John Sayers' Recording Studio Design Forum Archive

Hi guys!

let me start by saying, how great I already find this forum! Lots of useful information and everybody seems to be very helpful and kind.

In this thread I would like to present my concept for a rebuild of my room. I have been composing, producing and teaching there for about 4 years now, but my knowledge and money was very low at the time I first rented it, so the initial design was kind of mediocre. Now, equipped with the financial possibilities and a little bit more knowledge gathered over the years, I would like to both share this and hope, some people might contribute to it, as I still consider myself as a novice in acoustics.

Into the facts:

The room is in the cellar of of a small building in Munich, Germany. It is pretty quiet most of the day (back premises) and at night, being surrounded by thick concrete walls, so - from my experience - I don't have to worry for insulation and isolation too much. Most of my work is composing and teaching, so I decided not to do a room in room build. The original floor is problematic, as it has tiling all over, combined with floor heating. The ceiling is detached by roughly 10 cm throughout the room with a wood construction covered by drywall plates. It is not filled with wool!
Altogether, I'd say (surprise!) the room is to boomy in the low frequencies and has some flutter echo problems.
In 2008 I build a light dry wall (as seen in the pictures) to separate in two rooms. The smaller one originally used only as kind of a rest room. the dry wall is only something like 10 cm thick and has to have a door, so there is not too much isolation between the rooms.

This is, what I want to do:
- better isolation between the two rooms to be able to record other musicians with monitoring them
- more linear and better sound in the mixing room (this is also going to be stuffed with synths, PCs, tons of midi gear, etc., as I do much composing)
- better sound in the recording/teaching room (I know this is a bit small, but most of my work and need for space is in the mixing/composing room)
- better looks of both rooms to have good vibes going

In the blueprint you can see the room as it is - the yellow areas being what I plan to add. I will explain everything later in more detail. First of all, I would be very thankful to get feedback for the overall design and potential traps I might run into ...
Some quick facts about the yellow areas and what I want to do:
- on top I plan a soffit mount of the stereo speakers (adam p22a) - this gives me the most headache even if there is so much on that topic in these forums and the basics sound very simple - any hint on that would be VERY welcome - I'll go into detail later
- in the middle you see the already built (white) wall and the extension in yellow
- in the corners some bass traps
- at the stairs an extra wall and door too prevent slap back echo
- the floor is going to be covered by foot fall absorber, followed by chipboard with higher mass (concrete-chipboard???), followed by oak parquet, everything together about 4 cm thick
- the ceiling is going to be filled with rock wool, injected by blowing through some holes - a special technique I found in the internet to prevent me from having to build the hole ceiling newly ...

I am looking forward to any reply on this! Thank you.

Ok, round 2: I will go through the details, as I planned them:

1) The separation wall:

The first wall was already built 4 years ago. I remember selecting the room proportions for the bigger room selecting from a table ... hope I did not pick the worst ratio by accident. I slightly angled the wall because I had read that to be a good thing somewhere. However, this wall only consist of a ca. 50 cm wood construction filled with a layer of fibre glass and covered by one layer of gypsum cardboard on each side. Bass frequencies go through pretty much untouched ...

What I plan to accomplish:
- obviously: separation between the rooms: I do not expect to isolate a rock drummer ... I think this would require a room in room construction; I have a roland v-drum set there, so probably drums will - if ever - rarely be recorded (sorry, drummers - I know ...); I hope to be able to isolate acoustic guitars, string quartett and woodwinds pretty well
- from both sides I kind of expect some kind of low frequency absorbtion with this wall; am I rightfully doing so?

