advice needed in making a large space sound more neutral

[attilafaravelli]

First time posting on the forum.. after months of following it passionately.. please be indulgent:)

I'm part of a non-profit cultural organisation named Frequente http://thequietus.com/articles/20896-ar ... -interview. Our collective is dedicated to sound art and experimental music, we are based in Milano (Italy) and since 2 years, thanks to a benevolent landlord we signed a free loan agreement which allows us to use a spacious venue as our base. In this space we have our office and in the main big room we organise concerts as well as recording sessions.
http://www.standardstudio.it
It never gets used as a commercial recording studio but we use it to record our own music and to guest recording sessions by colleagues who come here for 'residency' periods during which to develop new ideas and record.

rilievo dettagliato pt.pdf

For some reasons not worth discussing here sound isolation has never been an issue for us.

Acoustic response-wise, when I first analysed the room I found it relatively smooth in the deep lows, I had read about this in the books:) but I only knew my own small studio with its typical uneven response.. I felt like the first intervention, as a start, could be to just scatter the sound to get a more diffuse field and to lower a bit the reverb time. Phase 1 of our 'acoustical treatment' has been to just get some wood 'leftovers' from veneer production; we found a factory which kind of gives away pieces of wood, no one wants them, in fact they are left to bend to rather crazy shapes on huge shelves under canopies for years. We got 600 kgs of it for 500 Euros:) Walnut and Eucaliptus.. Of course we were interested in the look of it. Also we wanted to differentiate the space from the typical white box look of an art gallery. I'd say that when there's people inside the space, during the events, the sound is quite acceptable. Not neutral at all, but ok. When it's empty it's of course another story...

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So this brings me to my questions:

right now we are at the phase 2 of our treatment (scattering-diffusion usually comes afterwards absorption but well..uhm.. ) and some professional advice would be very much needed and appreciated.

My very limited education on the topic, like most non professional designers I guess, is focused on treating small spaces (Rod Gervais book being an example of that). To quote Newell, "To build an acoustically large neutral room is not a particularly difficult exercise" and I can see why this is true but the point is that I wouldn't really know how to do it on a budget.
Maybe it's just NOT possible.
Also, we guest events with sometimes 200 people (very thick cheap fluffy absorption is not an option but in the wall ceiling corners and the ceiling).

As you can see, the reverb time is still waaay too much, especially in the low-mids.

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Cavo Spectrogram.jpg

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Cavo Waterfall.jpg

Shall I rather think about building some broadband slatted Helmoltz resonators (a la John Sayer) or broadband perforated panels (a la Jens Eklund) and put them in every corner? MDF is very cheap here like everywhere I guess..

or

should I hang 200 to 350 mm thick fluffy insulation open panels at a distance from the walls and in the corners, maybe covered with 80-20% reflection-absorption?

But more in general:
should I take care or not about the panels being too absorptive in the highs like one would in a smaller space?

or

should I treat the corners and the ceiling with hangers (hanged or not a la John Brandt) or maybe superchunks? But overall it seems to my not-expert eye and hears that the main issues here are not in the modal resonances region but somewhere in the low-mids.

Will reducing low-mids bring us to an unbearable long decay time in the mid-highs, highs?

I guess there's no easy answers to all that:)
But I feel confident that there's no better place than this where to ask:)

Attila

Edit: p.s. maybe 'neutral' sounds confusing in the context of this project.. I mean of course not perfectly balanced like a control room, and not even like a typical pop-rock recording studio. My aim would be to just tame a bit the low-mids and shorten by a small degree the overall reverb time to make the room presence less prominent in the recordings.

[Soundman2020]

Hi "Attila", and Welcome!

That's an interesting problem you have there!

But I feel confident that there's no better place than this where to ask:)

I must agree with that!

OK, you call it a large room, but it's not really that large acoustically. It's less than 60m2, so it would still be within the usable range for a control room, and it has modal support down to 18 Hz, which is nice, but the Schroeder frequency right now is likely around 180 Hz, since there's practically no absorption. So it's not really a large room, although it certainly isn't tiny!

First, to clarify: This room is only meant to be a rehearsal/tracking/performance room right? A live room, where musicians actually perform, practice, and are recorded? Correct?

Phase 1 of our 'acoustical treatment' has been to just get some wood 'leftovers' from the veneer production; we found a factory which kind of gives away some beautiful pieces of wood

What you have done there looks very nice, but it isn't diffusion! Acoustic diffusion refers to reflecting the sound in different directions, yes, but in a carefully controlled manner such that the reflections are spread out evenly, smoothly, and across a certain range of frequencies, ranging from the low cutoff to the high cutoff. The result is a smooth, even, balanced "dome" of diffuse sound, that has had all of the frequencies within its range scattered randomly but in a controlled manner, such that all directions get roughly the same level, phase, time, and frequency spectrum, with no lobing.

