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DIY Sound Diffusers - Page 3

post #61 of 310
Quote:
Originally Posted by coctostan View Post

Honestly, I've never even seen a discussion of diffusion in the modal region. Maybe it is because a 2 foot deep diffuser is about as impractical as it gets.

That's probably a very accurate assessment.

Quote:


BTW, from what I've read you don't want diffusers closer than 3x the lowest diffused wavelength. According to QRDude, a 24" deep N23 1D would need to be 15 feet from the listener.

I thought about that. For any diffuser, I think there's a spectrum from no diffuser (pure specular reflection) to recommended distance (perfectly even diffusion). If you sit too close, so what? It will be a little less even than "perfect" but a lot more diffuse than a specular reflection. You can't get any worse than just not having a diffuser.

And especially in the LF region, if your goal was to have an impact on modal breakup/prevention, I don't think you would really care if you're at the recommended distance or not. Standing waves occur as a buildup phenomenon anyway, and preventing that phenomenon would have a benefit for all locations within the room, even close to the diffuser (maybe even more so in this case, since near boundaries is where you get maximum aberration in response from modes).

But yeah, 2' deep is a lot. To really impact modes in an average room you're talking about more like 4'. And that has to be wide... like 8' or so. Good for a church... HT maybe not so much? Playing with an early concept for a future HT, I think I could live with giving up the space for one 4' x 8' by whatever height LF diffuser, and that is effective really only down to maybe 60-70Hz or so. In a room big enough to accommodate something like that, the Schrodinger frequency would be much lower, as low as 20-30hz perhaps. So there would still be unaltered modes, though much more sparse, lower in frequency (harder to hear), and perhaps dealt with by resonant damping targeting the stragglers.

But I think it's interesting in concept at least. Imagine a room without significant modes, or without having to struggle with too much HF absorption when enough bass trapping is used, or having to design a dozen diaphragm or hemholtz resonators to deal with specific frequencies.... a little broandband absorption would then go a very long way in further smoothing out the response.
post #62 of 310
I can't see the benefit in diffusion in the modal region. Down there you want to damp the ringing more than anything else. Modes when undamped decay at a greater rate and they add to the sense of bass volume, but with that add boominess as they ring out in time. When damped, their decay rate is increased so that you get a more even decay. So what you want is to absorb their energy. The result is that the room starts to transition to a bass sound that is more like you would get outdoors. However, room gain stays intact so you get the best of both worlds. Bass sounds subjectively tighter, faster and also it appears as if there is less of it. When you put enough of it in, and that is quite a lot, the result can be startling.

Now if you put diffusion in instead, I suppose what you will do if you succeed is alter the distribution of modes. Perhaps you will get smoothing through spatial averaging, since they will travel in more directions from reflection points, yielding less strong peaks and dips. It's hard to say. I would guess that the result would be similar to multi subs. However, the space taken up and the expense of doing it would quickly climb up. You need a big room to fit lots of those in and it's probably going to be expensive if you give up enough real estate for it to really work.

I would guess that the result would be less satisfying than simply trapping the room heavily. Far less expensive, far less obtrusive, far more effective.

Multi subs + placement with measurements + serious bass trapping + EQ = bass bliss

I have a DVD here that describes this idea well ... "weird science!" ... bit of a blast from the teenage past.
post #63 of 310
Quote:
Originally Posted by paulspencer View Post

I can't see the benefit in diffusion in the modal region.

Well, I can easily see the benefit, but I can easily acknowledge that it isn't a very practical solution.
Quote:


Down there you want to damp the ringing more than anything else.

I see two options - damp the modes, or prevent the modes. I have no hard evidence to prove it (obviously I haven't built and measured a room with lots of huge a$$ diffusers ), but I believe effective distributed diffusion in the modal region would absolutely remove longitudinal modes leaving only higher order modes, and would probably do a bang up job of "spreading" their energy over a broad range. Essentially take a few very problematic modes and create and spread that energy over hundreds of tangential small modes. If you smooth out FR and remove strong ringing, have you not effectively prevented modal problems?

Quote:


Modes when undamped decay at a greater rate and they add to the sense of bass volume, but with that add boominess as they ring out in time. When damped, their decay rate is increased so that you get a more even decay. So what you want is to absorb their energy.

