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I want them for my ceiling first reflection points, so wood is not an option (I rent). I'm thinking styrofoam is my only choice. The ploblem is that you can't just go to Walmart or Hobby Lobby and pick up a thin 2' x 2' sheet or 2" x 2" bars; they sell no pieces big enough out of which to cut the former, and it would take some precise cutting skills/tools to cut the latter from the blanks they do sell.Is anyone familiar with an online retailer that can supply these? I found one that will cut to your specifications, but they require a minimum $250 order.
Along those lines, how many folks here would be interested in a co-op purchase? I haven't done the math yet, so I don't know how many of us it would take to reach the $250 mark. I suspect it would take quite a few, because the pieces I required to make two diffusers cost less than $10. E-mail me if you're interested, and I'll organize this madness.
May the bridges we burn light our way....
Follow Ups:
Do yourself a favor first and hold a match to a piece of styrofoam. I'm not sure what'll happen (will it melt or will it burn?) but first you may want to ask yourself if you want whatever happens to occur, in large scale, on your ceiling if there is a fire. If you have any of those pole lamps it may not be a bad idea to stick a piece of styrofoam over one of them to see what happens (imagine re-arranging your room, moving a pole lamp under the foam, and accidently leaving it there for a few minutes. Pretty silly reason to burn a building down).I think you'll find there are a wide variety of foamed plastics available and all of them have very different mechanical, chemical, and (most importantly) acoustic properties. Finding the right foam will take some trial and error.
May I suggest trying absorption instead of diffusion? I know, I know, too much absorption is a bad thing... but it's soooo much easier than diffusion and more likely to be successful. You could modify Jon's exellent absorber panel designs. Just remove the wooden frame and make a big fiberglass/polyfill/burlap pillow to hang on your ceiling. Again, fire risk will be a concern so fireproofing the burlap may be a good idea. But you'll likely have more success at a lower cost than a home made skyline diffusor.
Styrofoam is not that 'solid' at lower audio frequencies, and it can actually become acoustically opaque at fairly high frequencies.This means that should you try to extend the depth past 6"-7", to get a wider bandwidth, it will not work. In fact, depending on how stiff/dense the foam is, it may not work down to the nominal depth of 7" (which is roughly working down to about 500 Hz)
Another consideration, is that the bass in the room can and will shake the styrofoam, causing the HF's to become modulated as the styrofoam moves.
This has been cured on DIY diffusors by coating the styrofoam with some epoxy, thick paint, etc. to help further stiffen and damp the styrofoam 'sticks', both in terms of the surface, adn of the 2" by 2" sticks moving relative to each other.
Finally, for the maximum performance out of a given DIY attempt, do not cut the ends facing out into the room off even, but rather, slice them at a slight angle (say 20-25 degrees), and rotate the angles from square to square to make them more effective (random) at HF's.
The depth of the RPG design is 6 3/8 inches. They claim a diffusion coefficient of .71 at 125Hz. I don't know what that means, but the diffusion coefficient they claim for the entire specified frequency range is not too distant (I guess) from the .71 figure. If it would take more than 7" to get lower than 500Hz, is RPG fudging the specs? Are they measuring an irrelevant factor? Perhaps you don't share their definition of effective wave-breakup?It's very curious. See my post below comparing RPG with the online diffuser calculator, for additional confusion.
Thanks for the slice angle advice, Jon. Done carefully, that would look really sweet. I was also thinking of using different colors of paint to add a pointilist desgin of some sort to the diffusors. If I knew how, I would cheese out completely and effect two eyes that follow you around the room!
May the bridges we burn light our way....
I can assure you that at 125 Hz, there is no significant actual diffusion going on with a depth of 7" or less.Perhaps the measurements included some anomalous effects at low frequencies, but my own real world experience as well as my understanding of theory tells me that it takes more than a 7" depth to truly diffuse 125Hz.
Have you seen pictures of the difussers inside the opera house in Hong Kong. I would post but I saw them before I converted to digital Very large wooden difusion panels. Beautiul on their own. Cover large portions of the interior walls Done up in gorgeous wood.