What you are doing is reflecting certain fixed frequencies in fixed patterns, with lobing, and without any changes in phase, timing, or level. You are scattering sound, yes, but not diffusing it, and not absorbing it. Diffusers also do scatter some sound, but only at the frequencies down to about an octave below the lower cutoff frequency, and even then it is reasonably smooth.

when there's people inside the space, during the events, the sound is quite acceptable. Not neutral at all, but ok. When it's empty it's of course another story.

Right! And there's a BIG clue to what your room needs!!! Because people both absorb and diffuse. The human body absorbs about 3 to 5 sabins, so if you have 100 people in that room, you have around 500 sabins of absorption in there, which is roughly equivalent to having open windows in the room walls, measuring 500 square feet (about 46 m2). So imagine how much better the sound would be if you could chop huge holes in BOTH of the long walls, measuring 6m long by 3m high, leading out to the garden, AND ALSO holes 2m long by 2.5m high in BOTH end walls... Because that's what you have when the room is half full of people. And when it is completely full, with 200 people, It's basically the same as taking away the walls entirely...

So, since you already know what the solution is, you just need to implement it another way! Since you can't kill a hundred people and hang them on the walls, you'll have to find a better way...

It turns out that the absorption spectrum of the human body rises with frequency: not much at all at low frequencies, to quite a lot at mids and highs. And from looking at your graphs, that's exactly what your room needs.

Also, people do provide scattering of sound, vaguely resembling diffusion, when they are arrayed randomly spread out, but not so diffuse (although still scattering) when they are bunched together.

right now we are at the phase 2 of our treatment (scattering-diffusion usually comes afterwards absorption but well..uhm.. )

Well, yes, but you didn't do any diffusion yet! Only scattering... So you might also need some diffusion, in addition to what I'm going to suggest below...

Maybe it's just NOT possible

Ahh, but it is possible! You already saw the solution, but didn't realize it...

Also, we guest events with sometimes 200 people

... and there's your answer!

very thick ... fluffy absorption is not an option

Ahh, OK, then you were right: treating your room IS impossible. Sorry. If you already rejected the treatment that you need, you have also rejected having the acoustics that you want. End of story.....

As you can see, the reverb time is still waaay too much, especially in the low-mids.

Yes. And the most effective treatment for that is absorption.

But I'd have to see your actual REW data file, and also to know how it was obtained, to give a more detailed answer. Please upload your MDAT file to a file sharing service, such as DropBox, then post the link here. And also explain how that data was measured: type of speaker, type of mic, location of speaker, location of mic, etc.

Also, it seems like you did not calibrate REW correctly, since your room seems to be about ten times quieter than the quietest acoustic laboratory on the planet... So please make sure that you calibrate it properly, using a suitable, calibrated hand-held sound level meter, for future tests.

Shall I rather think about building some broadband slatted Helmoltz resonators

For a small section on one or two walls, that's a possibility, but only if you already have enough absorption in the rest if the room.

or broadband perforated panels

Same thing. Slot walls and perf panel work on exactly the same principle, and if tuned the same with the same coverage, would give similar results. The slot wall would be better for diffusing, though. It does provide some diffusion.

and put them in every corner

Actually, that's the WORST place to put slot walls and perf panel devices! They should go where they are needed... on the walls.

should I hang 200 to 350 mm thick fluffy insulation open panels at a distance from the walls and in the corners

Ummmmm.... but didn't you just say that this is not an option???? Quote: "very thick cheap fluffy absorption is not an option". So now I'm confused! Is it an option, or is it not an option?

open panels at a distance from the walls and in the corners,

Open absorption panels, yes, absolutely... carefully designed and implemented. But they don't need to be at a distance from the walls. You are aiming at the mid-range here mostly, and the lower part of the highs, so that's where you need to be focused. You'd only need to space them away if you were aiming lower down the spectrum.

maybe covered with 80-20% reflection-absorption?

I'm not sure what you mean by that, but yes, your absorption will need to be partially covered with foils that selectively reflect highs and only allow mids and lows to be absorbed. There are equations and methods for figuring out the foil characteristics that you will need, and also the characteristics of the insulation you will need (density, thickness, absorption coefficients, etc).

should I take care or not about the panels being too absorptive in the highs like one would in a smaller space?

Yes, to a certain extent. As mentioned above. Choose the foil characteristics (thickness, density, coverage) to ensure that the right frequency range is reflected/absorbed, to the right extent. I treated a church like this a while back: 250 m2 floor areas, terrible response in the mid range.... two dozen panels with the correct characteristics in the correct location made the room usable and pleasant, even with no congregation present, and pretty decent sound with 150 people in it.

should I treat the corners and the ceiling with hangers ... or maybe superchunks?

