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Hi there "Scattered", and Welcome to the forum! :) Unfortunately, there was a glitch with the forum server yesterday, and your photos seem to have disappeared, with no trace! Please could you re-post the photos. Sorry about that!

A. Sound insulation: As much as possible. Playing my drums would produce around 115 dB(A)

For drums (and in reality for any very loud music) you should use "C" weighting, not "A" weighting. "A" is fine for quite sounds and for measurements in offices, houses, outside, etc., but for loud sounds with a lot of low-frequency content, you should use "C". If you measured a lelvel of 115 dBA, then that is going to be a lot higher on dBC.

(I've measured 10-12 dB(A) sound reduction with a 1 kHz test signal) ... So if I could achieve a further 40 dB(A) of sound reduction

When talking about reductions in sound levels, then you should not specify the weighting as that has no meaning. Reduction is a pure decibel measurement, without reference to weighting. In other words, if you measured 90 dBC on one side of a wall and 70 dBC on the other, then the difference is 20 dB, not 20 dBC.

The existing party-wall between me and my neighbour (300mm limestone blocks) has the following sound rating: Rw=55 dB.

Did you actually measure that, or are you just quoting a number from the manufacturer's specifications? Those are two very different things. The manufacturer tested his products under perfect controlled conditions in an acoustic laboratory, and built the test wall as well as possible. Your wall is probably not built the same, and is probably not providing the same amount of isolation. If you have not tested it, then you should do that. Play loud music from a full-range speaker at about 100 dBC inside your room, and measure how loud that is inside your neighbor's place, as well as in other locations inside and outside your own building. Anyway, assuming that you are getting roughly 55 dB, you say you want to get an additional 40 dB of isolation, for a total of 95 dB of isolation. That is an extremely, high level of isolation! The very best studio in the world gets 100 dB of isolation (Galaxy Studios in Belgium), and it cost them several million dollars to achieve that. To get that level of isolation the built massively thick concrete rooms, and floated each room on springs, like this: I'm guessing that you don't want to spend that much money on your drum booth! :)

The theoretical sound volume of my drums on the neighbour side would be 115-55-40=20 dB.

... which means that it would be about 100 times quieter than a quite bedroom at night! You do not need that much isolation. And it would be practically impossible to achieve it, without spending millions of dollars. You would not even be able to measure such a quite level, without spending thousands of dollars on extremely sensitive acoustic measurement gear... Most people consider that a level of 40 dB is inaudible, especially in a residential building where there is a constant level of background noise anyway.

Because of the door turning to the inside...

To achieve high levels of isolation, you need two doors, back to back. One opens outwards, the other opens inwards.

C. Budget: Budget is around €1000 for materials (studs, plasterboard, insulation, screws, etc.)

I don't want to seem harsh here, but that is not a realistic budget. There is no way that you can get 95 dB of isolation for €1000. You can't even get 60 dB for that. Your budget just is not realistic for what you want to do. Sorry. If you had about ten times the budget, you cold probably get 60 dB of isolation, if you are very lucky.

D. Acoustics: As dry as possible. I don't mind a "boomy" room with a lot of high frequencies being absorbed

That is not related to the isolation of the room: that is related to the treatment of the room. Isolation and treatment are two very different things. You certainly could have have good acoustic treatment in your room for €1000, but not isolation.

E. (Re-)movable: I don't own the apartment, it's a rental. So it has to be removable

This too is unrealistic. In order to get high levels of isolation, you would need to do some major construction. If this is a rental, your contract probably does not allow you to do anything like that at all! You should probably read your contract... Also, the amount of construction that you would need to do is massively heavy, and would likely greatly exceed the structural limitations of the building itself, even if you are allowed to do something.

1. Is it possible to achieve a further 40dB sound reduction within the given room space (100mm construction space)?

No. Assuming you really are getting 55 dB already, there is no way that you would be able to increase that to 95 dB in only 10 cm. of space. You MIGHT be able to get 65 dB of isolation, but that's about as high as it would be possible to get without spending huge amounts of money on very complex, massive construction. - Stuart -

So if I would "reconstruct" something like those booths, theoretically I would have an total isolation value of 52+30=82dB.

