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And what about the bass hangars ... Do they make any sense at this size and their position? I also wonder, if it it is necessary to have a closed compartment with only one entry area for air, opposed to just suspending those hangers from the framework, letting the air flow through the whole construction? In most some designs I saw hangers suspended from the ceiling just covered by cloth. So, it should be the same effect, as the compartment behind the speaker front, right?

i think some of this decision will be based on how much storage you actually need, since the more storage you place there, the less space there will be for absorption. that said, maybe the doors/panels covering the shelves could be tuned as broadband in nature so you use the air space (whatever is left) could be a cavity which does something useful - esp if the back side of the shelves is absorptive. you might find an MLV limp membrane trap in each corner adds some additional LF absorption over and above the porous absorption. for the porous approach - i'd use 4" of semi-rigid insulation (same 3lb up to 6lb) and backfill with pink (light) insulation (e.g. R30).

Thanks Glenn! So, if I got you right, with the shelf Doors this could be working, maybe comparable to or Even better than to the A design? I do not intend to stuff the shelves full of junk. Only some often needed items would go there, like microphones, guitar strings etc. everything else will go in the other room. That only leaves me with one of my older questions: what about the bass hangers? Would they be better than simple porous absorbers in this size for absorbing more bass energy? And one new question arises: how would you integrate the MLV into the corner traps? As a top layer? I understand limp mass the way that it does not have to be sealed somewhere. So would I just attach the MLV to the studs? I have looked for MLV on the internet, but I cannot find that it on the German market (not sure, if I am looking for the right name!). Is there something similar I could buy? from what I found about it I guess it is similar to those footfall sound insulation mats I used for my floor. Those are pretty heavy, too and seem to have properties and flexibility.

an idea on the back corners

Wow Glenn! That is more I could ever ask for! Thank you so much! I will investigate what kind of substitute for MLV I could get and see, if I can get this done ...

Hello again! After using the studio without any further acoustic treatment added, I have gotten some money together and finally installed a ventilation system to the rooms. It was quite difficult, but absolutely worth it, since I am going to stay there for the next 20 years at least. Then, I went further with the acoustic treatment. I built the back of the main room, as demonstrated in the last picture (with the storage in the center). I also built some covers attaching with magnets, so that I can easily access everything (including the ventilation pipes). Now, I have filled the super chunks with mineral wool and did some other testing. For the deeper parts (about 50 cm), I found out by experimentation, cheap, right mineral wool provides better results than the denser tamarack 50, so I filled everything with the lighter stuff. I will later also put a lot of mineral wool into the space behind the soffit mounted speakers, to get rid of more low frequency modes (especially around 50 Hz). Here are my REW results comparing every single step so far: I guess, most steps are pretty self-explanatory. The last step was to compensate for the baffle effect with the internal Adam EQ (-4 db below 150 Hz low shelf filter - there is no higher setting available). What do you think? Will the treatment be enough for the low frequencies, as soon as I add the insulation behind the soffit mount? What would be the most important steps behind it? My plan is: - put light insulation in the bags they come with behind the soffit mount and maybe some hangars, if they work (according to REW measurements) - build two slat wall pieces (or absorbers) to the right and left of the stereo speakers to finally close the soffit mount - buy or build some diffusors for the bare walls (left and right) to get rid of the flutter echo, maybe combine those with additional absorbers or slat walls, if there there is some additional mid absorption necessary - build some wood panels over parts of the absorptive ceiling and/or walls, in case I want to gain back some high frequency reflections What do you guys think? What I really don't get is why the insulation increased the hole at about 127 Hz instead of evening it out. What would I have to do to address that problem?

Ok! I am one step further. I did some experimentation with different mineral wool and found out, that a to layer of Termarock 50 over the light insulation in bass traps and remaining back wall treatment did some more low frequency reduction - both SPL and RT60. Now, for testing I put 4 unopened packs of light insulation behind the soffits. I had to squeeze some of them together, to fit them in, so it will probably become a lot better, when done properly later. It did further decrease the 50 Hz bump, but - shockingly - the 127 Hz hole moved up a little and got a lot worse. I can also verify its existing by ears, using a sine wave generator. Could someone please help me understand, where this is coming fro and what I might be able to do against it?

Silent bump ...