Right, or you could spread out their energy evenly over the entire bass frequency range such that there are no FR or time domain spikes. Broadband absorption can then fine tune RT60 much more easily since you won't have to worry about overdamping the HF.

Quote:


Now if you put diffusion in instead, I suppose what you will do if you succeed is alter the distribution of modes. Perhaps you will get smoothing through spatial averaging, since they will travel in more directions from reflection points, yielding less strong peaks and dips. It's hard to say.

Yeah, this is what I'm pretty certain would happen.

The rest of your post is spot on... the size is pretty enormous compared to the average HT room. There are more "practical" ways to deal with the problem, but diffusion isn't without it's upsides. When using broadband absorption, you have to be very careful with balancing absorption evenly and appropriately within the audio spectrum. No easy task! It often requires using FSK or other facings on LF absorbers and such. If you do hemholtz or other resonant damping, you could end up needing a dozen different sizes to treat just the most offensive modes, and probably multiples of each. Not exactly practical either from a size or cost viewpoint.

The common method of putting as much absorption as you can, using a face sheeting if necessary, and then using EQ to fix what is left is obviously the most practical and cost effective method for pretty much anyone! Hey, I never said a 4+ foot deep diffuser was practical. But it's an interesting concept in theory.

On the other hand, many people do have screen false walls with anywhere from one to several feet behind them. Most of these people will put up 2" or so of dense fiberglass absorbant panels. I think the idea of putting in diffusers up to 2' deep that operate down to 150Hz (still in the thick of modal problems in many real room) deserves serious consideration. I wouldn't forgo front wall absorption to do just diffusion... you could easily and cheaply build a 1D QRD diffuser 2' deep covering most of the front wall, itself covered in 1-2" of dense fiberglass panels. You'd absorb HF and midrange as desired, diffuse bass down to 150Hz laterally, and by having the fiberglass anywhere from a few inches to 2' out from the well bottoms, you'd achieve much better low frequency absorption and a more even absorption over a broader frequency range (due to the varying spacing of fiberglass from well bottoms) than the 2" on the wall alone would achieve. All for the cost of plywood, dimensional lumber, MDF or whatever. Doesn't have to be pretty behind a screen wall. Again... perhaps an idea worth considering for many people doing "from scratch" builds.
post #64 of 310
No doubt it would be interesting to see it done. Modal ringing would not be fixed with diffusion only, that requires damping as well ie absorption.

Interesting observation - I've opened a door in a corner and measured and listened. You can measure the difference (small), but it's not readily audible. Even with 2 windows open as well. 3 traps in my room in the other 3 corners however make a night and day difference.
post #65 of 310
A theoretical question:

Can standing waves develop in a closed space of any shape in a frequency range over which diffusers covering all surfaces are behaving as pure diffusers? What path can you draw that allows reinforcement to occur?

You say to rid a space of modal ringing requires damping. Does it, really? I'm not sure i know the answer in the strict sense. If we can answer the theoretical case then we can address whether there is any beneficial application to realistic rooms.
post #66 of 310
Btw, aside from working through mathematical modeling or FEA, this question could be handily investigated in a water tank.
post #67 of 310
Bigus ... you remind me of a certain Monty Python skit that includes your alias!

Good question.

Now let's suppose the entire enclosure acts as a bass diffuser. You won't get standing waves in this theoretical ideal, but because there is no damping they will still have a longer than ideal decay time. You would get a more uniform decay and response. Were you to combine absorption and diffusion in equal and large amounts I'd guess you'd go from a good result to a state of the art result, as well as a very strange looking impractical room. That's my guess, but things could in reality be quite different.

What do you think?
post #68 of 310
Pretty much my thoughts as well. So the real question is whether it ever makes sense in a real room to pursue diffusion in the modal region... Is the space required more effectively used in another way, like say a four foot deep chink of rigid fiberglass? It's a question I'll look into more as I have time.
post #69 of 310
Has anyone done the calculation to check if diffusion is even possible in typical rooms' bass region?