Actually, no you would not. What you are doing there is adding two logarithmic values, which isn't valid.... That's the same as multiplying, but two walls together won't multiply your isolation! You can't just add up the transmission loss values of individual barriers to get the total transmission loss, any more than you can add up the loudness of individual instruments to get the total loudness. If things worked like that, then a full symphony orchestra would put out many thousands of decibels.... :) If isolation worked like that, by simply adding numbers, then you could take just 11 sheets of drywall, each of which can block about 18 dB, and you'd be able to stop the shock wave from a bomb, which is obviously the loudest possible sound wave, at 194 dB... Once again, the very best isolated studio on planet Earth, is Galaxy studios in Belgium. They get about 100.7 dB of isolation, and it cost them many, many millions of dollars to achieve. If you don't believe me, then contact them yourself: +32 (0) 14 31 43 43. The following statement comes from there own web site ( http://www.galaxystudios.com/about-us ): "... we met with Eric Desart of the Gerber Group and later on Gerrit Vermeir, professor at the Universty of Leuven, already renowned authorities in their fields of expertise. They explained our wishes were far beyond anything that had been achieved until then. Nevertheless we believed it was worth trying. They developed industrial solutions to achieve the unheard insulation value of plus 100 dB. By using insulated bunkers for studios, and thus excluding external noise, they could achieve a much higher level of internal acoustic control. Moreover, cross-talk between different studio rooms could be completely avoided. The biggest problem was to insulate the lowest frequencies. To insulate frequencies as low as 20 or 30Hz, Vermeir and Desart calculated that the insulation capacity had to start a few octaves lower! This meant that it was necessary to find a way to start the insulating from less than 3Hz. Therefore, they decided to put concrete bunkers on special designed metal springs, to bring the resonance frequency for each room to less than 3Hz. The measured result (1994) is still a world record… 100,7 dB between each room. If what you say is true about just adding up the isolation from individual walls, then then could have saved a lot of money by just building two of walls like yours, rated at 52 dB, and they would have got 104 dB... :) But it doesn't work like that. Eric Desart was a member of this forum, but unfortunately he passed away recently. However, you can still find many, many posts by him here on the forum, and in some of those he mentions his work at Galaxy, and how he managed to attain such extreme isolation.

I'm trying to figure out my options an possibility's.

I think you missed my point entirely. I'm NOT saying that you can't build something to improve isolation! Not at all. What I am saying is that your expectations are unrealistic, and your numbers are unrealistic. There is no way you will ever get 85 dB of isolation in the way you propose: that's an unrealistic expectation. Perhaps you could get 60-something dB of isolation like that. But 60-something dB is still very good, and will mean that the sound of your drums will be very much attenuated, and will probably not bother your neighbors at all. That's what I was trying to explain: The problem seems to be with your understanding of decibels, sound, and noise, not with being able to build something that provides good isolation. For example, you mentioned that you wanted to achieve a level of 20 dB outside, but 20 dB is also unrealistic! There are very few places in the world where you can get such a silent level, and certainly not in a typical residential area! I'd guess that the background noise level outside in your garden is very probably around 35 to 40 dB, even at the quiet times of night, and possibly higher than that if you happen to live close to downtown or a busy street. So aiming to get a level that is 20 decibels quieter than the ambient level makes no sense at all. That's the problem here: decibels don't work the way you think they work, and they don't mean what you think they mean. If you were to set a level of, for example, 55 dB measured at 1m from the outside of your walls, that would be realistic, and achievable. That would require about 60 to 65 dB of isolation, which is also realistic and achievable with the plan you mentioned. But a level of 85 dB of isolation for a result of 20 dB sound level outside, is very, very unrealistic, and is not achievable. That's what I was trying to explain. You can get good isolation, but not 85 dB. - Stuart -