What I plan to do:
- first, I will add a second layer of drywall on the mixing side to increase the mass; do you think 2 is enough or rather 3?
- then I will build a wooden frame for a second (thicker) wall with an additional door; I cut the ceiling open over that new wall to break the resonance of the cardboards of the ceiling; a tricky thing is the ceiling wood frame itself: I suppose it would be better to cut it along the new wall as well, so that each rooms ceiling rest on its own wall; what do you think? is it worth the effort? does the frame still transfer a lot of energy to the other room if not disconnected? I would rather avoid doing it, as it is a bit tricky with the wall-frame being not in line with the ceiling-frame and the cutting could be potentially dangerous; however, I don't want to get disappointed later, when it is too late ... and statically it should work, I suppose ...
- the wooden frame of the wall rests on a footfall absorbing rubber layer - about 1,5 cm thick; I bought the pieces they sell for putting your drum set on it; meanwhile, I found out it might be to hard or to soft - I should have calculated the weight of the wall and picked a fitting kind of "sylomer" as the walls base. but I already started building that wall frame, so I would only redo that if it really matters a lot; what do you think?
- at both sides the wood frame is drilled into the concrete wall with massive screws; I suppose I can ignore the transfer, as the walls are massive and the whole studio is in the basement, hence, connected to ground soil; am I right with that?
- after I put the first layer of gypsum board on the wall I will put the insulation in ... huh? no, seriously! the guy who is going to inject the ceiling with rock wool told me, he could also fill my wall with it; it is going to be much denser, than I could achieve by doing it with fiberglass pieces; the question here is: denser better or not? to my knowledge it is better for isolation, but worse for absorbing, right? so, I think it will be better in this case ...
- of course, here a second layer of cardboard is planned, too; same question as before: third one? Or maybe later add some wood/absorber/reflector here?
- on the left side of the wall I already integrated a wooden frame for a bass trap (anticipating this will be a good thing) I will to that after the wall is finished on the other side; the reason I planned it that way is, there is not much room to the window you see in the picture; supposing the denser rock wool is better for the rest of the wall: what about here? is the fibreglass in a corner trap supposed to be denser or lighter? will there still be bass absorbtion if I put two layers of gypsum board in front of the wool? or is this alltogether not working as a corner bass trap? do i need air coming in their? I just don't understand, if low frequencies would go through the double layer of dry wall or rather be reflected ...
- on the upper right corner you see a gas pipe going through the wall; I already built that through the wall 4 years ago and it was no problem, however, considering my investment in isolation: is the transfer of noise through this pipe (copper, 2.3 cm in diameter) relevant to the equation? I cannot remove it, so, would a foam coating help here? Or is it to thin to be relevant?

2) Soffit Mount Positioning (this is actually how I found this site - took me an hour to find out how the english term is ... )

What is it?
This is probably the most complex thing to me, although the theory behind it is supposedly easy. This is what I gathered from here: speakers in correct listening position, rigidly bound to a big mass prevent sound energy to make the speaker "wobble" or excite any other masses to resonate. Combined with a bass trap (hangars?) and an MDF cover (which may not touch the speaker itself) this leads to a smoother low frequencies, but at the same time a slight bass boost, that needs to be compensated with the speakers EQ. Also, I like the looks of it and in the same step I can angle the ceiling slightly ... did I get that right?

What are my premises?
Current Speakers: four Adam P22A (I recently got an extra pair almost for free, so I only needed one for a 5.1 setup!!!) and a Adam Sub 10
Setup: 2.1 and maybe an option to add 5.1 later.
Purpose: Composing and producing with samples (orchestral)/synths, recording (myself playing guitar, piano, percussion, etc. and recording others from string quartet to small bands); basically, I need a very flexible enviroment, because I haven't got a clue, what future projects will bring; the mixing room is mainly used for producing, but should be fit for mixing at the same time.