I suspect the diffusers would have bigger dimensions than the room having said modes.
post #70 of 310
Any chance we can get back to the OP and how well my diffusers were designed and produced for the DIY market

I think they are spot, on
post #71 of 310
Actually, yes. Depending on whether you are shooting for the "pure diffusion" range, or just "effective scattering" which extends yet another octave lower. I believe the big difference there is whether the scattering supports equal energy lobes with repeating diffusers, which is the claim to fame of QRD diffusers... I doubt that would be important for modal range stuff.

So I did a quick and dirty 1D last night to see how it would fit in an actual room... scattering down into the upper 40's has a depth of some 64" and a period width of around 200", and could be effective up to well above 1kHz. Not tiny at all!

But then, it is conceivable that such a thing could actually fit in a real room. I wonder how many people here have screen false walls that are say 36" from the actual front wall? 48"? Probably most with false walls are in the 24-36" range if I had to guess... maybe closer to the 24" end. I'm also guessing you'd find quite a few of the really nice and perhaps larger theater builds might be more in the 36-48" range. If a single 1D diffuser along the axial mode of the room would have a significant impact on the length axial mode (and tangential modes, though less important), which seems to cause lots of headaches as front seating often falls near the middle of the room and the rear row near the back wall, would sacrificing another foot or two of room length to accommodate the diffuser behind the false wall instead of just a few inches of fiberglass be feasible?

So we sort of tackled the theoretical case. Let's define a more real-world case and ask whether it would be beneficial. Imagine a decent sized room being turned into a HT, say 18' x 28'. On the large end of the scale for members here. Presume a false wall is being built for the screen, initial plans were to have it 40" from the front wall to house typical monitor speakers with a little room left behind them. That could be increased to 62" if necessary and still leave plenty of length in the room for a comfortable HT. Given a 5 foot by 18 foot chunk of real estate in the front of the room, what is best way to utilize it? Multiple hemholtz resonators of various sizes? A chunk of solid dense fiberglass (huge chunk!) with FSK here and there to minimize overdamping of HF? Some mix of absorption and HF diffusion? Or a big 1D diffuser that breaks up some of the worst modes, with a 1-2" sheet of dense fiberglass in front to take care of HF absorption and give good LF absorption over a very broad frequency range?

I really don't know. I don't have the FE tools to try to mock up such a comparison, nor do I know if those tools would even effectively handle such a LF diffusion case. Would it have enough of an impact on modes to be a good use of the floorspace? Is it a waste without having similar diffusers on the back wall, and maybe side walls too? I dunno. I think it's interesting.

I think of things like the DBA (active double bass array) concept, or the passive version using limp-mass absorption along the back wall like is done in "the octagon." Would diffusion be as effective as limp mass on one wall? Again, I dunno. I'm not sure of a practical way to even investigate it without having a big empty room to experiment with and the time to toss together a large crude 1D out of standard lumber. It could be done if someone had an empty basement, measuring tools, spare change for a chunk of lumber, and lots of fiberglass panels lying around.
post #72 of 310
Thread Starter 
Okay, guys, I have some rough estimates on cost!

Keep in mind, he did say these were rough estimates, but fairly close. These are all figuring that the highest point is 7" tall on both designs.

A 24" x 24" diffuser made like the 1D type would be roughly $20. He said he was figuring the foam density as 1lb per cu ft. But he could always change that to 1.5 lbs per cu ft. if we thought it was needed. The price to set that production run up would be almost nothing because it's fairly easy to make the cutting shape.

Now on the 2D style with a bunch of raised squares. He said they could do a 24" x 24" piece but the mold would be pretty expensive. He recommended a 12" x 12" mold and the price per section would be less than $8.

He can get me the price of that mold. It won't be anywhere near as bad as injection molding prices because they don't need the same type of pressure to make the piece.



So there you go. If there's enough interest, I'm game. Obviously the 1D stuff would be cheaper because of molding costs.



Oh, and keep in mind, on the 1D stuff, he just assumed 24" x 24". He could make it any size we wanted. 18" x 26" or something like that might be easier and cheaper to ship.
post #73 of 310
Quote:
Originally Posted by Erich H View Post

Okay, guys, I have some rough estimates on cost!

Keep in mind, he did say these were rough estimates, but fairly close. These are all figuring that the highest point is 7" tall on both designs.

A 24" x 24" diffuser made like the 1D type would be roughly $20. He said he was figuring the foam density as 1lb per cu ft. But he could always change that to 1.5 lbs per cu ft. if we thought it was needed. The price to set that production run up would be almost nothing because it's fairly easy to make the cutting shape.