I went to the site (Universal Packaging) and used their calculator. My head must have been up my... well, you know. It would be more than $10 for two diffuser's worth of foam. However, not by much.I remember the longest bar on the skyline was 7" (see above for the accuracy of my memory). Using that length for each of the 144 "skyscrapers" on each diffuser (way too much, but a distinctly conservative way to estimate this), I calculated $24.30 for two diffusers made from 1 lb foam and $35.50 for two diffusers made from 2 lb. foam. I didn't calculate for 1.5 lb foam. The figures include the cost of two 1" thick 24" x 24" bases.
So, assuming a maximum height of 7", 2" x 2" bars purchased in 3' lenghts (gotta cut them ourselves somehow), the total styrofoam cost to build two diffusors would be between less than $24.30 and less than $35.50. Not bad, huh? That means it would only take probably about 15 of us to reach the $250 minimum purchase.
Sorry about the rough calculations and shoddy approach. If we can get enough interest in this, I can do exact calculations based on the actual dimensions of the diffusers and the foam density we can all agree on. I have the Master Handbook of acoustics, if we wish to use the designs there, or we can just mimic the skylines.
Again, please e-mail me if you're interested.
May the bridges we burn light our way....
This little ditty is extremely useful. All you have to do is pick your upper and lower frequency limits and it will tel how wide and long your diffuser will be, and also exactly how many pieces of each length are required.It provides a numbered grid so you know exactly where each piece belongs.
Hope this helps and let me know when you are ready for a group order or where everything can be bought, 'cause I'm all over it.
BTW, the main website has plenty of other useful acoustic calculators, check it out.
According to their website, the max height (from mounting platform - itself 5/8" thick) of the individual bars is 6 3/8 inches. As calculated from RPG's web-published specifications, each bar is 1.8854 inches square (a little over 1 7/8 inches). The RPG website seems to claim similar diffusion accross a frequency range that includes 125Hz and 4000Hz. The online diffuser calculator requires bigger squares and much longer bars to acheive diffusion (at what coefficient we aren't told). Is this due to the different designs or is RPG using additional design or special material?
May the bridges we burn light our way....
If the top of each piece is angled slightly, with an appropriate orientation of angles, then you would get a bit more effective diffusion than you would with flat tops on each piece. I think the calculator assumes flat tops and I don't know whether the RPG product has angles.In any event, the figure the calculator gives is theoretical and I suspect that the RPG figure is an actual test result. Acoustic tests sometimes return odd results. Theoretically you can't have an absorption coefficient greater than 1 which represents complete absorption but some materials when tested give results greater than 1 due to the effects of things like edge diffraction. There can be some odd things come out in the results of acoustic tests and the RPG quoted diffusion coefficient may "benefit" from such oddities.
I'm interested.. Was just thinking along the same lines how to diy
diffusers. Seem kind of pricey for what they are, but i guess for
audio they might be cheap.
I got an e-mail from another inmate, wherein I learned that Home Depot sells large, blue styrofoam blanks. He did the math and figured out that it would still be cheaper to buy online, so if I get enough names we're doing this! I only need two or three diffuser panels, but I didn't think about the possibility that others might want more or less. As such, please e-mail me confirming your participation and the number of panels you would be in for.If we don't get enough participants, I'll get the sheet from Home Depot. Then the issue is how to make accurate cuts. It looks like heated nichrome wire is the standard tool, but how do you get the wire to cut along a straight line and at a 90 degree angle to the foam blank?
May the bridges we burn light our way....
x
A bandsaw works very well for cutting styrofoam.Perhaps you could find someone in your area to cut it for you.If you use a hot wire cutter,you could make a guide,a few pieces of wood screwed or clamped together would do it.
I'd like to put together at least two, may be fourThat is, if the numbers are right
I sent you an E-mail
Julien
"There's someone in my head, but it's not me"
How about one of those foam mattress pads with an "egg crate" texture?
To build replicas of the skylines, the base should be stiff. Those mattress pads, in my experience, are too flimsy.If you're thinking about using them alone as diffusers, I don't think they would sufficiently break up the soundwaves. This is because the bumps are of uniform height and shape, and the material is not reflective enough. I assume they would provide a discernible amount of abosorption, however.
May the bridges we burn light our way....
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