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# DIY Sound Diffusers - Page 2

Anyone else playing around with the QRDude calculator? Or have done so in the past and have some experience with it?

I'm toying with the idea of a fractal (in the Diffractal) style 2D QRD panel in which the HF cutoff of the large pattern overlaps, matches, or is close to the LF diffusion cutoff of the smaller pattern, and in which the small pattern is meant to have a period width equal or smaller but very close to the column width of the larger panel.

I can't seem to get this to work out... probably a function of the mathematics but I want to make sure I'm not overlooking something. I started with 1D experimenting as that seemed easier but I have the same problem in 2D as well. What happens is that there is a gap between HF and LF cutoffs. As I try to extend the HF response of the large pattern it hits and passes the viscous limit, and as I try to lower the LF response of the small pattern its period width becomes too large to fit.

I'm wondering about implementing half round cylinders (or half-spheres?) on the tops of each block in a 2D panel with random orientation and perhaps size variation, but I have no idea what kind of periodicity problems this might create and I'm coming up predictably empty on being able to model the HF and LF cutoffs for such a hybrid system.

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So the answer to my question is pretty obvious.

High frequency cutoff is dictated by well width where the width is half the wavelength of the maximum frequency (assuming other conditions are met such that the width is the restraint and not plate frequency, etc.).

Low frequency cutoff is dictated by period width where it is equal to the wavelength of the minimum frequency benefiting from pure diffusion, again assuming other restraints do not take precedence.

Thus, if you set the period of the high frequency pattern equal to the well width of the low frequency pattern, there is always a half-wavelength, i.e. one octave gap, no matter the size you choose. You can make the high-frequency pattern smaller but then the gap increases.

I see no way around this mathematical limitation, except perhaps to split a high frequency pattern among say four low frequency wells. Since parts of the pattern will be at differing heights, I can only presume that this radically screws with the phase steering the principle relies on, and of course there is no tool available to model this.

I'll do some reading on frequency limits of cylindrical diffusors though I still would be concerned about periodicity problems and/or the inability to model it well. Perhaps varying the size and orientation of hemicylinders randomly would alleviate this problem? Dunno. Again, no easy modeling.

FYI - for anyone trying to design one of these that has a cutoff frequency in the kHz region wanting to extend the range to the top of the audible spectrum, I think a simply spraying of ceiling popcorn or the like with a coat of paint would do the job nicely, though this would also add just a bit of absorption at those frequencies. Fleckstone type paint would do a similar job but limited to very high frequencies and probably of little benefit.
Ah, I forgot to mention that since there is an octave or so of "scattering" below the pure LF diffusion limit, that one octave gap isn't completely empty. Not sure how big of an issue this would be sonically. Depending on how deep you want to make one of these things, that gap could be anywhere from 1kHz-2kHz up to several kHz range... a very sensitive/critical area.
You are correct about the gap, as well as this needing to be covered by the "scattering" range below the design frequency of the well insert. I did consider adding a "diffractal" wizard, but felt that RPG probably hates QRDude enough already. No point in ending up in court!
Quote:
Originally Posted by Bigus

Perhaps varying the size and orientation of hemicylinders randomly would alleviate this problem?

Do you think just varying the orientation would be enough? I assume you're talking about placing every other one either vertical or horizontal?

So would it make sense to put the 2D QRD's towards the middle of the wall and short 'hemicylinders' surrounding them in an altering pattern? Basically trying to get the cylinders closer to the corners of the room?

Do you guys think I should look into fabricating costs on these later this week? Or is this not something worth pursuing?
Quote:
Originally Posted by Erich H

Do you think just varying the orientation would be enough? I assume you're talking about placing every other one either vertical or horizontal?

So would it make sense to put the 2D QRD's towards the middle of the wall and short 'hemicylinders' surrounding them in an altering pattern? Basically trying to get the cylinders closer to the corners of the room?

Do you guys think I should look into fabricating costs on these later this week? Or is this not something worth pursuing?

