Acoustic improvement input for CR (Stockholm Sweden)

[Heavymaker]

Hi all! I'd appreciate some input on improving the acoustics on our control room as well as our live room. I'll start with the CR here. We initially built the entire studio with the help from an acoustician to be. He did his final report on the project and we built it from his specifications. However we find that it needs more work (as you will clearly see in the graphs). I'll post the master file from the sweetspot too so that you can fiddle around with it for yourself.

In the CR the two corners in the front are filled with fiberglass 75 cm each (measured diagonally). The front wall between them is covered with 10 cm of glassfiber and the entire front end of the room is covered with pleated cloth (se picture). We are aiming for LEDE. The back wall is covered with polys in a row (see sketch up outtake). There are no built in speakers though. We haven't bought and built that in yet. :: The back wall is made of gypsum plasterboards (2x13mm, fiberglass + air gap (8 mm) + 2x13mm plasterboard). Behind that there is a small corridor where the wall is made of concrete. I suspect that some of the lower bass is reflecting that wall rather than the polys (of course) and gypsum... (The ceiling is to 90% covered with absorbing ceiling tiles 8 cm from the ceiling c:a 4cm thick.)

Here is a couple of questions on that - if we are to treat the low bass problem - is it possible to do that "in the room" or do we have to do that in the corridor since the low bass is cutting through through the gypsum? How deep does the corner absorber have to be to make a difference at the 40 and 60 Hz RT problem? (see waterfall). It seems like we have to build ourselves in to a telephone booth to hit 0,3-0,4 RT-60...

I'll post and overlay too. REW complained about the measurement though. "The signal is not where it's supposed to be". Maybe I did something wrong and you are able to see some obvious misstake here. Anyway it seems like the peaks on 40/60 Hz are visible on both waterfall and SPL & phase are a problem. Any input on how to treat that - Helmholtz? Corner absorber? panel absorber? We have 4 panel absorbers specified by the acoustician too - design to hit 90 Hz. It seems that they are active a bit lower though.

The measurement is made with L+R speaker and a sub too. 1/48 smoothing. At sweetspot and overlays at different random positions.

Room dimensions 4mx(W) 5,80(L),x 2,22m (H) plus a non-parallell wall for half of the room (seen in one pic.) making the room c:a 40 cm wider in the front (it starts tilting halfway through the room). We´re opting for improvement. Neighbors are no problem (parking garage). And now question really:

1. How do we best "trap" the bass and where?
2. Any other input when you have seen the acoustic measurements?

I'm sorry but I had a hard time making my pics, and measurement data availible in this thread. I've really tried to read all the post before posting, but I haven's seen anything regarding how to post an .mdat (I've only read that you work with REW as one of your two softwares) I therefore post a link to GS where I was able to post them. I'm so sorry about that! post with relevant uploads here:

https://www.gearslutz.com/board/studio- ... ut-cr.html

Any input on improvement would be greatly appreciated!!

/Fredrik

[Soundman2020]

Hi there Fredrik, and Welcome to the forum!

We initially built the entire studio with the help from an acoustician to be. He did his final report on the project and we built it from his specifications. However we find that it needs more work (as you will clearly see in the graphs).

I'm glad you said "acoustician to be", because he certainly isn't one yet! As you already found out. There's a lot of things wrong with that room, but fortunately most of them can be fixed.

The only question is, are you prepared to do what is needed to fix the room? Unfortunately, it is not as simple as just putting some treatment panels on the walls. The room is way too bad for that. It is a mess, and it needs some big changes to make it usable... It was not designed correctly (you already know that!!), but it is possible to fix it.

(as you will clearly see in the graphs)

Well, the graphs show a very small part of the problem, and even then they don't show much that is useful: You need to look at all of the data in the MDAT file, and you need to analyze it carefully, to see what the issue are. I found the file you posted over at GS, and it helps a bit, but there's clearly a problem with how the measurements were done to make that file. The tests need to be repeated, properly. More about that below.....

In the CR the two corners in the front are filled with fiberglass 75 cm each (measured diagonally).

What type of fiberglass? What density? What brand? Do those traps run from floor to ceiling? Are they completely filled with fiberglass, or is it only across the front? How thick is it? Are the framed? If so, wood or metal? Are there any slots of holes in the frames? Is the insulation wrapped in plastic, or not? If so, how thick is the plastic, and what type?