First things first: I start with the best speaker position. Here, I set up a stereo triangle. The first problem I have: Where do I place them exactly? With free standing monitors i would try to find out the perfect position by listening (and maybe measuring). But this is probably irrelevant, as soon as the mount is completed. So, there has to be a way of calculating that in advance, has there?
Obviously, they have to fit behind the mount. Regarding the stereo triangle I read, that sometimes a 90 degree angle can be valid to have a broader stereo image and more space between the speakers. This seems to apply to my room size as well. In my 3D model something in between looked practical. But how does it sound? Impossible to say for me, until the whole thing is built.
Also, is there any way to define the sweet spot of distance? Until now I listened from about a meter, but this was due to spacial limitations and the room being poorly treated. To my knowledge, P22A are designed as nearfield/midfield monitors, nearfield being 0.7 - 2 meters. And how do room height and height of the tweeters correlate? How do I do that? I am puzzled ...

At the same time, I think about expanding my 2.1 setup to 5.1, as only one speaker is missing:
However, the wider I put my listening circle, the wider back I have to pull it into the room for the center being in place, resulting in a bad listening position because of the window reflections (this window will only get a a curtain as treatment, as I have to open it quite often for air ventilation). Also, there seems to be a problem with LS and RS at 110 degree, when the radius of the circle is coming close to 2 meters. I more than 110 degree also valid for a 5.1 setup. Excuse my ignorance here, but I never set up 5.1. Would you recommend putting 5.1 into the equation? Alternatively, I could get smaller free standing speakers for 5.1 later and stick with 2.1 with my adam p22s ...
I have even more questions regarding the soffit construction, but to make it shorter I make a break here ...

Hmmm, am I doing something wrong? I think I complied to all rules. Please tell me, because at some point I will have to start building to get everything ready. Until then, I would like to get an idea, if my plans are any good at all. I'll keep going, hoping someone will take the time ...

Welcome FriFlo,

A skewed wall is fine in the tracking room, but it wil most likely cause you problems in the mixing room. The mixing room should be symetrical. You might want to consider building the new wall on the mixing room side, and making it straight.

Thanks for the reply!
Hmm, is this really VERY important? I already started on that wall (as I told, I regret that I found this forum a little to late ...). So I would probably have to build a third wall ... I thought, when I create a lot of diffusion in the back of the room there wouldn't be a big difference. Also, since my room is not 100% symmetrical, the angle of the wall might look greater in the picture, than it really is. It is somewhere beyond 7 degree.
I would prefer to solve that with treatment as most of the walls frame already stands, unless you guys say, it is absolutely crucial.

Here a picture of the current state:
As you can see, these would be the problems when building the additional wall on the other side:

- I have to remove the door frame and turn it by 180 degree, which is very tricky
- I already started with the frame and - even worse - already cut the ceiling open - at this point it would be easier to continue with the wall and build an additional structure on the other side afterwards - maybe up to the door ...
Something like this?

You don't want to build a third wall as that would countermine whatever isolation you have between the rooms - the infamous third leaf! You could however make framing and fill it with rockwool as in your last picture. This will then make the rear wall predominantly absorptive which would counter or offset the problem of skewed reflections.

So, if I understand you correctly: You would agree, that building the second wall on the other side would be an over-kill of effort in my case and leaving the plan as it is could be compensated well with treatment? If so, this is great news for me ...
So, what does that third leave do acoustically? I never heard that before ...
Looking at my last picture: Should I build that absorber (wood frame filled with fibreglass, coated with fabric???) like in the picture: assymetrical to make it parallel with the other wall? Or rather just build a big absorber on the back wall? To my knowledge, a slight angle of an absorber doesn't effect it's result, right?

Before getting into some of the other details, let's look at your isolation wall between the two areas. What you need is a wall conforming to MSM (Mass-Spring-Mass) principles. You might want to search for topics regarding this theme. Basically what you want is a layer of mass (as much mass as possible) then a sealed layer of dead air or better loose insulation to act as the spring layer, then another layer of mass. Again, the entire construction must be airtight.

From what I understand, your existing wall consists of a frame with drywall on each side. This is already two leaves of mass!