I was actually talking about making the "fractal" high frequency part (ala RPG) hemicylindrical instead of another QRD. Looking into it, the relationship is the same... cylinder of a certain diameter has the same LF cutoff limit as a QRD with that period width. So no better than using QRD (worse, as far as periodicity problems, though perhaps with more HF extension which I think is really a non-issue since it can be tackled with "rock-wall" texturing).
Here's an example of a fairly broadband N7 in N7 fractal QRD design with usable frequency range from 150Hz to 7kHz. It isn't small, at 4' x 4' x 2', but other than the depth is certainly in line with other commonly used room treatment panel sizes. I think you'd find that depth will be required to do anything at all with 150Hz with either 2D QRD or broadband absorption. You can get by with a little shallower with an optimized 1D QRD panel. And of course diaphragm or hemholtz resonators can be much thinner but are effective only to target specific modal issues, not offer broadband shaping.

With some textured surface, this should cover from 150Hz to 20kHz. There's a "gap" from about 1kHz to 2kHz where there isn't pure equal energy diffusion but there is scattering. Right at 1kHz this is the least effective... nearer 2kHz it will be very close to pure diffusion. If you were to say cover an entire wall with this type of thing, I think perhaps a decent solution would be to create another 2D QRD solution that is 1/4 the LxW of the larger pattern (so 2' x 2') which should have very good coverage of that "gap" region, and if you really wanted you could put tiny fractal high frequency QRD panels in each well to give response up to a much higher frequency. Then by using 4' x 4' and 2' x 2' panels you could create a tiled pattern that covered the larger wall surface. Personally, I don't think I'd bother.

I also wonder about using something like this and then filling certain wells with dense skinned and unskinned (depending on frequency) fiberglass like FRK OC703 or OC705 to provide absorption at specific frequencies. This would monkey with the phase steering but I have no way of knowing right now by how much. If wells chosen were dispersed throughout the panel/wall, it may not cause any significant problems. ? What it would offer in return is the ability to select wells of specific depth (both in large and small patterns in the nested design) to damp those frequency ranges and thus give pretty fine grained shaping of the room's RT60 over a broad frequency range.
And yes, I do realize that cutting some 2400 blocks of whatever would be quite some undertaking... or multiples of that if you covered a larger area.
Bigus-

That is some cool stuff. If you can figure out an easy way to pull it off let us all know.

Erich-

I honestly don't know how many people would fork over even a small amount of money for these. You'd probably get more traction over on the Dedicated Theater board. There is much more interest in room treatments there. You might also get some traction on AudioCircle with the tweakers.

It would probably be worth a shot especially with the pub you are getting with the WGs and flat packs.

I would ask for an estimate on a 1D which should be very easy, and a 2D which might also be doable. Both should satisfy the DIY crowd...hell simple 1Ds are sold by RPG and GIK to pros studios.
Quote:
Originally Posted by warpdrv

starkiller wins - with the double benefits !!

Best design and implementation award !!!

Thanks.
Well, I guess it's worth getting a price. Never know, it might be really cheap. I aiming more towards getting the DIY crowd cheap stuff that we design, but no reason others can't take advantage of it.

I'm going to contact the company mid week just to see what they say. What kind of sizes make the most sense? I'm thinking it would be better to do something like 18" x 24" because shipping might be cheaper than 24"x24". 12"x12" seems a bit too small and odd shaped if these are going to be 5"+ deep.
Quote:
Originally Posted by Erich H

Well, I guess it's worth getting a price. Never know, it might be really cheap. I aiming more towards getting the DIY crowd cheap stuff that we design, but no reason others can't take advantage of it.

I'm going to contact the company mid week just to see what they say. What kind of sizes make the most sense? I'm thinking it would be better to do something like 18" x 24" because shipping might be cheaper than 24"x24". 12"x12" seems a bit too small and odd shaped if these are going to be 5"+ deep.

For a 2D, from what I understand, it will need to be square. A N23 4" deep 2D looks pretty good to me. That would be a 23"x23"x4" piece.

If you need something like 18x24, you would need to do a 1D, but with 1D diffusers you typically want to stagger them on the wall and 18x24 won't allow this. 12x24 would since you could basically make your 24x24 pieces. Actually it would be 11.5"x23" for a N23 4" deep 1D.

I'd be thrilled with either setup since I have a fair amount of wall space I'd use these on and going commercial would cost a fortune. I would consider cutting 1" foam insulation myself but it is low on my long list of DIY wants.
Would expanding foam work? Thinking why not build one form and then fill it up to make each panel. There has to be something it doesn't stick to to use as a barrier.
Excuse the legs Lol. Check out this angle. That's a 48"x48" "Balloon" above the listening position. A spectral decay plot is the only way to really tell if things are balanced......I think. I made these for pennies. Like I said in the other post where my plans are, tedious at best......