Now on the 2D style with a bunch of raised squares. He said they could do a 24" x 24" piece but the mold would be pretty expensive. He recommended a 12" x 12" mold and the price per section would be less than $8.

He can get me the price of that mold. It won't be anywhere near as bad as injection molding prices because they don't need the same type of pressure to make the piece.



So there you go. If there's enough interest, I'm game. Obviously the 1D stuff would be cheaper because of molding costs.



Oh, and keep in mind, on the 1D stuff, he just assumed 24" x 24". He could make it any size we wanted. 18" x 26" or something like that might be easier and cheaper to ship.

The 1D stuff would work. I imagine I would be interested in 4 pcs of 24" x24" ( or the comparable size) diffusor when available.

Erich, thanks for coordinating this.
post #74 of 310
Thread Starter 
Keep in mind, prices are probably going to be less. He basically said they shouldn't be over the price he quoted. It sounded like they might be cheaper but he wanted to error on the more expensive side.

And obviously they'd be much less if the design didn't go up to 7" deep (which is pretty deep). If it was 1/2 that deep, it might be half the price which would be $10 for a 24"x24" panel. That's pretty darn cheap if it's a good design. Some cheesy Auralex stuff that's less than 3" thick is twice that much and only 21" x 21".

The setup cost on the 1D stuff should be cheap enough to allow us to have a few different designs. So maybe we could make the panels 12" x 12" but have 4 different designs. That way it could be varied on the wall.


I also had another idea that could make a 2D cheaper. If he runs the 1D though the machine, but then turns the piece sideways and runs it through another design, that could give much more variation in the cut. Instead of just straight lines, you'd have squares of varying depth. I'm not sure he can even do that, but it seems like a good idea IF the design could be figured out.
post #75 of 310
Those are good prices. Do you know what material is used? EPS? The density is of little concern acoustically. A rigid foam be reflective in the frequency range we intend to diffuse regardless of density. It might be a concern for product stability or durability. That I know nothing about.

I vote for a 24" wide 1D. That allows for a N23 design with 1" wide wells. IMO 4-5" deep is the sweet spot because it is pretty unobtrusive on the wall but still gives a nice wide range of diffusion. It is no coincidence it is what RPG and others typically sell. 7" would perform slightly better but WAF goes way down IMO.

For a 1D the width and height need to be even multiples of each other so they can be staggered in orientation. So either 6", 12" or 24" high to match the 24" width.

Can he make these with dividers?

If I understand it correctly, I don't think your idea for the 2D will work. I don't think that would give us the correct 2D pattern, but maybe I just don't understand.
post #76 of 310
Thread Starter 
Quote:
Originally Posted by coctostan View Post

Those are good prices. Do you know what material is used? EPS? The density is of little concern acoustically. A rigid foam be reflective in the frequency range we intend to diffuse regardless of density. It might be a concern for product stability or durability. That I know nothing about.

I vote for a 24" wide 1D. That allows for a N23 design with 1" wide wells. IMO 4-5" deep is the sweet spot because it is pretty unobtrusive on the wall but still gives a nice wide range of diffusion. It is no coincidence it is what RPG and others typically sell. 7" would perform slightly better but WAF goes way down IMO.


Can he make these with dividers?

Not sure about the exact material, we just talked about polystyrene.

What do you mean about "dividers"?


Is this design something that you can draw up from a side view so the guy can see the height and width of each raised edge? If so, I'll send it to him and see what he says. If you want 2 different designs, draw up both and we'll see if he can do both.

I'll try to see what's the best size for shipping. 16"x16" might not be bad. But I thought you said they had to be a specific width in the end. Not sure.
post #77 of 310
"Dividers" are full height thin-walled pieces between each well. If the design is 7" thick, then each divider is a full 7" tall (minus backing thickness), with well depths varying.

This is pretty helpful for proper phase steering, especially for 1D designs (I believe it is less critical for 2D though it still helps for larger well sizes).

For an example, look at the pic I posted earlier in the thread... this is a 2D version, but you can see that the large size diffuser dividers while the small ones in each well do not.
post #78 of 310
Thread Starter 
Wouldn't the dividers be extremely fragile with a simple 1D design? 2D with them in a grid type fashion would be fairly strong, but I think shipping a 1D with full dividers wouldn't work too well.
post #79 of 310
Whatever you guys decide please count me in .