Ok, I think I have gotten another step further towards a good sound! Bass trapping in the back (except for the absorbent doors in front of the shelf) and in the front behind the speaker wall are finished. I experimented with soft and rigid mineral wool and hangers. In the back under the shelf the hangers seemed to work not too well, so I put a layer of soft inflation before them (20 cm with a distance of 30 cm to the back wall and one hanger layer behind it). Inside the front I treated mainly with soft insulation as well some hangers in the front. Those actually did a little good to the ow end, more so did placement of the sub and crossover setting of the speakers. Meanwhile, my subwoofer came back from repair, so I could only test the changes after the bass traps in the back were in place. This is what I achieved so far: I am not sure, wether I can do a lot more for the low end, I guess the treatment on the side walls will help it a little, if they are deep enough. Also the absorbent doors will help a little with the first mode (the Null at about 33 Hz). But I think it is normal, that a studio of that size cannot become all flat in the low end. Clearly, to me the biggest problem in the lows seems to be the null at about 130 Hz. Meanwhile, I think I figured it out! With the desk remove, it this cancelation moves a lot in frequency (up). So I figured, it is the reflection from the desk that does this. Kind of like SBIR, but with the desk. Or You could see it as a room mode between desk and ceiling. close to the desk or ceiling above the desk it gets really loud and in between more or less canceled (judged by ears). So, I figured, this can be improved by angling the desk (as planned anyway) and having some extra absorber above the desk (my ceiling is 10 cm rigid rock wool, but there I could add another layer, maybe basotect. To confirm the theory, I put a 7 degree angled board on the desk and compared directly: 5 db less cancelation tells me, I might be right with my theory! Am I? :-) I also put boards before the openings in the soffit wall, leaving some space open. This seems to wok quite well, considering this is where slat resonators will go. It smoothed out around 100 Hz quite a bit. All in all, I think it will not get perfect, but quite good considering the space and the compromises that had to be made for the field of work (composing). So, apart from desk and ceiling, my plan is to build some slot resonators next, trying to deal with the remaining problematic frequencies. Maybe some thin laminate/parquet panels to the ceiling in the back of the room to get some more liveliness back. But then again ... I think I target a 200 ms RT60 over all and that is where I am almost at in the highs. I would really like to hear your opinion about this! Maybe even someone else than Stuart's! ;-) I will later add a link to the REW files.

overall that is looking pretty decent. the deep null may be due to the ceiling/floor and/or some combination of that and side walls. so getting the clouds into place and the side walls done should that that down quite a bit as well.

Thanks Glenn! So, I thought of one last idea (apart from the shelf doors with 10 cm basotect): I could build an absorbing door (actually just a frame with door handle) before the entrance door filled with 10 - 20 cm of basotect. Closed, its front will have a distance of about 50 cm from the door. That should help even more with the lows. Further, I think I know how to build those slot resonators by now. My Idea is to build the front as an MDF board, glue laminate to that and saw the slots in. Would this work as well as using slats? I think it will be easier with my equipment and it should look nice, too. I could also use MDF and varnish it. But this is only an optical consideration. As far as I understand, the frequencies (or frequency-areas) I tune those resonators to should be problematic ones in the listening spot, that have a modal peak at the location the resonator goes to. Is that right? Also, the narrower and fewer the slots, the less broadband the resonator will be. I am yet not sure, which way to lay these out: Look for some peaks (e.g. 130, 230 and 260 Hz) and figure out the parameters to hit those three frequencies with three different slat withs? This would be less broadband, but it should work, if those frequencies all have peaks at the position of the resonator, I guess. However, people keep telling here, that it is too hard to really hit specific frequencies with a resonator. Does that also apply to a slot resonator with multiple frequencies? The only way to make it more broadband would be the amount of insulation (the Q), right? But on the other hand, if you follow the formula, you cannot really reach deep with a broadband slot resonator, except if you build a very deep one, which I do not have the space for. You kind of have to make pretty narrow slots to effect frequencies under 200Hz. Example: Slot width. 4 mm Depth from wall. 120 mm Slat Depth. 30 mm Effective depth of Slot 36 mm Slat width. 160, 117 and 37 mm Absorption Frequency. 130, 230 and 261 Hz Would a slot resonator like that work, if those frequencies have peaks at the positions of the according slats? Or is this too hard to build and need thousands of adjustments until it is about right? In that case: How would you plan the slot resonators for my room?

Polite bump ... :-) It would be terribly nice, if someone could tell me, how to proceed. I plan doing the last bits in August/September, but I would really like to be able to plan ahead as soon as possible.