.....of course that's a panamorphic lens in front of the projector....it's CIH or die ! CinemaScope rocks !

kg, looks like you live in the Matrix and have a pixelation problem
Nice. What material? I don't really see a qrd pattern in those panels. Looks random. Is it? I'm sure you get good diffusion but I suppose the whole equal energy lobes goes away. Not sure how big a deal that is since I see very few people implementing multiple repeated panels.

As for the above poster, a closed cell foam would work fine especially if coated with a sealer or painted for higher frequencies but would likely be ineffectual at bass frequencies. Anything made with a one or two inch block would probably be fine.

As for construction ideas for my fractal pattern above, if I were to pursue it I'd look into having the small pattern injection molded or perhaps cast from a pattern. Assemble the large pattern as open fins and glue the all patterns at the correct height in each well. Flip over and pour full of foam, beaded epoxy, bondo, something. At least that's my off the cuff idea long in advance of actually considering constructing one.
For those that are interested in making a mold to do several of these you may want to check out this site http://www.freemansupply.com/

I've used a lot of their products for making molds for carbon fiber stuff. They have some nice video tutorials as well.

Think I'll just stick with wood as it would be the easiest but heaviest without it being machined.

I'm still unsure where all these should be placed in a rectangular room going by one of the prefered ratios.
Quote:
Originally Posted by Bigus

As for construction ideas for my fractal pattern above, if I were to pursue it I'd look into having the small pattern injection molded or perhaps cast from a pattern.

The mold needed for injection molding a part that big would be approaching or surpassing \$100,000 just for the aluminum mold. No kidding, maybe even a lot more than \$100k.

I think this would have to be done with styrofoam. Casting it in any type of plastic would be very expensive. Even if you made a silicone mold, the mold would only cost about \$500-\$600, but filling that with any type of plastics would be well over \$100 each.

If you want to help figure out the best mathematical design, I'll see what the foam guy will do, that's of course if he can do anything for us. I'm hoping he can.
Of course I was talking only of the smaller nested panel which is less than 7 inches square. I have the ability to machine my own mold (well, have it done cheaply for me as a favor) and have some experience in mold design. This still would not likely be the most economical alternative even if molding a couple of hundred of them. Finding time on an injection setup would not be trivial either.

If casting, I see no reason to use plastics when there are many more suitable options. On the cheap and heavy end, even plaster would work well. A fill lightened resin would be a good choice. For this freqeuncy range a pourable foam would be fine but as each is a well plate fir the low frequency pattern each casting either gas to be effective at reflecting low frequencies or you'd have to fill the back side of the wells in the large pattern with a much denser material. Hell, I've investment castes stuff from aluminum before but that's pricier than resin and this shape isn't the best for investment work.

My swag is that each small panel could probably be cast in resin for ten to twenty bucks.
There is a good BBC paper on a modular skyline diffusor like those shown here. Keep in mind that some of the "diffusers" shown in here are actually not diffusers. A diffuser will take sound from any direction and break it up so the reflection occurs evenly in all directions. A curved panel simply causes high frequencies to bounce off at different angles, but the reflections are still specular in nature. There is a video on the real traps website where they put a mic in front of different surfaces to show how they sound.
I think all this is well and good, I like my diffusers, but aren't we creating an answer to problem yet presented. No two rooms are alike, thus we need a problem, then we can create a solution.

First you need to get some measurements of a given room and while we are headed in the right direction we still don't have a problem yet.

What would really be a solution is to develop a frequecy specific program to create a specific solution in regards to diffusing sound.

The picture of the bottles on the shelf is funny , but an example of a partial solution that may just require a small tweak in another frequency range. By plinking down a broadband diffuser may cause real problems.......

Now, what method to detecting problems are we gonna use
Quote:
Originally Posted by kgveteran

I think all this is well and good, I like my diffusers, but aren't we creating an answer to problem yet presented. No two rooms are alike, thus we need a problem, then we can create a solution.

First you need to get some measurements of a given room and while we are headed in the right direction we still don't have a problem yet.