Probably not necessary. If the wall panels are designed, built, and located correctly, then you should be getting enough low-end trapping as well. It might be necessary to do something in some of the vertical corners later, but I'd first get the walls and ceiling done.

it seems to my not-expert eye and hears that the main issues here are not in the modal resonances region but somewhere in the low-mids.

Possibly, but the room is too live at present to show much low-end detail, and in any case your graphs are way too low resolution to be useful. Any modal activity is probably masked within the general "mush" in the low end, and as well as hidden by the very low res graphs. As I mentioned above, I'd need the actual MDAT file, and I'd need to know how it was measured. Most people don't realize that measuring the decay times at different points in the room can give you very different results, for example...

Will reducing low-mids bring us to an unbearable long decay time in the mid-highs, highs?

No, not if you do it it EVENLY! If you just throw a few things around at random, the probably, yes, but if you have the treatment plan designed and implemented carefully, bit by bit, then it should produce more even decay times. That's not to say that they have to be absolutely even and perfectly smooth, as for a control room (since this is NOT a control room)! This is a live room, so it does need to be live, and does not need to be perfectly even: it can have it's own character, but at the same time should also be pleasant to be in, both with just a couple of people in there, or when full of people.

I guess there's no easy answers to all that

I guess that depends on whether or not you consider "absorption panels with partial foil facing, carefully designed, built and installed" to be a simple answer!"

You might also need something in the ceiling, and also some mid-range diffusion, but that's a different story...

The key here is to NOT over-do it. You only need a little less than "enough" here, since those human bodies will provide the rest when they are present. You want the room to sound "almost decent" when empty, "good" when partly full of people, and "great" when packed with an audience. If you make it sound "great" when it is empty, then it will be too dead when full. Don't over-kill it.

It would be good if you could post photos of what the room looks like for a typical event, with people and musicians in it, to see how it is laid out, and what it looks like.


- Stuart -

[attilafaravelli]

Hey Stuart,

thanks so much for your time! I was hoping for one of the clarifying answers of yours that I've been appreciating so much since I discovered this forum.
It's 2 in the morning in Italy right now, I'll reply tomorrow!

bests,

Attila

[attilafaravelli]

Hello again Stuart,
all of what you say is very informative and I'm glad that I posted our project here, and so are my associates. I guess the problem with the internet and all the available resources today is that people like me think that by reading a few books and articles + thousands of pages on the forums can get you to understand acoustics but as your answer shows, to get an idea of a project as a whole it takes a lot more than some fragmented notions:)

This room is only meant to be a rehearsal/tracking/performance room right? A live room, where musicians actually perform, practice, and are recorded? Correct?

Yes you are correct. We never use it for mixing. The music which is performed and recorded here is usually acousmatic (music that is specifically composed for presentation using speakers) or completely acoustic, or a mix of both. Some of what is proposed lacks the frontality which is typical of jazz-rock-pop gigs in the sense that we often distribute in the room smaller speakers which add to the main P.A. On some occasions the audience even move around in the room to listen from different perspectives.

What you are doing is reflecting certain fixed frequencies in fixed patterns, with lobing, and without any changes in phase, timing, or level. You are scattering sound, yes, but not diffusing it

Sure, I was inaccurate, what we got might be the opposite of a diffuse field. The reflections from the woods are very dis-uniform. The idea for this installation came by reading the Morpho butterfly wings story in Alton Everest's Master Handbook of Acoustic (Italian translation, I haven't seen this in the original ver. In the diffusors chapter Everest talks about the shimmering colours of this butterfly which in the end has brown wings (if looked at with a microscope) and this is an example of iridescence through structural coloration (not from a specific pigmentation).
Also we've been fascinated by this Eliane Radigue interview (seminal electronic composer)
https://vimeo.com/8983993
''In terms of the loudspeaker, the room, the positioning of the speakers, the way in which the sound has to fill the whole space, so that each listener, wherever they may be, feel comfortable in this musical bath... 4 loudspeakers arranged in a cross-there's no stereo. Then, according to the acoustic response of the room I can point them in different directions. Sometimes the technicians tell me, 'that's anti acoustics'... But what do I care about anti acoustics if it's what I want! Which is to avoid directionality of sound for the audience. No need to say 'seat in the centre, that's where the sound is best'. No, you can be in the smallest corner and hear the totality of what is going on. There is perhaps a slightly different story in each part of the room, but it is none the less a total story. It's like looking at the surface of a river. There's an iridescence around the reefs, but it's never completely the same, according to the way in which you look, you see the golden flashes of the sun or the depths of the water. In a swimming pool you can see the reflection of the ripples on the bottom or have a vision of the whole and let yourself be carried away by what I call 'dram gazing', or fix on a detail and make your own landscape.

When there's many people in the room I feel like the woods are more effective in creating these distorted specular reflection, probably because the room sounds less muddy..