Just need to figure out how many I need and where?

BTW whats the ETA?
post #80 of 310
Thread Starter 
About the dividers on a 1D again. And I know this may not be 100% perfect, but it still might be better than not doing this:

From left to right, what if the first and last raised cuts were the highest on the board. If the panels were square you could place them on the wall and simply rotate every other one. That would give you the grid like dividers. I realize it might not be the perfect method, but if the dividers are more important than the other aspects, it might be a good way to go to balance it out.

But that's just a quick thought.
post #81 of 310
Quote:
Originally Posted by Erich H View Post
About the dividers on a 1D again. And I know this may not be 100% perfect, but it still might be better than not doing this:

From left to right, what if the first and last raised cuts were the highest on the board. If the panels were square you could place them on the wall and simply rotate every other one. That would give you the grid like dividers. I realize it might not be the perfect method, but if the dividers are more important than the other aspects, it might be a good way to go to balance it out.

But that's just a quick thought.
I was doing a little research on polystyrene and it seems it is highly flammable and it is not supposed to be used without a thermal barrier or fire retardant. I'm not sure I'd want a bunch of this stuff on my walls to be honest.
post #82 of 310
Thread Starter 
I'm sure he would use any type of foam we wanted. I can ask the guy if there's a different product because all he deals with is different types of foam.

Isn't that what the other companies use......Styrofoam? When you asked what it was made of, I just assumed expanded polystyrene. Maybe the guy had something else in mind. But as far as I know, most of these are made from that material. What else would you want to use?
post #83 of 310
Quote:
Originally Posted by coctostan View Post
I was doing a little research on polystyrene and it seems it is highly flammable and it is not supposed to be used without a thermal barrier or fire retardant. I'm not sure I'd want a bunch of this stuff on my walls to be honest.
You got me curious after posting that. Here is what I found. I'm not saying you are wrong just saying that this does not agree with what you are saying



Expanded polystyreneExpanded polystyrene (EPS) is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. Familiar uses include molded sheets for building insulation and packing material ("peanuts") for cushioning fragile items inside boxes. Sheets are commonly packaged as rigid panels (size 4 by 8 or 2 by 8 feet in the United States), which are also known as "bead-board". Thermal resistivity is usually about 28 m·K/W (or R-4 per inch in American customary units). Some EPS boards have a flame spread of less than 25 and a smoke-developed index of less than 450, which means they can be used without a fire barrier (but require a 15 minute thermal barrier) according to US building codes. A growing use of EPS in construction is insulating concrete forms. The density range is about 16–640 kg/m3.[4] The most common processing method is thermal cutting with hot wires.[12]

Maybe you were looking at another form of polystyrene and not expanded?
post #84 of 310
Thread Starter 
For what it's worth, polystyrene is used in a MASSIVE amount of stuff, not just as styrofoam. They even shred it up and use it as insulation in homes.

I'll still ask the 'foam guy' if he has any other recommendations.
post #85 of 310
Quote:
Originally Posted by Erich H View Post
For what it's worth, polystyrene is used in a MASSIVE amount of stuff, not just as styrofoam. They even shred it up and use it as insulation in homes.

I'll still ask the 'foam guy' if he has any other recommendations.
Yeah, its everywhere!

I just dont think that there is an issue with it at all. And it will need to be painted in most cases I would think.

Although it does combust rapidly it is hard to ignite innitially. So I'm sure that would be a problem even if it wasnt on the walls.
post #86 of 310
Quote:
Originally Posted by Erich H
Wouldn't the dividers be extremely fragile with a simple 1D design? 2D with them in a grid type fashion would be fairly strong, but I think shipping a 1D with full dividers wouldn't work too well.
The dividers are actually more important for a 1D design than for a 2D. In fact, without them the diffuser doesn't act at all like a qrd should, not to mention being less effective in general at diffusing period. Not that they won't have some effectiveness, it just won't be up to their potential.