What you need to do is remove the drywall from one side of the existing wall (the side where you want to build the new wall!) and beef up (perhaps reuse the old drywall) the remaining side. You didn't mention the thickness of the existing drywall, but basically 12mm thick is too thin, even if you add a second layer. You should be using at least 16mm thick drywall, preferably at least two layers and ideally with green glue between the layers - although the latter is not absolutely necessary if it's not available or within your budget. Personally I would recommend using a double layer of 18mm Fermacell which has considerable more mass than ordinary drywall, and should be available in your area.

So now you'd have your original wall with a double or triple layer of plasterboard on one side (the mixing room side), and the bare frame on the other. Light mineral wool or glass wool should fill the frame. You don't want anything too heavy or massive for the insulation - this is the spring layer!
Now you should have your new wall frame adjacent to, and separated from the old frame. These wall frames should in no way be mechanically connected to each other, and ideally , at least the new frame should be isolated from the ceiling and floor using sylomer, neoprene or even "randdammstreifen" and the bolts would also be best isolated with rubber tubing along the shaft, a rubber washer under the head, and into plastic anchors (dubels) in the floor and ceiling. All edges on both walls should be well caulked airtight This frame too should be filled with wool, then finally covered in a couple layers of plasterboard. Remember too that the plasterboards must be taped and mudded.

Obviously it's going to be PITA if you have to take down the framing again (unless you do want to address the decoupling issues). So here's an alternative. You could remove the plasterboard from the mixing room side, and either use it or thicker stuff to "beef up" the remaining layer on the live room side by cutting it and affixing it to the plasterboard between the studs. This would leave you with a proverbial inside-out wall which will be fine because this side of the wall is going to be absorptive treatment anyway.

The key thing to understand here is that you want two, and only two mass leaves in the overall complete isolation wall. As additional leaves are added (e.g. MSMSM), the isolating properties of the wall are reduced. Two leaves is the ideal - consult any competent source of acoustic information!

Ok, this is really helpful! Thank you so much! A few things to be sure:

1) Just to get the whole idea right: you are saying the MSM thing won't work, when I have mass (gypsum) - wool - mass - wool - mass? So, I'd rather need mass (more) - wool (more) - mass (again, more)?
If I leave the additional wall frame on the tracking room side, I would have to remove the gypsum boards on that side of the wall. Instead I add at least one layer of gypsum board to the mixing side of the wall (what was already planned).
Afterwards, I use the same principle on the other side. My wall would look like this: Right?

2) I assumed that two layers of 14 mm gypsum (these are the ones I already bought) add up together as one 28 mm mass. Or not? I suppose that green glue is to join these boards even tighter, right? So, can I work with those 14 mm boards or will I have to buy new ones? Regarding the glue - it looks expensive:
http://www.amazon.com/Case-Green-Glue-N ... B000SKWD8Y
Is there an alternative, which is more accessible (and maybe cheaper) in Germany?

3) Regarding the fibreglass: That guy who is going to inject my ceiling with stone wool could also do that with the wool. This way, it would be still loose material, but packed tighter than with doing it bay hand. Do I understand you right that looser is better in that case?

4) Regarding airtight: When I plaster each drywall separately, is that air tight, or are additional treatments needed. Also, to decouple the ceilings above each room, I had to cut the ceiling open. So, the Air inside the wall will always be coupled to the air inside the ceiling, even if I manage to get the wall tight. Is that a problem?

5) About the decoupling the new frame: Is the drummers mat on the floor going to do the job? Does the drilling to the side walls cause a problem? If so, how else do I get the required stability?

6) Finally, what do you suggest regarding the framework in the ceiling? Do I have to uncouple (cut) that between the two rooms?

Oh, I just cross-posted! Regarding your new idea: Don't forget, that the door is the weakest link in the whole thing. I built the extra frame to add another door, primarily. But i am not sure, if i understand that new proposal right, anyways ...