The human body absorbs about 3 to 5 sabins, so if you have 100 people in that room, you have around 500 sabins of absorption in there, which is roughly equivalent to having open windows in the room walls, measuring 500 square feet (about 46 m2).

Wow that's very impressive. I had no idea that we absorb so much. People here sit on the floor, who knows whether a human being closer to the pressure zone absorbs more or less:) I guess that depends if we are more like resonators with our cavities and empty skulls:) or more like porous absorbers with our hairs and clothes?
Anyhow here it is how the space looks like with people. Red picture is during the concert.

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But I'd have to see your actual REW data file, and also to know how it was obtained, to give a more detailed answer. Please upload your MDAT file to a file sharing service, such as DropBox, then post the link here. And also explain how that data was measured: type of speaker, type of mic, location of speaker, location of mic, etc.

Yesterday I took many new overzealous measurements along a grid (1,5 x 1,5 m) in the room. The mic. 80 cm high (like sitting on the floor) 1.7 m (average person) and 2.5 m (AB and ORTF mains), then wall to floor and wall to ceiling corners.

https://www.dropbox.com/s/4vpwe8djrw78k ... e.zip?dl=0

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that's too much data I know, I can upload just a couple, in case. I'm just unsure about what can be useful or not.

About the setup, our main speakers are now being repaired, 2 old studio monitors from the 80's, Genelec 1024B that a client of mine gave me as a gift, long long ago. We call them the sarcophagus':)

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I wouldn't use them for mixing.. but they sound much better than a regular P.A. and they came for free.
For these measurements I had to use a P.A. system with subwoofer which is just decent, very uneven in the response but that's what we are able to afford until the vintage Genelec's come back and I fear that's gonna take a while. The sub was positioned on the floor as close as possible to the short wall on the opposite side of the door, the 2 satellites were each at 1 meter from the side walls and 40 cm from the rear wall, 2.05 m high. I used one of my 2 Sennheiser MKH 8020 omnidirectional microphones, there's no cal anywhere but it's very linear and has pretty impressive specs. I tried to be more accurate with the SPL but I'm not sure if I got it.
Before measuring I removed the few (7) fluffy fiberglass (6000 Pa.s/m2) panels you can see in the pictures from my previous post. I didn't expect it would have made a hell of a difference but it actually did..

Actually, that's the WORST place to put slot walls and perf panel devices! They should go where they are needed... on the walls.

I see, I was just thinking about the broadband corner slotted HH resonators (Alton Everest, Rod Gervais books). Do you consider it a bad idea in general or in a room like this?

Any modal activity is probably masked within the general "mush" in the low end, and as well as hidden by the very low res graphs.

That's very very interesting, I had never considered that!

No, not if you do it it EVENLY! If you just throw a few things around at random, the probably, yes, but if you have the treatment plan designed and implemented carefully, bit by bit, then it should produce more even decay times.

We'll follow for sure your suggestion and go down this route! We'll have to do it as you say, very carefully, but good to know that it's feasible without spending a fortune!

To get an even distribution of porous absorbent in the room, would it make sense, in your opinion, to use the empty spaces between the woods, or shall we be more systematic and geometrical?

Shall we build big panels or smaller ones. If the diffraction from a panel's corners makes it bigger than it is..

maybe covered with 80-20% reflection-absorption?
I'm not sure what you mean by that, but yes, your absorption will need to be partially covered with foils that selectively reflect highs and only allow mids and lows to be absorbed. There are equations and methods for figuring out the foil characteristics that you will need, and also the characteristics of the insulation you will need (density, thickness, absorption coefficients, etc).

I was referring to open panels (not sealed) filled with insulation + 5cm wide slats, 2 cm slot, 2.5 cm thick. The thin foil panel would be much easier to build, and lighter (drywall only walls here, very fragile).

The porous-multilayer absorber calculator that I use
http://www.acousticmodelling.com
cannot simulate the effect of a thin foil. Would you suggest any other tool? Glen Kuras (Gik Acoustics) suggests cardstock. Others just the fiber board's own protective foil..
Both the acoustic response of a thin foil and the specific cut off frequency that we need for the absorbers in our room are quite baffling:)

So you might also need some diffusion, in addition to what I'm going to suggest below...

I've seen people successfully making diffusors out of hard styrofoam and foam board insulation products (as far as the material is reflective at the frequencies of interest). That would be best for our fragile drywall walls. I agree that we mainly need absorption, but I'll look into that as well. I just discovered this

http://www.oliverprime.com/prd/

which allows some very complex, long sequences..
I guess the only place where to put a diffusor in our room, a place where people wouldn't end up too close to it (rendering it acoustically irrelevant) would be the ceiling..