The graphs seem to show a major lack of bass trapping in the room, so I suspect something wrong with the way the traps were designed, or built, or positioned.

The front wall between them is covered with 10 cm of glassfiber and the entire front end of the room is covered with pleated cloth (se picture).

The cloth is not doing much for you, and is not needed. It looks nice, yes, but you could take it out if you wanted. It's probably not doing any harm either, except that it is taking up space where your speakers should be. Part of the problem with the room response is way the speakers are set up, and perhaps the speakers themselves. So you could keep the cloth, probably, but not where it is right now.

We are aiming for LEDE.

LLEDE Really LEDE is a very old design concept dating from the 1970's! It is no longer used in modern studios, for many reasons. It has been abandoned, and replaced with better concepts. Among those reasons: A true LEDE rooms is very tiring on the ears, and people get "fatigued" after long sessions. Or even short sessions at high levels. It also isn't very accurate, and it modifies the direct sound field, even though it is not supposed to do that. It also does not provide a suitable neutral and diffuse sound field after the direct sound, that decays smoothly. The original idea was good, but it turned out to not be practical. Studios today are no longer designed using LEDE. They are designed using the more modern extensions and modifications of LEDE, such as RFZ, NER and CID: Theose are are much better concepts. Your room needs to be modified to fit one of those. Professional studio designers do not use the original LEDE concept any more these days for control room design. It just didn't work very well. I'm surprised that someone would even try to do that today.

The back wall is covered with polys in a row (see sketch up outtake).

That's a mistake too! Even for a true LEDE design, that's not the correct way to treat the rear wall. Rear wall diffusion, yes, but not just three very large poly's! It needs controlled, tuned diffusion, commonly done with Schroeder, QRD, Skyline, or something similar.

I found the single SKP image from SketchUp over at GS, and it's clear that the polys on the rear wall are way wrong for that room. That needs to be fixed.

However, that image does not match the photos at all, so I'm wondering if that is even the same room!? Please post the real SKP file that shows the actual room, exactly as it is right now, and also post some photos of the rest of the room, especially the rear and side walls, as well as the floor.

The back wall is made of gypsum plasterboards (2x13mm, fiberglass + air gap (8 mm) + 2x13mm plasterboard).

You only have 8mm between the two leaves in your back wall???? Then you have a big problem! That wall will need re-building. The gap between the two leaves needs to be at least 100mm, and 200mm would be better. And it needs to be filled mostly with good quality acoustically useful insulation, such as 30 kg/m3 fiberglass, or 50kg/m3 mineral wool. With only 8mm gap in there, you have a major resonant cavity that is not isolating, and is also messing up the room response. On top of that, you don't have any absorption at all on your rear wall, so it is mostly reflective across a broad range of frequencies... not a good situation... That's a big part of your problems.

I suspect that some of the lower bass is reflecting that wall rather than the polys

It certainly is reflecting off both the wall and the polys, but not just the bass. The mids and highs see that entire rear wall as a huge bumpy curved mirror... It's a big disaster.

The rear wall should be absorptive, not reflective. If the room is big enough, then it could be both absorptive and diffusive, but never just reflective. Big mistake. (That isn't even part of LEDE design...)

With modern RFZ design, the basic concept is to shape the room such that ONLY the direct sound from the speakers reaches the ears of the engineer, and there is no room sound at all arriving at his ears until at least 20 ms after the direct sound gets there. And when the room sound does arrive, it is diffuse, neutral, and at least 20 dB quieter than the direct sound. Your room is NOT doing any of that. It is coloring the sound greatly, and reflecting it, and not controlling it very well at all. And it is not neutral.

(The ceiling is to 90% covered with absorbing ceiling tiles 8 cm from the ceiling c:a 4cm thick.)

That needs to be fixed as well. That's another major problem: You need a cloud up there, partly absorptive, and probably it will need to be a hard-backed cloud placed at an angle, to help fix the modal issues. What you have up there is a big problem. It's not just that it isn't any use, but rather that is causing some of the problems. It has to go.

Here is a couple of questions on that - if we are to treat the low bass problem - is it possible to do that "in the room" or do we have to do that in the corridor since the low bass is cutting through through the gypsum?

ALL of the treatment goes inside the room! Treatment outside of the room does nothing useful at all for the room! After the sound has left the room, it isn't going to come back inside again, so any "treatment" that you put outside the room is not going to have any effect on what happens inside the room, acoustically.