I'm not really sure that is the case. For absorption, absolutely since each room is different, has different modes, responds differently to treatments, etc.

For diffusion... it's almost a "too much can't hurt" thing. I'm sure there is a point where it might, but let's at least think about what we are talking about. Diffusion scatters sound from surfaces... that's all. It has the effect of reducing modes by not allowing the standing waves to form properly, and this is always a good thing IMO. And it also has a small effect on lowering RT60, which IIRC is secondary to the reduced ringing at modal frequencies.

But it doesn't have a radical effect on RT60, so you're safe in applying as much diffusion as you want to any room without mucking up global response and behavior of the speaker/room system. And it reduces modes at all frequencies at which the diffuser is effective, and again I can't see where you would cause problems with having as much as you can fit.

The only room specific part is exactly where you mount them. If you believe Toole's latest book, and want pure reflection and not absorption or diffusion at lateral first reflection points, then don't place diffusers there. Pretty simple. Anywhere else in the room, if you have the space on the wall, I can't see how diffusion the reflected sound would ever be worse than allowing it to reflect directly.

Anyone else have any thoughts on this? I guess you could say that some rooms due to size might benefit more or less from diffusers reaching down really low, but they are so big I don't think they would ever be designed into a "one size fits all" approach. My 4' x 4' x 2' design only goes down to 150Hz, and I think we can all agree that all rooms would benefit from modal prevention down to at least that low.
Quote:
Originally Posted by kgveteran

I think all this is well and good, I like my diffusers, but aren't we creating an answer to problem yet presented. No two rooms are alike, thus we need a problem, then we can create a solution.

First you need to get some measurements of a given room and while we are headed in the right direction we still don't have a problem yet.

What would really be a solution is to develop a frequecy specific program to create a specific solution in regards to diffusing sound.

The picture of the bottles on the shelf is funny , but an example of a partial solution that may just require a small tweak in another frequency range. By plinking down a broadband diffuser may cause real problems.......

Now, what method to detecting problems are we gonna use

KG, I'm curious what methods you used to determine your application of diffusion.

I do agree that no two rooms are identical but many are similar, especially for HTs and studios. Some generalizations can be made which will allow generic tools (such as diffusers and absorbers) to work in many applications. Ideally, each solution would be based on the unique room's empirical data and subsequent analysis and the end result would be completely custom acoustic tools for the situation.

RPG, Real Traps and GIK seem to do well selling the more generic tools. I'd like to see if Erich could produce some DIY friendly diffuser products. They wouldn't need to be as polished, robust or tested as the commercial offereings and thus less expensive. Most likely it would be a simple QRD. The implementation of QRDs is quite well documented and I think many people could put them to good use.
Quote:
Originally Posted by kgveteran

I think all this is well and good, I like my diffusers, but aren't we creating an answer to problem yet presented. No two rooms are alike, thus we need a problem, then we can create a solution.

First you need to get some measurements of a given room and while we are headed in the right direction we still don't have a problem yet.

I know I addressed this earlier, but after some thought I think I can do it a bit more simply:

1) All rooms have wall/ceiling/floor surfaces that have the common problem of specular reflections. You can absorb or diffuse them. Absorbing requires knowing the response before and after to know what you have and what you have done. Which leads to...

2) Diffusion does not significantly alter the measured response except that if operating in a low enough frequency band will reduce ringing from standing waves. What measurement do you propose would be necessary to know what "problem" is being addressed other than number (1) above, and what change in that measurement would signal that you are doing good and not harm to the room's acoustics by adding diffusion?

I guess what I'm getting at is that rooms with hard flat walls are a significant departure from the ideal setting for good audio. Hard flat walls = generally bad. The only time that differs is at the lateral first reflection points depending on whether you agree with Toole or not. I personally think "wides" in a 9.1 or higher layout is a better way to address the issue he raises. That being said, anything you can do to mitigate the negative effect of hard flat walls is good. Adding as much diffusion as possible allows you to turn them from hard and flat to hard and non-flat. Great starting point, can be done nearly ubiquitously on every surface in the room that is practical (which is ultimately the limit). You can then start adding target absorption to deal with the mix of RT60, early reflection, and modal issues that remain. That is where measurements are most useful. I certainly don't think it would hurt anything at all to know what the bare room response was in multiple locations, I just don't see how I'd really use that information to change my plan on addition duffusion, which would essentially be to put as much as space allowed and was practical.