Dividers don't have to be ultra thin or anything... they just usually are when the construction material is wood for obvious reasons. I'd prefer an n11 or n17 with 1" wide wells and half inch or so dividers than an n23 with 1" wells and no dividers.
post #87 of 310
Quote:
Originally Posted by audiovideoholic View Post

You got me curious after posting that. Here is what I found. I'm not saying you are wrong just saying that this does not agree with what you are saying



Expanded polystyreneExpanded polystyrene (EPS) is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. Familiar uses include molded sheets for building insulation and packing material ("peanuts") for cushioning fragile items inside boxes. Sheets are commonly packaged as rigid panels (size 4 by 8 or 2 by 8 feet in the United States), which are also known as "bead-board". Thermal resistivity is usually about 28 m·K/W (or R-4 per inch in American customary units). Some EPS boards have a flame spread of less than 25 and a smoke-developed index of less than 450, which means they can be used without a fire barrier (but require a 15 minute thermal barrier) according to US building codes. A growing use of EPS in construction is insulating concrete forms. The density range is about 16-640 kg/m3.[4] The most common processing method is thermal cutting with hot wires.[12]

Maybe you were looking at another form of polystyrene and not expanded?

I read that as well and what I'm reading is that it would be against US building code to leave EPS exposed without a thermal barrier. What constitutes a thermal barrier? We are talking about putting a substantial amount of this on the walls. Probably in 4ft x 6ft sections.

I'm all about being frugal and getting the best performance but not at the risk of my family. That is just me.

If we could confirm that it is safe to use and meets code, I'm all for it.
post #88 of 310
Thread Starter 
Auralex and other brands say they're made from EPS. I'm guessing that codes allow it or they couldn't sell it.

Polystyrene is in so many things that if you removed it from your house because of fire hazards, you'd be tossing out hair brushes, tooth brushes, milk jugs, computer speakers, CD cases, food containers, etc, etc. EPS is basically just expanded "puffed up plastic" filled with air pockets.

They sell it with no backing at Lowes and Home Depot for insulating your house. I assume if it was a huge fire hazard, they wouldn't use it for insulation. Plus some companies out there shred Styrofoam and blow it in houses for insulation as well.


There are a lot of sites floating around that bash the heck out of EPS. But those are normally tree hugger sites that can't get past EPS from decades ago. One thing they hate is that it takes a long time to break down and it's too expensive to recycle. But it's be proven over and over again that it's very safe and because of it's light weight it cuts down MORE on pollution just by the weight savings in transporting it. Those people stack up huge piles and say "look at this going in the landfill". But once it's compacted by the 100 ton dozers on the landfill, an entire room full of EPS would probably fit in a bucket.....because all the air is pressed out of it.



But as mentioned, I'll ask him what the options are and what he thinks about the fire issue. I agree that if it's a major issue, then we shouldn't worry about doing this. But most of those diffusers are made from the same foam.
post #89 of 310
Quote:
Originally Posted by coctostan View Post

I read that as well and what I'm reading is that it would be against US building code to leave EPS exposed without a thermal barrier. What constitutes a thermal barrier? We are talking about putting a substantial amount of this on the walls. Probably in 4ft x 6ft sections.

I'm all about being frugal and getting the best performance but not at the risk of my family. That is just me.

If we could confirm that it is safe to use and meets code, I'm all for it.

Yes if your walls used this as insulation there would need to be sheetrock on top of it that would stop the fire for 15 min. But in the application that it is going to be used in a coat of the correct paint would meet building code. But, hanging this on the wall without anything would also most likely meet code since its not part of the construction so to speak. If you had a fire in the theater room without these on the walls I'm pretty sure you would know about it and therefore not stay located in the room or enter the room. These are not going to be highly flameable. They are going to be very combustable though. It will take a dangerous fire on the wall next to them before they would catch. There is a huge difference between highly flameable and highly combustable. Most rooms that have fabric on the walls are probably in the risk as well but all that means is that once a dangerous fire starts its going to be hard to put it out which is the case for most fires that arent treated imediately anyway.

Edit. I just would like to add that I am by no means trying to talk anyone in to this. I am only stating what I believe to be correct as I did graduate with an archecticual engineering degree.
post #90 of 310
I have no expertise in the area. I just wanted to make sure this was a safe product before Erich ventures too far and gets screwed.

I'm not trying to scuttle this. I'd probably be his first customer.
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