[Soundman2020]

I guess the problem with the internet and all the available resources today is that people like me think that by reading a few books and articles + thousands of pages on the forums can get you to understand acoustics but as your answer shows, to get an idea of a project as a whole it takes a lot more than some fragmented notions:)

Well, you are actually on the right path, and you would probably get there eventually, but I have the impression that you want to get there faster, and avoid further frustration!

But you are right about the internet: there's a HUGE amount of "stuff" on it about acoustics... the problem is that a large percentage of it is garbage, from the "Oh gee, look what I did and I think it sounds cool" folks, who really don't have a clue what they are doing, and would not know one end of a measurement mic from the other! It's hard, sometimes, to figure out which stuff is garbage, and which stuff is actually true.

Also, control rooms are relatively easy to design and treat, since the goal is "neutral", and there are methods, design concepts, guidelines, rules, and techniques for achieving that. But live rooms are art: they are supposed to have "character" and "life" and "warmth" (or something else, if you wish), as desired by the owner and expected by the occupants. Live rooms are a whole lot harder to do than control rooms.

in the sense that we often distribute in the room smaller speakers which add to the main P.A. On some occasions the audience even move around in the room to listen from different perspectives.

OK, so you need life, and character, bot not too smooth and even, or else the entire room will sound the same. If you want the audience to interact with the room, then some parts of it should sound different from other parts.

Or you could even make it variable. Design variable acoustic devices that all you (or the audience!) to change how the room sounds, by opening, closing, flipping, sliding, rotating, rocking, or otherwise moving parts of acoustic devices. That's probably a lot more expensive than what you want, it seems, but it is possible. It would need careful, intelligent design, which would not be cheap itself, then careful construction, as you don't want people breaking your devices, or your walls!

I was inaccurate, what we got might be the opposite of a diffuse field.

It's probably a scattered field, but not diffuse. It's hard to describe the difference non.technically, but you can hear it: if you walk around the room, or even stand still and move your head a bit, you'll hear noticeable changes in the intensity and direction from which the sound appears to come, and sort of "focused" and "specular" in some places. But with a diffuse field, you won't hear much change at all as you move around, except perhaps very close to the diffuser.

The idea for this installation came by reading the Morpho butterfly wings story in Alton Everest's Master Handbook of Acoustic

I haven't seen that, but I get what he is trying to say, I think. The problem is that you seem to be trying to implement it on a scale that is too large, and all of your scattering surfaces are vertical.... Nothing is horizontal. Most seem to be concave, few convex...

and this is an example of iridescence through structural coloration

Right, but that's an entirely different principle with light. It would be nice if sound waves behaved like light waves, and they do in some aspects, but bot in others. Diffraction gratings work for light, but not so well for sound. Sound does diffract, yes, but trying to use the resulting interference patterns to create "coloration" does not work too well. Not to mention that our eyes do not work the same way that our ears do, in just the same way that a camera lens does not work like a microphone does...

when there's many people in the room I feel like the woods are more effective in creating these distorted specular reflection, probably because the room sounds less muddy..

Right! Once again, you are clearing up the "mush" that is masking the good stuff, and improving clarity. With the very long reverb times you have in that room, all of the clarity is lost: there's just too much going on, bouncing around the room all over, when it is empty, but when you control that better with absorption, you can hear what's REALLY going on with your "scattering sticks". The room itself is overwhelming the subtleties of what that is doing.

Wow that's very impressive. I had no idea that we absorb so much.

Yup. But it varies widely ("3 to 5" is a change of 60%!), mostly based on clothing. "3 to 5" is average, typical, but not the limits. In winter, when people are wearing or carrying thick jackets, scarves, gloves, and wearing several layers of thick, soft thermal clothing, it's probably more like 6, maybe even 7, but someone standing there almost naked in their underwear, it's more like 2, or even less. There's also a difference in how the people are "arranged" in the room: Pressed up close in a tight crowd, the effect is lower, but spread out more thinly with space between them, it's higher. Sitting, standing, walking, laying down, arms held up high, arms hanging low... it changes. Subtle changes, yes, but still there.

People here sit on the floor, who knows whether a human being closer to the pressure zone absorbs more or less

Less, but not due to where they are: due to how much surface are of their bodies/clothing they are exposing to the room. Standing up, legs spread, arms raised, it's nearly 100%. Sitting down, it's a lot less.

I guess that depends if we are more like resonators with our cavities and empty skulls:) or more like porous absorbers with our hairs and clothes?

We don't resonate much: we are mostly water (about 60%), and it's not so easy to make a sack of water resonate... According to a research paper on this from a few years back: "The influences of a human on the sound field in a room can be compared to that of a hard ellipsoid with the same volume, and a length to radius ratio of 11, and density of 910 kg/m3, wrapped in a weakly scattering layer of absorbing material". So basically, for acoustics, we are tall egg-shaped blocks of wood, covered with a layer of fiberglass insulation... Not very flattering, but that's about it!