I get the impression that we're not yet quite on the same page, so let me try and answer your questions.

1. Only MSM. One leaf of gypsum board will have to come off the old wall. In my previous post I indicated this would probably be easier to do on the mixing side, as you already have framing constructed on the tracking side. The remaining leaf of gypsum board on the tracking side of the old wall will have to be beefed up, but again this is no longer possible from the tracking room side - again because of the new framing. So, the beefing up will have to be done from the mixing side, and attached to the other side between the studs of the framing. Forgive the crude drawing: 2. 14mm is better than ordinary 12 and in this case would suggest to use 3 or 4 layers for each leaf. Although your reasoning seems logical by adding the thicknesses together, it is only partly the case, because the resonance of the individual boards will still be present. (check out Stuart's answer to my own question in this regard http://www.johnlsayers.com/phpBB2/viewt ... 5&start=45)
Green Glue is in fact not glue at all, it is a specially formulated CLD (Constrained Layer Damping) compound for use between layers of drywall to reduce sound transmission. It cannot be used to glue the plasterboard together! It is not an absolute necessity but rather viewed as an added option - a "nice to have", but as you already noticed, it's expensive and not so readily available here. There is another similar product but again also not cheap and doesn't have the same proven test documentation to back it up.

3. You don't want the insulation packed tight. Loose and light - the fluffy stuff is generally preferred over the more rigid wool panels for use between the plaster leaves.

4. The MSM structure must be airtight. You said you cut the ceiling, but you must be referring to a drop ceiling, the wooden framing must go up to the actual ceiling. All edges (floor ceiling walls) should be caulked.

5. Using decoupling materials as I proposed in the DIY example in an earlier post or purchase decoupling braces (e.g. Mason Industries). If you can get a stethoscope, or contact mic, you could check for transmission by tapping to see if your rubber decoupling works. Otherwise, if you can see that it compresses somewhat under the weight of the wall (remember the layers of drywall will add considerable weight), it's probably sufficient.

6. Again, decoupling is always good, and the method of shrouding the screw /bolt shank plus rubber washer into a plastic anchor does work.

you are saying the MSM thing won't work, when I have mass (gypsum) - wool - mass - wool - mass?

Correct, because that would not be MSM: that would be MSMSM. That's a three-leaf wall, which will always isolate WORSE than the equivalent two-leaf wall, all other factors being equal.

So, I'd rather need mass (more) - wool (more) - mass (again, more)?

Yes. That's MSM.

Afterwards, I use the same principle on the other side. My wall would look like this:

That's one way of doing it, if you have access to both sides of the wall. Brian is suggesting another way of doing things, which is very useful if you only have access to one side of the wall.

2) I assumed that two layers of 14 mm gypsum (these are the ones I already bought) add up together as one 28 mm mass. Or not?

Not!

Well, OK, let's say "yes and no": Sure the total mass is the sum of the two individual masses, but that's not the only issue here. It's a bit more complex than that. Two sheets of thin drywall do not act the same as one sheet of thick drywall, since the panel has properties other than just pure mass: thickness, rigidity, flexibility, resonant frequency, coincidence dip, etc. All of those change for different thicknesses of panel. So, for example, if there were such things, then two sheets of 8mm drywall would NOT work out the same as a single sheet of 16 mm drywall. The wall made with two sheets of 8mm would not isolate as well as the wall made from the single sheet of 16mm.

I suppose that green glue is to join these boards even tighter, right?

Nope! As Brian pointed out, is not really glue at all. That's just the name of the product, but does not describe what it does. Maybe they could have chosen a better name!

What it does is not to glue the sheets together, but the exact opposite: it keeps them apart! It creates a thin layer of rubbery "stuff" with tiny air gaps in it, which acts to damp certain types of resonance in the wall, and that increases the isolation.

Regarding the glue - it looks expensive:

Yup! But you might not need it: how much isolation do you need?