My Idea is to build the front as an MDF board, glue laminate to that and saw the slots in. Would this work as well as using slats?

That would work just fine. It doesn't matter how you build them: as long as the dimensions, spacing and cavity depth are correct, that's all that really matters.

As far as I understand, the frequencies (or frequency-areas) I tune those resonators to should be problematic ones in the listening spot, that have a modal peak at the location the resonator goes to. Is that right?

Not really... First, you'll find it pretty hard to tune a slot resonator to typical modal frequencies: you have to be very precise, since modes are very narrow bandwidth, and you have to hit them spot on to get any useful effect. The slot size would likely be very, very thin, and even tiny errors would cause big shifts in your tuning. You could tune for broadband (by angling the slats further away from the wall at one end, so that the cavity changes depth), but that makes the entire things less effective. Then there's the issue of location: you'd have to ensure that there is a pressure peak for the mode you are trying to treat, at the front face of the slot that is trying to treat it. If the pressure peak is far away, the resonator won't do much for it. And you'd also have to be certain that you are using the correct wall! It's not much use using a side wall to treat vertical mode... Then there's the issue of volume: in order to be effective, the cavity volume associated with the frequency you are trying to treat needs to be at least 1% of the room volume. If you need to hit ten modes, that implies that you'd have to take up 10% of your room with resonators! In general, I don't try to use slot walls to treat modes: not a good way of doing it. Slot walls are great for mids and highs, but just too much hassle for lows. It's much better to just go with porous absorption, panel resonators, limp membrane resonators, or some such for treating the modes.

Also, the narrower and fewer the slots, the less broadband the resonator will be.

Not true. The slot width does not affect the frequency range: it just affects the frequency. To make it broadband, you need to vary the depth of the cavity behind it, or vary the width of the slot, or vary the thickness of the slat. That said, it all depends on how you are tuning the wall: If the open area is a small percentage of the total area (less than a couple of %), then the slots act more individually, each one tuned to its own frequency. If the open area is a fairly large percentage (say 5% or more), then the wall tends to act more like a single broadband unit, rather than a set of individual resonators.

However, people keep telling here, that it is too hard to really hit specific frequencies with a resonator.

Right! Those people are very correct. Modes are very narrow, and hitting them exactly is very herd to do: You need very high precision workmanship, extreme accuracy, a way to test that you really did tune it right, and a way to change the tuning easily if you didn't. Now, add to that the fact that it is pretty darn hard to get accurate acoustic measurements of the exact modal frequencies anyway, and you start to see that it's not very probable that you'll be successful.

The only way to make it more broadband would be the amount of insulation (the Q), right?

The amount of damping affect both effectiveness and also bandwidth. It's a tradeoff: more insulation inside the cavity means more effect, but it also de-tunes the sharpness of resonator, making it more broadband and less effective at any one frequency... contradiction, I know, but that's the way it is.

But on the other hand, if you follow the formula, you cannot really reach deep with a broadband slot resonator, except if you build a very deep one, which I do not have the space for. You kind of have to make pretty narrow slots to effect frequencies under 200Hz.

Yup. And therein lies the rub, as the saying goes....

Would a slot resonator like that work, if those frequencies have peaks at the positions of the according slats?

You are talking about room modes, so the frequencies will be the same anywhere in the room. However, the intensity of the problem will change from place to place. At some locations, you'll hear one mode way louder than the others, while a few inches away (or a few feet away) that one won't be much of an issue, but others will be problematic. It's not that the frequency changed: it's that you moved into and out of the nulls and peaks for different modes. If you are experiencing an issue where the frequency changes as you move around the room, then it isn't a modal problem: it is SBIR, which is something else entirely.

Or is this too hard to build and need thousands of adjustments until it is about right? In that case: How would you plan the slot resonators for my room?

Simple! Don't plan it as a bass trap! :) Instead, plan it for dealing with mid-range issues, and just do more useful conventional bass traps, such as superchunks in the corners, or hangers, or limp-membrane traps, or panel traps. Etc. Personally, I prefer broadband porous absorber bass traps with some type of reflective membrane in front, to keep the highs in the room and only treat the lows. - Stuart -