Anyhow here it is how the space looks like with people

Red picture summer, other pictures winter? Notice the difference in the clothing: for the lower two pictures, there's a LOT more absorption than in the red picture: people standing, thick clothes, vs. people sitting, less tick clothes. I'll bet the red photo concert sounded a lot more lively and bright than the other two...

Yesterday I took many new overzealous measurements along a grid (1,5 x 1,5 m) in the room.

Wow! That's a lot of data to work through! I'll try to take a look at SOME of that, but to analyze it all would take many days, and my paying customers would not be happy if I stopped working on their projects to do that!

2 old studio monitors from the 80's, Genelec 1024B

I haven't seen any of those in a while! They were pretty radical back then, and still sound pretty good today. Nice!

Sennheiser MKH 8020 omnidirectional microphones, there's no cal anywhere but it's very linear and has pretty impressive specs.

That's fine. Plenty good enough for what this type of analysis.

There's definitely modal stuff going on by the way: At least at 60Hz, 70Hz, 86Hz, 95 Hz, 112 Hz, 167 Hz, and several others. That's just rough estimates, based on a random sampling of your data...

I see, I was just thinking about the broadband corner slotted HH resonators (Alton Everest, Rod Gervais books). Do you consider it a bad idea in general or in a room like this?

Careful... sometimes things can look like they are slot resonators, but they actually aren't. One key point is if the device is sealed air-tight at the back. If it is not, then there's no resonance. In that case, it's just an partially reflective, slightly diffusive absorber... In your room, I would probably try to avoid tuned traps: they are notoriously hard to tune: much harder than the text books make it seem! Plus, each devices can only be tuned to one fundamental issue, needs to be very large to control it significantly, and must be placed at the pressure node for that problem, on the correct surface of the room... even if that happens to be in the middle of a window, door, or some place where you would NOT want to put it.... I'm not a big fan of tuned resonant devices. I only use them rarely, under unusual circumstances. And you can't use them much for treating modal issues anyway.... they have to be huge, and cover a large surface area to be effective...

You room needs absorption and diffusion in the low mids, and the upper part of the low end. It's "boomy", I would say. Hollow-sounding. I would plan on treating mostly around 180 Hz, plus two octaves up and one octave down. You will probably need some lower trapping as well, but you'll need to be careful to not kill the high end.

Any modal activity is probably masked within the general "mush" in the low end, and as well as hidden by the very low res graphs.

That's very very interesting, I had never considered that!

Now that I have the data, I can see it clearly. It was hidden mostly in the low resolution, but also a bit in the "mush". It's typical of a small room's modal issues.

To get an even distribution of porous absorbent in the room, would it make sense, in your opinion, to use the empty spaces between the woods, or shall we be more systematic and geometrical?

You are not aiming for perfectly smooth acoustic control, since this is an artistic space. I would go with a more random distribution of panels, between many of the "scattering sticks", as well as on the ceiling surfaces. You can probably be a bit creative here. You need to get the right total amount of absorption for the room, at the right frequencies, but it does not need to be distributed evenly, because that's not what your room is for! It's a performance space: I would suggest making one end more live, and the other more subdued. Perhaps make the front end "softer" (where the performers / musicians normally are), and the rear more live, reflective. I'd put the absorption generally higher up on the walls, since the people are lower down, and the diffusion mostly on the end walls, lower down (ear height). But feel free to experiment! After all, that's what your room is for! Experimental music. So the acoustics can be "experimental" too. Not in the sense that you are "experimenting" with it (like you are doing now, with your "scatter sticks"!), but rather in a controlled but artistic manner. Have enough total absorption and diffusion in the room to get the decay times under control, with a basic layout that ensures there are no terrible spots and does not change, then try out a few variations on distributing the rest of the treatment. Call it "artistic science" if you want: Based on the science of acoustics (rather than guessing or experimenting) for the underlying foundation layout of the acoustic response, then with variation to get the final result that you want.

I would SERIOUSLY consider variable devices, if that were my room. . . .

Shall we build big panels or smaller ones.

I would do both. A few larger panels that stay mostly fixed in place, and several smaller panels.

I was referring to open panels (not sealed) filled with insulation + 5cm wide slats, 2 cm slot, 2.5 cm thick

So not tuned. Basically just absorbers with some reflective surfaces, aimed to reflect only the high end.... And if you built them all the same, then you'd be absorbing/reflecting the same frequencies everywhere...

The thin foil panel would be much easier to build, and lighter (drywall only walls here, very fragile).

Right, but there must be studs behind that. Hang your panels ONLY where there are studs. NEVER on drywall alone.

The porous-multilayer absorber calculator that I use cannot simulate the effect of a thin foil. Right.... Well, you could model it as a very thin limp membrane, but that's not really going to work very well. Better would be perf panel with no holes...