The internal hard, solid, rigid surfaces of the room define the acoustic boundary. All of your treatment goes within that. Any sound that gets through that isolation wall and leaves the room, well... it has left the room! It has GONE! And it won't come back. Unless there's an even more massive barrier beyond that might reflect some of the bass energy back again, then there's no point to putting treatment outside the room. And if that really is the case (if you really do have a more massive barrier outside of the room isolation wall), then that just means that the room was designed and built wrong. In which case, that needs to be fixed as well...

There are two very different aspects to acoustics: isolation, and treatment. Isolation is what stops sound from leaving the room, and treatment is what makes the room sound good again, on the inside. They are very, very different things. Isolation blocks sound so it cannot get out. Treatment is inside the room, and deals with the sound that didn't get out, as it bounces around inside the room, causing problems...

How deep does the corner absorber have to be to make a difference at the 40 and 60 Hz RT problem? (see waterfall). It seems like we have to build ourselves in to a telephone booth to hit 0,3-0,4 RT-60...

Not really. Absorption is still effective at 7% of the wavelength that is its treating, for normally incident sound, and 3.5% for randomly incident sound. That's why even relatively thin absorption can treat very deep frequencies, provided that it is the right type of absorption, in the right location, and mounted correctly...

What are the real acoustic dimensions of the room? What are the dimensions, locations, and materials of the existing treatment? How are they mounted?

However, the bad news here is that you cannot fix your room by just putting some treatment in it. The problems are too complex and severe for that. The good news is that it CAN be fixed! Just not with a couple of simple panels. It needs a complete re-resign. (I hope you didn't spend a lot of money on the original design by the "acoustician to be", because the design you got is not worth any money at all, really. It looks nice visually, but is wrong acoustically.)

Also, I suspect that the REW test was not done correctly: the data shows that you did the test at an average level of around 135 dB, which would make you about the loudest studio on the planet! If that really was the level you did the test at, then you won't need a studio at all, since you are now totally deaf: your ear drums were probably ruptured by the testing.... So it seems that you didn't calibrate REW correctly.

The impulse response graph also shows major comb filtering, which doesn't seem likely at all from that room so I think you messed up with the measurement, and the waterfall plots show some pretty major issues going on all over the place, not just at 38.3 Hz and 60.9 Hz. I think that the test was not done correctly at all.

So please repeat the REW testing, but this time calibrate REW properly before you do the test, using an accurate hand-held sound level meter for the calibration, and do all your testing at 85 dBC for each individual speaker, which should then show about 94 dBC for all three speakers at once. Also, you need to do four tests in the same MDAT file: One for just the sub by itself, one for just the left speaker, one for just the right speaker, and one for all speakers at once. Do not change anything at all between tests! Don't adjust anything, and don't even move the chair! Just turn off the other speakers, and turn on the speaker being tested.

REW complained about the measurement though. "The signal is not where it's supposed to be"

Exactly! You have to CALIBRATE it first. Follow the calibration procedure in the manual, and make sure that the signal is not clipping anywhere, and that there are no other sound sources in the room while you test. In fact, it is better to use the "Start delay" function, to give yourself time to get out of the room and close the door, plus another few seconds, before the actual test starts. Make sure the room is dead silent before you test. No fans, computers, people, music, or any other noises. And make sure your mic is carefully fixed in the correct position. And make sure it really is an acoustic test mic! You can't use an ordinary mic.... it must be a proper test mic. (I use a Presonus PRM1 most of the time, but there are several good measurement mics from other manufacturers that are not expensive. And they can also be used as instrument mics, too! So it won't be wasted money.)

Any input on how to treat that - Helmholtz? Corner absorber? panel absorber? We have 4 panel absorbers specified by the acoustician too - design to hit 90 Hz. It seems that they are active a bit lower though.

No, not Helmholtz resonators: They are VERY hard to design and even harder to tune. Much, MUCH harder than the "expert" websites and books tell you about. Room modes are very, very narrow band issues, and trying to tune a resonator to exactly that frequency is really difficult. They also have to be BIG! Very big. Forget that. And once again, your room cannot be fixed with just a couple of treatment devices. It needs to be modified in order to be usable.