3) Regarding the fibreglass: That guy who is going to inject my ceiling with stone wool could also do that with the wool.

Brian already mentioned this, but I'll repeat it: blowing in insulation is not a good way of isolating walls, acoustically. You run the risk of creating both flanking paths and voids, and also of uneven distribution in the cavity. All of those are Bad Things That Should Be Avoided.

but packed tighter than with doing it bay hand. Do I understand you right that looser is better in that case?

Correct: you do not want it packed tight at all! You want it loose. Just enough insulation to fill the cavity, and no more. You should NEVER pack insulation tightly if you need isolation.

4) Regarding airtight: When I plaster each drywall separately, is that air tight, or are additional treatments needed.

Yes, more is needed: you need to seal the framing to the floor with acoustic sealant, at least 3 seals (five is better). Same where walls meet: the frames must be sealed together. Same where the walls meet the ceiling: acoustic sealant in the gap. Very important. Sealing is critical to isolation You need two perfect, hermetic seals: one for the outer leaf, one for the inner leaf.

Also, to decouple the ceilings above each room, I had to cut the ceiling open. So, the Air inside the wall will always be coupled to the air inside the ceiling, even if I manage to get the wall tight. Is that a problem?

If I understand you right, that is fine: But do you mean that the air between the inner-leaf ceiling and outer-leaf ceiling meets the air between the inner-leaf walls and outer-leaf walls?

I do not expect to isolate a rock drummer ... I think this would require a room in room construction;

Ummmm..... don't look now, but everything we are talking about so far IS for room-in-a-room construction! That's exactly what you are doing.

I slightly angled the wall because I had read that to be a good thing somewhere.

Well, yes it is a good thing, but you angled the wrong wall! For a control room, the rear wall should be parallel to the front wall, or at least symmetrically non-parallel, if possible, oar worst case, designed specifically to not reflect sound back to the engineers ears too soon or at too high a level.

the wooden frame of the wall rests on a footfall absorbing rubber layer - about 1,5 cm thick; I bought the pieces they sell for putting your drum set on it; meanwhile, I found out it might be to hard or to soft - I should have calculated the weight of the wall

Unless you did all the calculations, then the chances are pretty close to zero that it is not doing anything useful. For example, what is the total mass of that wall, including everything that the wall supports? And what
deflection are you getting on the rubber under that load? And what deflection does the manufacturer of that rubber recommend for optimum float? If you didn't figure that out carefully, then I suspect that you just have an expensive but useless decoration under your wall...

but I already started building that wall frame, so I would only redo that if it really matters a lot; what do you think?

How did you attach that wall to the floor underneath? Did you use bolts, screws, nails or something else?

- at both sides the wood frame is drilled into the concrete wall with massive screws; I suppose I can ignore the transfer, as the walls are massive and the whole studio is in the basement, hence, connected to ground soil; am I right with that?

Actually, it is the other way around: If that wall is firmly attached to the side walls, then nothing is decoupled, and sound will transfer very well between rooms. Sorry.

the guy who is going to inject the ceiling with rock wool told me, he could also fill my wall with it; it is going to be much denser, than I could achieve by doing it with fiberglass pieces; the question here is: denser better or not?

What do you mean by "dense"? Put that in numbers. And what type of insulation? There is an optimum density for each type of insulation. denser is not better.

5) About the decoupling the new frame: Is the drummers mat on the floor going to do the job? Does the drilling to the side walls cause a problem? If so, how else do I get the required stability?

One at a time: "Is the drummers mat on the floor going to do the job?" No. "Does the drilling to the side walls cause a problem?" Yes. "If so, how else do I get the required stability?" You decouple the walls, by building a complete "room in a room". You CANNOT isolate two rooms unless you decouple everything: Just building a wall between them will NOT achieve good isolation, unless you also cut all the flanking paths. And the only way to do that, is to build the room correctly, as four decoupled walls with a decoupled ceiling on top.