But there's a much easier way:

There is a reflection curve for foils (plastic is a foil too, acoustically) that defines the point where foil of any given surface density will be 80% transparent to sound (allowing 80% of that frequency through to the other side, reflecting back 20%, or coefficient of reflection = 0.2). The curve rises to about 99% reflection above that, at about 6 dB/octave, and falls off to practically 0% transparency below that, at the same rate:

F = 90 / m

F = The frequency at which the foil transmits 80% of the sound
m = The surface Mass of the foil in kg/m2

effect-of-plastic-sheet-foil-on-porous-absorber.jpg

porous-absorber-with-foil-covering--4.jpg

Both the acoustic response of a thin foil and the specific cut off frequency that we need for the absorbers in our room are quite baffling

Plastic sheeting works very well. It is available in various thicknesses at most hardware stores, and you can layer it too. But there's no "specific cutoff frequency" for a foil: just a gentle curve, which is exactly what you want...

I've seen people successfully making diffusors out of hard styrofoam and foam board insulation products.

Hmmm.... yeah, I've seen that too, but serious diffusors are usually made of wood... especially if you have them where people can bump against them... Styrofoam is fragile. It dents, bends, breaks...


- Stuart -

[attilafaravelli]

It took me a while to get back, I've been away for a couple weeks.

Needless to say that your second reply was also very informative, thanks again Stuart for your time and dedication.

control rooms are relatively easy to design and treat, since the goal is "neutral", and there are methods, design concepts, guidelines, rules, and techniques for achieving that. But live rooms are art: they are supposed to have "character" and "life" and "warmth" (or something else, if you wish), as desired by the owner and expected by the occupants. Live rooms are a whole lot harder to do than control rooms.

I agree with you but I feel like bring into focus how, for our project at least, there's more to it than art, I mean that we are interested in a not-neutral and not-flat acoustic environments starting from an anthropological perspective which is well rooted in hystorical-scientific notions.
In this seminal book:
https://mitpress.mit.edu/books/spaces-s ... -listening
there's a few chapters about the evolution of human hearing system during hundred of thousands of years.. The authors underline how the fact that we now spend most of our lives inside closed spaces (houses-offices-recording studios..) doesn't mean that it's always been like that... in fact our hearing apparatuses have been 'growing together' with a world where 'stuff' was going on in the open.. where nothing is still nor linear, air always moves with breeze or wind, warm and cold air currents diffract sound, moving objects like leaves and grass and flowing water reflect sound in complex and ever changing patterns. It seems like we as humans have the ability to discern between repeating and not repeating sections of white noise..

From the above book:
"Richard Warren and colleagues (2001) studied long segments of noise from the perspective of auditory memory—remembering the particular attributes of particular segments of sound. No matter how random, every sound has its aural personality: unique gaps and local pitch. In their study, both experts and nonexperts learned to rec- ognize noise segments that were asynchronously embedded within a continuous ran- dom noise. They recognized the target segment regardless of when it appeared, often remembering it for as long as 10 to 20 seconds. In his study of what is being remem- bered, Christian Kaernbach (1993) concluded that ‘‘when the auditory system is pre- sented with repeated white noise, it will enhance details of this noisy structure which we otherwise would not perceive. . . . White noise seems to be filled with a lot of such potential features. . . . The physical basis for perceiving such features does exist, so per- ception should be possible. . . . As soon as the feature pattern reappears [multiple expo- sures] the features are taken to be informative [recognized].’’ A segment of noise is perceived as noise only on first exposure. Christian Kaernbach, Erich Schro ̈ger, and Thomas C. Gunter (1998) found that, among those individuals with an aptitude for perceiving small segments of repeated noise, there were observable manifestations in their measured brain potentials."

So, in the end, I guess there should be a pretty rigorous approach which is not about creating a pleasant or warm sounding space but which respects and exploits our amazing abilities
, as hearing animals, to work toward complexity, movement, differentiation. We probably lacked the acoustic knowledge and experience to get the sound that we want in our space but the principles behind the project are quite the contrary to the freedom that one would associate with 'art', there really is the intention to look for a space which is acoustically more permeable and feels less like an enclosed box with some embellishments:)

I like the idea of variable devices is super intriguing. We are working on some ideas, thanks for the tip!

Sorry again for sending so many measurements, I'm well aware that you are a professional, and I wish we could afford to hire one sometime (of course you are first on the list:)) .. I just was unsure about what's useful or not and I had no idea that the REW files were so heavy.. I'm very grateful that you tool a look at some of them, and we are working considering the problematic frequencies that you point us to!!

And thanks for the formula you shared to calculate the foil to be put in front of the porous material. May I ask you where you got it? I'm eager to learn!! and would really be interested in studying more about these topics.

