John Sayers' Recording Studio Design Forum Archive

to be effective for a specific fq a porous absorber has to have a quarter of its wavelength distance to the hard surface behind it. If I would want to decrease the most problematic area of modal resonance of about 50Hz, the distance to the wall would therefore have to be around 170cm!!!

Well, that's only partly true, and only for "normally incident" sound, which means sound waves that strike the front surface of the absorber head-on, at an angle of 90°. For waves that hit it at any other angle, then the wave will travel through a lot more insulation, and a lot more air, before it reaches the wall behind. So in effect, it "sees" the distance as being much greater, and therefore the absorber works down to much lower frequencies than you'd expect for waves that are not "normally incident". A wave hitting at an angle of 30°, for example, "sees" a path that is twice as long, so it would get the same effect as a wave hitting head on, but down to half the frequency. When you consider that most waves in a room are not arriving "normal" to the surface, you can understand that the effective frequency is a lot lower than you'd think.

Also, the effect doesn't suddenly drop to zero for frequencies lower than 1/4 wave length: Even for normally incident sound it's a gradual roll-off, and absorption is still very effective down to much lower frequencies. Yes, it is optimal for frequencies at the 1/4 wave distance, but for frequencies lower than that (longer wavelengths) you are still getting very useful absorption. Check the absorption coefficients for 4" OC-703 mounted flat on the wall, for example, and you might be surprised....

There's also the additional point that you don't have to have extremely thick insulation to get useful absorption: In fact, even at 7% of the wavelength you are still getting decent absorption.

That's why porous absorbers in general are recommended for bass trapping: They can be effective down to very low frequencies. And that's also why superchunks are an excellent choice: the front face is angled, so the apparent depth is greater due to most sound arriving at non-normal incidence; there's a lot of depth (a wave arriving at a glancing angle sees upwards of 60cm of absorption), and the corner location makes them between two and four times more effective than just plain absorption over a flat wall.

I guess you imply that the low frequencies around 50Hz will not be affected by that and that I will probably never loose any of these? What would be the way to get rid of them?

The superchunks in the corners will take care of most of that, and the absorption on the back wall will do the rest.

Here's a couple of graphs from a room I treated last year, showing the difference in response before and after adding a 4" panel of 703 to the rear wall, over a 30 cm gap. This room already had superchunks in the rear corners, but still had some issues with the length mode.

Before 703: And after installing the 703: That's just how the frequency response changed. The time-domain response tells you a lot more about how effective this is.

Water fall plot, before 703: And afterwards: Note particularly what happened to the modes at just under 80 Hz, and just under 40 Hz. They smoothed out quite neatly. And that was just from a single panel of 4" 703 a few inches in front of the rear wall. (Later treatment hit some of the other issues you can see in the graphs, but that was a very clear example of how effective absorption can be at very low frequencies.)

Helmholtz resonator? I got quite a lot of people telling me not to go into building these, as they are very difficult to tune ...

Yup. Very true. Don't go there.... Modes are very tight, very narrow band resonance. Hard to hit with a fixed tuning device. If the tuning can be varied easily, then you stand a better chance, but you still need a huge air volume inside in order for the resonator to be effective. Absorption is much easier.

VPR devices do work well, but they are fairly new and the math isn't well understood yet. Or at least, there isn't a lot of verifiable independent research out there about how they work, except for some done in Germany, I think. Unfortunately, as you mentioned, the research institute patented the device and clammed up on their research, releasing only bits here and there, and since it is patented, nobody else is bothering to build them and test them in different configurations, and in independent acoustic labs. Until that happens, I think I won't be buying any. Or building them for my clients either!

Porous absorbers, on the other hand, are a well-known, well-understood device, tested endlessly all over the world, with widely available results. There's no secrets as to how they work....