Thanks Stuart! Ok, I get it. So, where some of those slot resonators would be good, is for the mid frequencies. If you look at the pictures of my room response - which dimensions and location would you choose? If you look at page 3 of this thread, you can barely see on the edges, I have two openings of about 60 cm. I planned them for a slot resonator, but I could also put a porous absorber there. A lot of proplematic low frequencies seem to have peaks there. Other space for slots would only be the side walls next to my ears until the bass traps in the back. Would it be good to cover all this wall area with mid FQ slot resonators? And would you angle those for being more broadband? The way I understood John Sayers approach to slot resonators is, that I will get a FQ range just by using 3 different slat widths. But how I understand you, I have to angle it? Still I would have to choose a frequency area to adjust to. Which one would you choose for my room? The thing with porous absorbers is, I have used them and see your point! They are very effective. But I already have taken too much highs out, so I think I shouldn't build any more plain porous absorbers than now. What would you do, to get more highs back? Cover some areas with thin slats or boards with holes (in this case not built like a resonator, just to get some high frequencies back)? If I am wrong again: what is you analysis of my room response with the stuff I already did? Are the results as expected judging from the measures I took? What is still most the problematic and how can I tackle it? What would you do with the remaining space for treatment (side walls and the two openings in the speaker walls)?

For the areas closest to your ears, I would go only with porous absorption. Slot walls do several things at once, and you don't want that happening around your ears. By all means do slot walls on the sides behind you, but not in front of you.

And would you angle those for being more broadband? The way I understood John Sayers approach to slot resonators is, that I will get a FQ range just by using 3 different slat widths. But how I understand you, I have to angle it?

You can do it either way, or both at once! Here's how it works. A slot wall where the open area (ratio of "slot" to "slat") is less than about 10% acts like a bunch of individually tuned Helmholtz resonators, each with a fairly high Q. The lower the amount of open area, the tighter the Q gets, and the more specific the frequencies are. If you go the other way, with more open area and less slat area, once you get up around 20% open area, the wall acts pretty much like a single device that is tuned to the average frequency, and has a lower overall Q. In other words, a broadband device. However, even with a tightly tuned wall, (small open area), if you angle the slats so they are further away form the backing at one end than the other, then you are also making the device more broadband, since each point in the slot is tuned to a slightly different frequency, due to the different cavity depth behind it. So you can play around with those to get the effect you want.

Still I would have to choose a frequency area to adjust to. Which one would you choose for my room

I would first install all the bass traps and pure porous absorption, as well as the clouds and most of the furniture, before deciding on that. Once all of that is in place, do another measurement with REW to see how things are looking, then decide which frequencies (or range) needs the most attention.

But I already have taken too much highs out, so I think I shouldn't build any more plain porous absorbers than now. What would you do, to get more highs back?

Plastic! :) Cover the front face of your bass traps with thin plastic. That will reflect back some of the highs into the room, while allowing the bass to get through to the traps. You can change the thickness of the plastic as needed. You could use something very thin, like the plastic film that painters use to protect furniture while they are painting ("drop cloth"): it is tissue-thin, and only reflects back the very highest frequencies. Or you could use thicker plastic, such as the type that builders use under foundations to prevent water getting through. That reflects back most of the highs and even some of the mids. Or you could go with even thicker membranes, such as very thin plywood. There are lots of ways of preventing bass traps from "sucking out" the highs. - Stuart -

Soundman2020 wrote:

Plastic! :) Cover the front face of your bass traps with thin plastic. That will reflect back some of the highs into the room, while allowing the bass to get through to the traps. You can change the thickness of the plastic as needed. You could use something very thin, like the plastic film that painters use to protect furniture while they are painting ("drop cloth"): it is tissue-thin, and only reflects back the very highest frequencies. Or you could use thicker plastic, such as the type that builders use under foundations to prevent water getting through. That reflects back most of the highs and even some of the mids. Or you could go with even thicker membranes, such as very thin plywood. There are lots of ways of preventing bass traps from "sucking out" the highs.

Ok! Now I am about to start the last phase! So, plastic ... I have some problems with that: The biggest area of porous absorption in my control room is the ceiling, which is 10cm of stone wool covered with cloth. If I really want to get back some of the highs, I need to cover the ceiling with a layer of plastic. Then there is some areas in the back (mainly the bass traps) that are covered with cloth, which could also be covered, but most effective should be the ceiling. I am afraid of humidity and potentially mold. So, I looked for "breathable" foil and there are some available. The other thing is the look of it. I think it will be quite difficult to attach the foil over the tissue wrinkle-free and even if that works, I don't think that will look very good ... having a room as nice as possible is also something I want to achieve. Lately, I talked to a friend, who works for a company that produces tissue for wheather tight clothes. The material is water proof but permeates air. He might be able to get me some oth this tissue, which is commercially not available. Would that be a possible sollution? What other suggestion could you give me to solve the aspect of beauty of the surface and potential humiity problem? @Stuart: When you say thin plywood: how thin would that be? Something like veneer? Is there any practical thread here, where somebody has gotten rid of the high frequency over-dampening and has some pictures how he did it? Everything I found always has to do with cloth ... never seen any thread with absorbers with a plastic cover! At least not as the top layer.

never seen any thread with absorbers with a plastic cover! At least not as the top layer.