Talking about thin foils, I found an informative post of yours on the forum where you tell about a project where you designed some diamond shaped plywood (veneer) structures to be put in front of thick porous absorbers. You say it was probably 2mm thick wood. Let's say an average weight for wood would be around 700 kg per cubic meter (pine), a 2 mm slice of pine would be about 1,4 kg. If I use the formula a pretty low frequency comes out: 90/1,4=64,28.
There must be something wrong with my calculations:)... ?? Or is it common practice to build devices with such a low cut off frequency, maybe your client's room was huge and his Schroeder frequency was very very low?
I'd really like to use wood for our broadband absorbers to create a continuity with the woods (pure aesthetic reasons) but I've been considering Tyvek as well:
http://www.dupont.com/products-and-serv ... -wrap.html
it comes for very cheap, on one side it's white (of course:)) and I kinda like how it looks, I built some backpacking equipment out of it and it's incredibly rugged. Weight would be about 110 g per square meter and it would bring the cut off frequency to about 818 Hz.

Would it make sense to you?

[Soundman2020]

we now spend most of our lives inside closed spaces (houses-offices-recording studios..) doesn't mean that it's always been like that...

Right: Which is why our ears and brains are keyed to the floor. Wherever you go, your entire adult life, the floor is always the same distance away from your ears when you walk around, in doors or out. Your brain understands that "signature" of sound bouncing off the floor very well, which is one of the major reasons why studios pretty much always have hard floors and soft ceilings: To provide the brain with the best acoustic clues about the room. If you've ever tried playing guitar in a room with a thick, soft, plush carpet on the floor and a concrete ceiling above you, you'll recognize just how unnatural and annoying that is, and how hard it is to play well like that. But if you play the same guitar in the same sized room with a hard floor and a more diffuse ceiling, it sounds so much better and more pleasant...

Way too many first-time studio builders think that carpeting their floors is a cool way to make it sound good... wrong!

In other words, whatever else you do in your room, make sure that you keep the floor hard and reflective. It provides an acoustic "signature" that everyone's brain is very much used to, and familiar with, and find pleasant...

We probably lacked the acoustic knowledge and experience to get the sound that we want in our space but the principles behind the project are quite the contrary to the freedom that one would associate with 'art',

OK, but as long as you do take into account the basic principles of acoustics for performance spaces, then you should be OK.

I like the idea of variable devices is super intriguing. We are working on some ideas, thanks for the tip!

You might find this interesting, from a room I designed for a customer a few years ago:

Variable-acoustic-01--panels--construction--half-open-SML.jpg

Variable-acoustic-02--panels--construction--fully-open--SML-ENH.JPG

Variable-acoustic-04--room--completed--SML-ENH.jpg

variable-acoustic-05--acoustic-rt60-plots-all-positions-t20.jpg

Don't try to copy that! It won't work for your room. That device was tuned especially for that specific room (and I'd prefer to keep the details to myself anyway, as that design was paid for by customer). But you can see how it changes the high-mid response a lot, the low end response quite decently (and in the opposite manner), but does not touch the low mids at all. Your problem is rather different, and you NEED variation in the low mids. That's where all of your problem are! So that device would be useless in your room, but works great in the room that it was designed for. Your device would need different tuning, and likely a different basic design. I just put it out there, so you can see that variable devices do work, and can be designed to deal with specific problems.

I wish we could afford to hire one sometime (of course you are first on the list:))

Maybe you can... But talk to John first! He should be your first choice, not me. It's his forum, and I'm just a guest here. In any case, I'm fully booked for the next few weeks...

I just was unsure about what's useful or not

It's all useful! The more data you have, the better it is for getting a complete picture of the room's acoustic behavior. But the more you have, the more complex the analysis becomes as well...

May I ask you where you got it? I'm eager to learn!! and would really be interested in studying more about these topics.

I originally got the whitepaper from here: http://www.mech.tottori-u.ac.jp/crlab/ but the link no longer works, and the site seems to be very different now. Since I don't read Japanese, I can't tell you what it says, but perhaps the domain was sold. Sorry! As far as I know, the original was described in the book ""Studio Akustik" by Andreas Friesecke, but that's in German...

You say it was probably 2mm thick wood.

2mm is not really foil any more. That's more like a panel. I wasn't intending that design for that purpose, but I would imagine that yes, such a piece of wood will probably be 80% transparent at 65 Hz, with the usual curve rising from there. But for a flat panel that thick over absorption, it would probably be better to consider it as a panel trap, and use the equation F = 170/SQRT(md) (imperial units: I don't have the metric version on hand).

but I've been considering Tyvek as well:

Tyvek should probably work OK.

the cut off frequency to about 818 Hz. Would it make sense to you?

Depends on the overall design of the device, the size, the angles of incidence, and the placement in the room! It's not just the material that matters, but how you use it.

- Stuart -