Absorption can do the job. But resonators and diffusers cannot! Not at low frequencies. Part of the modification that you need to do, is to get rid of those useless poly-cylindrical diffusers on the back wall, and replace them with thick, suitable absorption. Want proof? Just stacking up rolls of fiberglass insulation along the back wall is going to make a huge difference, so if you want to try that to prove that this is the right way to proceed, then please do! That's not the FINAL solution, of course, but it will at least get you comfortable with the concept that the room is all wrong at present, and that absorption on the rear wall is a big part of the solution. Just go to your local building supply store, and buy several rolls of ordinary fiberglass insulation (it is usually pink fluffy stuff). The density needs to be about 30 kg/m3, or maybe a bit lower. For the test, leave it wrapped in the plastic, and just stack up several rolls across the back of the room, floor to ceiling, then run a REW test (and a listening test). After you have done that and understand that this is the right way to go, then I'll show you what you need to do with that stuff, as the final solution. The above test won't fix the problems, of course, but it will make enough of a difference to show you that the basic concept is correct. But you can use what you buy for the test, to actually treat the room later. It won't be wasted money.

The measurement is made with L+R speaker and a sub too.

What sub (what brand and model)? What speakers (what brand and model)? Where is the sub located (position in room)? How are the controls on the speakers and sub adjusted (cross-over frequency, bass roll-off, high lift, etc.)? All of that is very important.

At sweetspot and overlays at different random positions.

The random positions are not needed. don't waste your time with those. The only REW test position that is really important, is the exact position where the engineer's head is located while he is mixing. Set up your acoustic measurement mic at exactly that position, at the exact height of his ears above the floor, with the mic angled upwards at an angle of about 45° and pointing exactly straight forwards. Carefully measure the precise location of the tip of the mic in the room (with a good tape measure). You need to know that precise position, so you can get the mic back to the exact same point for all future measurements. You need to be very accurate here: the mic must ALWAYS be in that precise location, accurate to within a few mm, for all tests in the future. If not, then you cannot compare results.

It can be useful to have some tests done in other positions too, and I might ask you to do that, but the REALLY important position that really matters, is where the engineer's head is. Do all your initial tests there.

Room dimensions 4mx(W) 5,80(L),x 2,22m (H)

That makes no sense at all! You clearly have some major modal issues, which are very obvious in the REW test, yet they do not correspond to any of those room dimensions! Not even close. So either you got the dimensions wrong (maybe you did not measure them to the actual acoustic boundaries of the room?), or you have some very big, very resonant objects in the room that are totally messing up the response. Or you have some other, very strange acoustic problem in the room....

What is the floor made of? Is it floated or solid?

For those dimensions, you should have modal issues at 29.7 Hz, 43.1 Hz, and 78.3 Hz, (plus others) but there is ZERO evidence of problems at those frequencies in the REW data. Nothing at all. That data does NOT come from a room with those dimensions. It seems to come from a room where one of the dimensions is 4.55m, and another is either 5.66m or 2.33m (hard to say which). It's also hard to figure out what the third dimension is, but there's a slight hint it might be about 8.4m, or maybe 4.2m. However, most certainly that data did not come from a room that has hard boundary dimensions of 5.8m x 4m x 2.2m. That much is clear.

plus a non-parallell wall for half of the room (seen in one pic.) making the room c:a 40 cm wider in the front (it starts tilting halfway through the room)

So the room is not symmetrical? In other words, the left half is not a "mirror image" of the right half? If that's the case, then that needs to be fixed too. Symmetry is critical for control rooms... I'll be able to see that on the REW data for the individual speakers.

1. How do we best "trap" the bass and where?
2. Any other input when you have seen the acoustic measurements?

As I mentioned in a few places above, the good news is that your room can be fixed. The bad news is that the "fix" is not simple. It is not just a couple of panels that you can hang to fix it. It's a lot more complex than that.

Any input on improvement would be greatly appreciated!!

First you need to calibrate REW correctly, then repeat the measurements in the way I outlined above, then you need to provide answers to all of the questions above. Post all of that here, along with some more photos of the room, especially the back wall, and I'll take a look at it (What would REALLY help, is to have an accurate SketchUp model of the room, showing exactly how it is right now, with all the details.)

I can't promise that I'll be able to respond fast or in great detail (my paying customers get first priority) but when I get time I'll take a look at all your data, and make some suggestions on how I would go about fixing your room.

Then after we fix your control room, we can take a look at the live room and fix that too!


- Stuart -