That's because the plastic never is the top layer!!! The plastic is not visible. You put it over the insulation, BEFORE you put the cloth on the front. So you never see the plastic. If you are concerned about humidity being trapped in the insulation, that should not be an issue. You can leave small holes in the plastic, or better still, cut holes in the sides of the frames that hold the insulation in place. You don't need a lot of hole area to allow the insulation to "breathe". For the bass traps at the rear, cut the plastic into broad strips and leave narrow spaces between the strips so it can breathe.

When you say thin plywood: how thin would that be? Something like veneer?

Correct. Just a couple of mm thick, maximum.

Is there any practical thread here, where somebody has gotten rid of the high frequency over-dampening and has some pictures how he did it?

I have done that for one of my customers in Canada, but unfortunately I can't post the photos as he hasn't given me permission to do so. But it works! What I did there was to design some geometric "pyramid" shaped diffusers, which he built just in front of the insulation in the bass traps, behind the cloth. So you can't see them in the room. I think we used something like 2mm veneer, if I recall correctly, glued and sealed along the edges. It worked very well to get some life back into the room. If you have worries that your room is suffering from this problem, then do a REW test and post the MDAT file here, so we can analyze it. - Stuart -

Alright! :-D I was already flabbergasted, you would advice me to put painters foil over my clothing! :-) I just thought, the cloth themselves where enough to take away some high frequencies and would need to be covered wirth plastic! Then, this is very easy! Thank you, I will try that out!

So, here I am back again ... :-)

gullfo wrote:

overall that is looking pretty decent. the deep null may be due to the ceiling/floor and/or some combination of that and side walls. so getting the clouds into place and the side walls done should that that down quite a bit as well.

Recently, I had time to build some more absorbers and since the reflections of the surround monitors to the left and right were the biggest problem from the already "decent" room, I decided to build some simple porous absorbers to the side walls: I used two kinds of Rockwool for these: Termarock 50 (rigid, similar to 703) and Sonorock (soft, similar to pink fluffy). The the angled compartment right to the speaker is 12 cm deep (40 cm open in the bottom for the bass traps in the back to be effective, backed by 2cm thick mdf board. It is filled with 10cm of Termarock 50. Then there are two absorbers, each 65cm wide. They go from floor to ceiling and are filled with 10cm of Termarock 50 (to the wall) followed by 10cm of Sonorock. They have 3-4cm distence to the wall behind. The back is held by thick plastic foil, the front is covered by very thin foil (thicker one could be applyed later, if needed). The 45 cm in the corner are just to have a smooth transition to the corners it is just 5cm of termarock 50 backed with more where there was space. Before finishing those with textile, I did measurements, of course: https://www.dropbox.com/s/8z6s2maanzthz ... .mdat?dl=0 - "Stereo no Treatment" was the room with just 10cm of termarock 50 on the ceiling and the soffit construction done, but no further absoption - "before side abs" was the room after all the basstrapping was done (see earlier in this thread) - "with side absorbers" is obviously the side absorbers added, as I just described them The RT60 is now down to about 100 milliseconds +/-10 except for around 300Hz, 800 Hz and under 100Hz (which I guess would be nearly impossible to control further for a room of that size). SPL-wise, there is no significant improvement for the stereo speakers, as I see it. To my suprise, the narrow hole at around 130 Hz got even worse! Have you got any idea, why it could get any worse by adding absorbtion? "10cm SonoR in shelf" was a test I did with 10cm of sonorock in the shelf behind the listening position. THis kind of improves it a little bit, but is still worse than without the side absorbers. Any ideas how to solve this are welcome! The other thing I wonder is what RT60 to target. After all the Absorbtion it seems all to tend towards 100 milliseconds. Isn't that a little bit to much on the dry side? Even for a small room like this? Of course, I will add more foil with different strength to regain some of the high reflection! But I could also add laminate boards to some parts of the basstraps and absorbers, wherever there is no early reclection happening. Kind of like a slat reonator, but with more than 20% open, so no helmholtz, just to get some more life into the room. Maybe, I could lift up the RT60 to 200 - 300 milliseconds this way. What would you recommend? Or is that just a matter of taste? Before I tackle that, I would rather see about a cloud above the listening position and the Absorber behind me (in the shelf). The biggest problem seems to be the 130 Hz, still. What else could I do about it?

