigate the problem you're working against first to not throw a pile of spaghetti onto a wall and see what sticks.
In the case of drums it means to get a feeling for the kind of noise to be reduced. Is it more like a specific tone from a single tom which is carried over the most? This could mean that a thin wall is starting to resonate at the same frequency, carrying the sound into the buildings structure. Some sort of dampening put onto this wall could reduce the noise enough to reach your goals. Is it more the attack of the hit itself like e-drums produce (much more than acoustical drums) then maybe just putting the set onto a floating panel could help in that case. Play around with the source of sound and have someone else hear/measure what's going on in the spot meant to be quietened. Maybe taking a tom from its stand while hitting it to distinguish between the different ways the sound travels through the air/structure. Which leads us to the next chapter:
#1 Air-Borne vs. Structure-Borne
For a better general understanding it's important to distinguish between air-borne and structure-borne sound. Air-borne sound declines quite fast, doubling the distance reduces the sound level by a half, which means 6dB. Compared to structure-borne this is a comparably fast decline, since, depending on the material, structure-borne sound can travel even miles without a hassle.
If you stimulate a hard structure which is attached to another hard structure (e.g. a wall which is attached to a floor) the sound travels directly into the adjacent structure and gets carried over to the next structure and the next... This is how people on the 10th floor still hear the drummer practicing in the basement - the sound travels directly through the walls, standing on each other, up to the roof. Even a pipe can make your sound travel into the neighbors building, or a wooden timber, once activated, sends it over to another room.
The same is true with air-borne sound. If you don't stop the air from transporting the sound waves they'll travel very far. This in fact means that a wall missing 1% of its surface area is considered as non-existent acoustical-insulation-wise.
The absolutely best method in absorbing sound waves is making them transition between structure-borne and air-borne as much as can since this passage eats up the most energy.
Anecdote: while building the Klangwerk (a bunch of studio and rehearsal rooms I'll come back to later on) my friend was standing in the next room, removing some screws from the ceiling. The walls were already built, doors open, so we had to yell very hard to understand each other. I was doing some measurements for the air conditioning. And suddenly something metallic fell to the floor in my room. I could hear it absolutely clearly. And then another piece, and another one. So I was getting nervous because it sounded like directly behind me. Turned out it was the concrete floor, carrying the sound the falling screws made,
directly into this highly insulated room. One couldn't tell any difference between their sound in the other room and the one I was hearing.
Conclusion: we lent a huge buzzsaw to cut the concrete between all the rooms - in the middle between the walls - to stop the sound from traveling between the rooms, which worked perfectly well. No falling screws anymore. You'll see it in the video at around 3:39.
#2 Mass
This is one of the more important concepts for acoustical insulation - the mass a structure-borne sound is supposed to pass for insulation should be huge. Everything else works contrary. This means a stone wall is very good for this, even concrete can do a good job. Although a wall made from single stones is the much better choice since all stones are kind of structures on their own while the cement between them helps in stopping the structure-borne sound while traveling. Compared to a concrete wall - a single structure, transporting vibrations without any hindrance. If you build a room in a room one should prefer at least simple dry walls, better choice would be fiber reinforced panels - and even do it double-ply at best. The material doesn't like to resonate and is very heavy.
In your case you've set up thin, non-dampened walls (even made from wood?) which act like a huge resonance chamber, like a guitars body, a drum shell or a pianos baffle. So they start vibrating, resonating (amplifying) at frequencies depending on the material, size and thickness, maximizing your problems. I guess they are attached to the floor and the inner ceiling directly (a couple of average screws are enough) which makes the vibrations travel into the adjacent structures without any hindrance.
For a floor it's the same - a concrete slab carries vibrations very far (as stated above), soil eats them up nearly completely.
#3 Dampening
There's a reason for the selection of images googling "
acoustical insulation". And even the
wikipedia article states that this is the most important way of insulating. It has various effects. One of the more important is dampening the adjacent walls to reduce their motivation to vibrate, like a moon gel or a towel/pillow attached to bass drum heads is doing. This way you reduce resonating of mass. Another effect is a constant travel between air-borne and structure-borne sound within the material itself. It also hinders the air particles to flow freely. And it brings even more mass to the game. If the construction keeps a distance between the wool and the dry wall, it might be a good idea to help with some styrofoam, directly attached to the panels, to reduce the resonance (you'll see it in the video at ~2:40).
Dampening in order to reduce reflections inside the room also helps a bit, but that's not that much in comparison with all the other concepts.
#4 Delinking
One of the more important concepts. It's all about separating structures from each other to hinder structure-borne sound to travel between them. If a sound wave hits a wall which is directly attached to the ceiling, it sends the sound waves it receives directly into the ceiling, which then adds up to even more noise in the room above. If a room-in-room is built, directly mounted to the walls/floor/ceiling of the outer room, it doesn't make too much sense since it sends all the vibrations received directly into the outer walls, too. If the inner rooms walls/ceilings/floors are connected to each other it also adds up to a resonant sound chamber. This means one has to de-link everything as much as possible.
#5 Room-In-Room
E.g. building a wall for a room-in-room construction, it should stand on something insulating, maybe a rubber strip or something like a thick carpet or the like to separate it from the floor. Use the least possible amount of screws to attach it to the building. It's better to hang the ceiling first,( using acoustically optimized joints at best) and then to construct the walls underneath it, standing on rubber. The wall should also be delinked from the ceiling by using some soft material between both of them. Attaching the panels to the walls construction should also happen in a delinking manner - put some strips of carpet onto the construction first and then screw the panels onto it. All of this doesn't have to cost a lot, it's just that following the concepts with whatever solution gives the best results.
#6 Doors
A single door doesn't help that much. Normally doors are hovering over the ground, leaving a small gap between the doorstep and the leaf. You should avoid this gap at all costs. Having a 2cm gap under a door of 2m height the door can be considered as non-existent sound-insulation-wise. Doors should have a rubber seal around them (or the frame) to not let
any air flow between the inside and the outer world. Adding a second door, the frames of both doors shouldn't have any kind of contact (floor, lintel, doorstep) between them. If one of the doors is an acoustically treated one (heavy!) it's even better, but normally two average room doors will do a tremendous job for you.
#7 Windows
At least in Germany there's more or less sound-proof windows all over the place, mainly in order to reduce energy costs. Nice side effect is a quite good acoustical insulation. But there's even special, acoustically optimized windows available. For insulating a sound booth a single window is not enough, unfortunately. The cheap solution is to build panels with insulating material attached, being put into the windows openings while rehearsing (to be able to let in some fresh air if needed), the way more expensive - but highly effective - version is to add a second window, a bit smaller/larger that the original one to let them open inside each other. The glass should be made at least from two plies, better three. Also there's acoustically treated window frames available. Very expensive, should only be applied if that's the real problem. Otherwise it's just throwing money down the drains.
Klangwerk
As said, currently me and a friend of mine are building a 400m² facility of studios and rehearsal rooms inside a 500yr old barn made of 1m thick stone walls. This is the fourth pro-grade studio I built in my life so there's at least a little knowledge about this topic. We already achieved a nearly full acoustical insulation between two rooms located directly next to each other, since the goal was to have a grind core band rehearsing in one room while having a bunch of pupils doing recorder lessons in the next one.
Mission accomplished.
Currently I'm cutting a video showing the making-of-Klangwerk, which is in the making still. It can be considered around 20% done. I've uploaded a first version to youtube so you can have a glance into what we were doing for the last 20 months or so. I don't say one has to do it like this to achieve his goals but it gives a good impression of how to put all those concepts from above above into place.