In fact, I think a great (but probably very expensive) product would be a replacement for traditional drywall that had a QRD profile molded in to whatever material was used. Make it in standard dimensions, say either 4' or 8' long and maybe 16", 24" or 32" wide to handle common stud spacings, constructed to allow overlapping joints that can be easily sealed and compatible with channel mounting systems. Perhaps the deepest wells could protrude somewhat into the joint cavity to keep the total profile depth down. Not an easy product to bring to market however as it would have to go through the process of being tested and approved for residential wall construction, they would certainly be very expensive - much more probably than drywall construction with diffusers added after construction (though you could potentially tackle issues of isolation, soundproofing, and diffusion at once). I suppose you could add standard receptacle and light switch mounting locations/options. i.e., this would be a very complex and expensive product to bring to market and I'd have absolutely no interest in tackling it. My point is that I think such a product could be used universally in any HT construction to great benefit in a very generic manner in every situation without having any measurements or analysis of the room before installation.

Hmm... maybe that wasn't a simpler way to address the question after all?
I'm planning to build some RPG BAD panel replicas for my theater. Unfortunately it will be probably a year before I am ready. The patent gives good detail on how to construct one:
http://www.freepatentsonline.com/5817992.html
Seems all that is really needed is a hand drill and a lot of patience. They don't require the huge depth of QRD or skylines in order to diffuse down to a reasonably low frequency.
I'll have to read the white paper on the BAD absorbers (and give more than a quick glance to the patent). From first inspection, it looks as if they don't offer diffusion down low but rather absorption, with the diffusion kicking in with frequencies having a half wavelength smaller than the digital element.

Is this your impression? Not a bad idea for a thin panel, but it doesn't do the magic for modes that a true LF diffuser does.
Quote:
Originally Posted by Bigus

Not a bad idea for a thin panel, but it doesn't do the magic for modes that a true LF diffuser does.

There have been several mentions of LF diffusion.

I don't get it; seems like required dimensions make them impractical for most people.
My room is a live end ( back of the room, seating area ). Front stage is dead. I use the terms loosely.

I made 3" deep first. They work, but I learned that more depth allows them to scatter at a lower frequency. That's why I went with 7" diffusers. You can actually hear your voice change as you walk from the back to the front of the room.

Absorbing makes the biggest difference, diffusing is pure icing on the cake. To keep it balanced bass traps really round things out.

I guess the simple term is ringing. Once it's gone you really notice how much of an issue it really is
Quote:
Originally Posted by noah katz

There have been several mentions of LF diffusion.

I don't get it; seems like required dimensions make them impractical for most people.

Depends I suppose on where we draw loose boundaries for where LF starts and what is impractical for most people. The panels I modeled above playing around are 2' deep and are effective down to just under 150Hz. There's a lot of modal action going on at 150Hz-300Hz in most rooms and breaking up the standing waves there I think we can agree would be of benefit. 2' may be impractical for many... useful behind a screen false wall, perhaps no where else for most people. Targeting 80Hz up would do wonders for most rooms, but with a depth of a few feet I don't think they would work for much of anyone... me included, even if I were designing a room from scratch.

Each room would have to have a serious look to see how deep you could afford to go with diffusion. Maybe most people wouldn't be able to go deep enough to affect the modal region... but for those who can I think it would be of great benefit.
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. Maybe it has no real benefit over simple absorption using bass traps. I honestly don't know. Bigus, throw one together, I'd love to see your results. 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.

Getting back on topic for Erich, I still think some styrofoam 1D or 2D QRDs 23x23 and 3-6" deep would be a nice offering. Basically RPG's most basic offerings. I think a 4" deep QRD gives predictable diffusion from about 800hz to 6600hz, is easily shippable and mountable. The home studio, 2-channel and HT guys would buy them up quickly if the price was right. They wouldn't have to be as "polished", durable, tested or cutting edge as the commercial offerings but if they could give 75-90% of the performance for 25-50% the price they would be popular. Even small pro studios would likely buy them.

BTW, RPG sells their 2D QRD Hemiffusor for about \$280 per 24x24x4" panel...and that is their polystyrene version. Commercial diffusers go up in price from there. You can see why they are fairly rare in HTs.
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