- Stuart -

That is good to know, Stuart. Thanks!
Unfortunately, the German forum with the VPRs is closed, because the admin has somehow disappeared. But I managed to email one of the VPR builders asking, if they were worth the effort. He said, only for locations, where you can only give up small space. If had the opportunity to do it with larger porous absorbers, he would go that route, so I will forget about those VPRs for now, although I found them really interesting!
So, going porous all the way, I still have a few unanswered questions:
1) Since the quarter of the wavelength is only half of the truth, what is the way or formula to determine what kind of depth of insulation to choose to reach my problematic frequencies by what degree? Or is it simply as thick as I can afford (space and price).
2) Most people seem to recommend lighter (and cheaper) materials, like fluffy glass wool or sonorock for thicker insulation. But I also find people who would still recommend Termarock 50 or 702 for deep corner traps. Is there any significant advantage in using the more expensive materials for this? Some people even claim those might deflect more at higher thickness. I already understand I would have to prevent sacking.
3) What about layering different materials from light to heavier, like 40 cm of sonorock followed by 10cm of Termarock 50? Does that show any advantage?
4) Do hangars make any sense at the size I described earlier? Or would you rather go with all porous absorption? I was thinking about approximately a 2m x 1.2m x 0.5m compartment on bottom level.
5) What about leaving some airspace behind the insulation vs. filling the entire space. I understand, I could reach deeper frequencies by using the same amount of material, but as in 1) I am clueless what would be a good ratio of not loosing to much room vs. efficiency of LF absorption.
6) What about some shelve followed by absorption?

By now, I have two types of basic ideas how I could design the entire back wall:
A: The entire back wall with a framework filled with insulation covered with molton.
Variables:
- distance to the wall, thickness and airspace
- probably triangular corner traps of greater depth
- bass hangar

B: Structured back wall with a unified depth and storage integrated.
- corners filled entirely with squared columns of rockwool (about 50cm x depth)
- bass hangars at bottom level in the center (or optionally just insulation there as well)
- top level just insulation with full depth
- center piece the storage shelf
For the shelf I have two ideas. It could be 10-20cm of insulation covered by cloth, followed by the shelves, but not the entire area (about 2m x 1.2m). Where there are no shelves I could go all the way with insulation.
Alternatively, I could imagine using only 10cm of insulation, followed by cloth and shelf. In this case I thought about putting absorbing doors in front of the shelves, like a wooden frame filled with 10cm (or more) basotect (or other foam). In theory, this should work nicely, as there is a great depth of insulation in the corners and at bottom and top level, being effective with the low frequencies. For the centerpiece there should still be some useful absorption pretty low down, as the distance of the absorbing doors to the wall will be fairly high (50-60cm approximately). I am not sure, however, if this works as I expect it to and about how to choose the parameters here (thickness of absorbing doors, distance to the wall, mineral wool behind the shelf or not, etc).
Although I would probably loose a lot more depth that way: a little storage compartment would totally make that worth it, while the space gained with A would not be as valuable from a practical point of view. Of course I see the disadvantage of B, which probably is that it is not as predictable as A.
Very much looking forward to your opinion!

To make the over all picture easier to understand, here some designs.
This is approximitely how I would approach design A without bass hangars:

frame to hold textiles fill with mineral wool cloth front I went for a depth of 40cm of mineral wool. This of course could be decreased, but I really want to go for these low frequency modes, I still cannot figure out the depth required to hit them! I might also do some other modifications like improving the ability to open the door entirely and adding some absorption to the door, but this is just a crude sketch of how it might be done. Not to forget: At the ceiling there already is 10cm of termarock 50.
I like about this design, that it is acoustically probably the most efficient to dampen the modes. I don't like, that all my equipment will have to be in the recording room. So, in the next reply I will post my design B soon to come.

And here goes version B:

A similar frame, but in with a depth of 50cm. I already improved the space for door opening in this version. Adding different layers of Rockwool, 50 or 10cm depth. The bass hangars are optional and could just as well be regular mineral wool. Finally, the 30cm deep shelf (backed by 10cm of insulataion and textile). Those are covered by two paneltrap doors (I thought of a light frame and basotect covered by textile), which can be opened to access the self. All visible parts will be coverd with textile before putting the self in.

Well, which design is better suited for absorption. I get that B is less predictable in its result, but for everything I know about absorption, B should absorb even more low end, since it is 10cm deeper over all. Did I overlook anything?

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.