6) Finally, what do you suggest regarding the framework in the ceiling? Do I have to uncouple (cut) that between the two rooms?

Yes, but you have to do it correctly!


- Stuart -

Thanks for chiming in as well, Stuart! I really appreciate that! And please excuse my ignorance ...

Still, a few questions remain regarding that wall:

Unless you did all the calculations, then the chances are pretty close to zero that it is not doing anything useful. For example, what is the total mass of that wall, including everything that the wall supports? And what deflection are you getting on the rubber under that load? And what deflection does the manufacturer of that rubber recommend for optimum float? If you didn't figure that out carefully, then I suspect that you just have an expensive but useless decoration under your wall...

What is the formula to calculate the needed material for floating? I tried finding it in the forum search unsuccessfully. I gathered from your answer, the mass of the wooden frame including all gypsum boards attached must be a variable in it, the rubbers deflection strength is in combination with the area of the rubber.
However, when I cut the ceiling wood construction between both rooms, the mass of that side of the ceiling will also partly have to rest on that wall frame. But how can I calculate that "partly"?

"Also, to decouple the ceilings above each room, I had to cut the ceiling open. So, the air inside the wall will always be coupled to the air inside the ceiling, even if I manage to get the wall tight. Is that a problem?"
If I understand you right, that is fine: But do you mean that the air between the inner-leaf ceiling and outer-leaf ceiling meets the air between the inner-leaf walls and outer-leaf walls?

I mean the air between both wall leave will be connected to the air between concrete ceiling and detached ceiling, because there is a gap between both wall wood frames where the ceiling is cut (see my photo). Does that destroy the MSM principle?

"I do not expect to isolate a rock drummer ... I think this would require a room in room construction;"
Ummmm..... don't look now, but everything we are talking about so far IS for room-in-a-room construction! That's exactly what you are doing.

I thought it would only be a room-in-room construction, if the whole system was closed (floor, ceiling, 4 walls). Are you suggesting the whole thing won't work at all for insulation, when I don't do that (continue the wall around the whole tracking room and to a new ceiling? Don't get me wrong: I do not need "perfect" isolation. I will be enough, if I can monitor people in the recording room without too much cross talk.

"Regarding the glue - it looks expensive:"
Yup! But you might not need it: how much isolation do you need?

Great! So, I will take that out for the sake of my budget.

"3) Regarding the fibreglass: That guy who is going to inject my ceiling with stone wool could also do that with the wool."
Brian already mentioned this, but I'll repeat it: blowing in insulation is not a good way of isolating walls, acoustically. You run the risk of creating both flanking paths and voids, and also of uneven distribution in the cavity. All of those are Bad Things That Should Be Avoided.

Ok, I get it. For isolating the wall I better do it by hand, which is fine, because it is cheaper anyways. But what about the ceiling? There, it is more about absorbing the sound and about putting more mass on the gypsum boards to prevent them from resonating at low frequencies. Also, I really cannot redo the ceiling, which I would have to do, to put fiberglass in by hand. My Question: Is this approach of injecting material suited for the ceiling then? What material is best blown in there? I gathered here, the more mass, the better, hence, the guy recommended rock wool.

"4) Regarding airtight: When I plaster each drywall separately, is that air tight, or are additional treatments needed."
Yes, more is needed: you need to seal the framing to the floor with acoustic sealant, at least 3 seals (five is better). Same where walls meet: the frames must be sealed together. Same where the walls meet the ceiling: acoustic sealant in the gap. Very important. Sealing is critical to isolation You need two perfect, hermetic seals: one for the outer leaf, one for the inner leaf.

What product/material do you refer to for that "acoustic sealant"? Especially, how can I seal off the air between the wall and the detached ceiling?

How did you attach that wall to the floor underneath? Did you use bolts, screws, nails or something else?

Nothing. The frame just rests on the rubber mat on the floor.