The other thing I wonder is what RT60 to target. After all the Absorbtion it seems all to tend towards 100 milliseconds. Isn't that a little bit to much on the dry side? Even for a small room like this?

Yes it is. It's too low for that size room. There's too much absorption overall, so you'll need to use thicker foils, or maybe make some parts into slot walls (taking off the plastic in those areas first, of course!).

Maybe, I could lift up the RT60 to 200 - 300 milliseconds this way. What would you recommend? Or is that just a matter of taste?

It's a matter of room volume, and purpose. For that size room, for a control room, the decay time should be around 180-220ms, rising to maybe 400 ms below 60 Hz, and dropping to maybe 130 ms above 4kHz. That's an average of the specs by ITU, EBU, and IEC. The ITU version of the equation is: Tm = 0.25 (Vr / V0)^^1/3 where: Vr : volume of room V0 : reference volume of 100 m3. So for your room: Tm = 0.25 x (43/100) ^^ 0.3333 Tm = 0.188 seconds Tm = 188 ms

Before I tackle that, I would rather see about a cloud above the listening position and the Absorber behind me (in the shelf).

Yes! You need those, for sure.

The biggest problem seems to be the 130 Hz, still. What else could I do about it?

It seems to be your 4.0.0 axial mode, which is a bit strange since the other lengthwise axials dont show up very much, but if it is your 4.0.0 the that deep, thick absorption in the rear wall should help: You should see a significant change there. Do that first, plus the cloud (make it hard-backed), then do another REW test and see how it is working out. - Stuart -

Soundman2020 wrote:

Tm = 0.25 (Vr / V0)^^1/3 (...) Tm = 188 ms

Thank you! That is exactly what I have been looking for. And it matches what I am hearing: The bass is a lot more defined with all the absoption in place, but it is quite dry.

Yes it is. It's too low for that size room. There's too much absorption overall, so you'll need to use thicker foils, or maybe make some parts into slot walls (taking off the plastic in those areas first, of course!).

Since putting foil befor the absorbers is cheap and easy to try, I did that with the corner traps and some of the other deep absorbers that are primarily for low absorption. I could significantly raise the RT60 between 1 and 10 kHz, but it didn't affect much below that and I came to the conclusion that I need something with more mass. I thoght about: a) Doing slats in places, where there are no direct reflexions happening, maybe using a sequence to get a little diffusion. The slats could be integrated into the frames I already built for the textile. Kind of like a scatter plate in the GIK-products. b)Perforated Panels: I figured it would be a lot easier to cut 5mm boards, fix them to the textile frames and perforate them in a meaningful way according to Chris Whealy's perforated panel spread sheet (I could get much better results in theory, than with the slats). However, I am aware that both the slat wall and the perforate panel spreadsheet are for sealed absorbers and in my case it is just for covering 20 - 50 cm deep porous absorbers, give back some mid and high frequencies and stay broadband. c) Using thicker foil: I used the thickest foil I had left, which is about 0.2 mm thick. But of course there are other products, that are thicker, up to 2mm at my home depot. I just don't want to buy a lot of it, as it will not reach down to 100 Hz and I need to raise the decay in those areas, too, in order to get a flat RT 60 of about 180ms from 60 to 4000Hz. I think, I partly answered half of my question myself: It is certainly a combination of hard surfaces, where they are not bad for reflexions, and foil. But I can't decide, whether pursuing the idea with the perforated panels is as valid as the scatter plates ...

Yes! You need those, for sure.

Definitively the absorption behind me, I agree! But I already got 10cm of (plain) insulation on the whole ceiling, which is only 2,27 high, remember? So, I would like to avoid putting an additional ceiling cloud there. But I will do it of course, if it can significantly improve the remaining problematic frequencies, like the hole at 130Hz. The most I can do is to put additional 10 cms of light insulation there, which could help with those 130 Hz, if it is related to any horizontal problem (like between desk and ceiling). It is unfortunately not as easy to experimentally fix it to the ceiling. But I will do that of course! :-)