[nathanm]

Somebody posted this awhile back in the Decware forum and I never really got a satisfactory answer, plus the company itself never bothered to answer my e-mail about these doohickeys.  Anyway, here we've got foam only it's cut in the shape of a diffusor with the flat nooks and crannies.  Now, I always thought that diffusors are supposed to be made out of hard, reflective material instead of absorbent.  Like for example auralex makes them out of polystyrene and RPG out of wood but these blokes are using foam.  So which is more effective?  I realize that even with foam there is going to be some diffusion anyway regardless of shape, but to me this particular method doesn't seem to make sense.

http://www.hsfacoustics.com/diffusion.htm

[Carlman]

I work for a company that can make something exactly like that but with the hard, closed-cell type foam.  I've been wondering if it'd work for sound diffusion... especially since I could get it fairly cheap.

It's a good question and I'd like to know as well.... if the closed-cell foam can be made to a useable shape for any sonic benefit, I'll be glad to start a group-buy for it.

-C

[8thnerve]

I work for a company that can make something exactly like that but with the hard, closed-cell type foam.  I've been wondering if it'd work for sound diffusion... especially since I could get it fairly cheap.

It's a good question and I'd like to know as well.... if the closed-cell foam can be made to a useable shape for any sonic benefit, I'll be glad to start a group-buy for it.

-C

You'd probably be better off just to make the front surface closed-cell, and make the rest open-cell.  That way you'll get the benefit of both absorption and diffusion.

[nathanm]

I work for a company that can make something exactly like that but with the hard, closed-cell type foam.  I've been wondering if it'd work for sound diffusion... especially since I could get it fairly cheap.

Reeaaaly....what's "fairly cheap" out of curiosity?  Can you beat $6.25 per square foot?  Auralex's Metrofusors are about $150 for 12 2' square panels.  Egads!  I guess it's not an atrocity, but geez a 4x8' sheet of polystyrene costs you about 20-30 bucks or something at Home Depot.  (then of course you need to buy a hot wire cutter for a few hundred.  Ugh.)

http://www.music123.com/Auralex-MetroFusor-Sound-Diffusor-i13296.music

I wouldn't mind just getting a bunch of 4' x 2" square bars of EPS which I could at least cut with my hot knife and make a knockoff RPG Skyline, maybe. (you wouldn't believe what they are charging for those suckers!  It's enough to make a guy run to screaming to egg cartons)

Can they make real dense stuff?  Cause to be honest on my last DIY diffusor project I decided that the big lumpy EPS was kinda lama even though it would do the trick.  Small pellets would be preferable to get a nicer edge I suspect.  Of course, getting all the nooks and crannies pre-cut would save oodles of work, obviously.

[Rob Babcock]

I've been mulling an idea over for quite some time:  Could you take some well-shaped foam and coat the shaped side with spray-in truck bedliner?  You can buy the stuff in buckets to spray on with a spray gun or in spray cans at hardware type stores.  I really don't know if it's comparable to the professional products that companies like Line-X use, but if so you could get a really rigid & reflective side on the outside and possibly leave the backside absorbant.  Does this seem practical at all?

I think if this approach works you could make knockoff Skyline-stype diffusers the same way with blocks of styrofoam.  You can cheaply buy 4' X 8' sheets of 'foam insulation up to 3" thick- this could be cut and assembled without a lot of work.  Then spray the sucker with bedliner.

Is this possible or absurd?

[8thnerve]

I've been mulling an idea over for quite some time:  Could you take some well-shaped foam and coat the shaped side with spray-in truck bedliner?  You can buy the stuff in buckets to spray on with a spray gun or in spray cans at hardware type stores.  I really don't know if it's comparable to the professional products that companies like Line-X use, but if so you could get a really rigid & reflective side on the outside and possibly leave the backside absorbant.  Does this seem practical at all?
...

Sounds like a pretty good idea if you can get the bedliner to adhere to open-cell foam...

[Carlman]

Reeaaaly....what's "fairly cheap" out of curiosity?  Can you beat $6.25 per square foot?  

We'll see.  I'm ordering some 'sample material' now to see if it even works.  I can't imagine it would be that expensive but, we'll see.  

I've got to get some material tested also for possible bass traps.  I was thinking a closed-cell foam sandwich type thing might work.  

-C

[satfrat]

As an example of what you guys are talking about, I picked up some of Furutech's RWL-1 Tuning Panels left over from CES that are made of hard styrofoam. It's basically 4 boards of a 24" x 45" box with a 3' depth. Inside is a one piece styrofoam that has 7 long rows of 1/2' thick styrofoam spaced 2 1/2' apart. The depth of these rows are like from 2 3/4" from each side to 3" in the middle as to create a bow. A tight covering of silk cloth is stretched over these rows to finish off with. I have 3 of these panels, one on each side of my main speakers for first reflection and one on the ceiling between the mains for the same purpose. Ceiling application is TOO easy because of the <6lb. weight. I have found these panels to do a good job of diffusing the sound and actually amplifying the soundstage that effectively hides my Lorelei's even more than they aleady have on their own. There really seems to be something working here with the combination of hard styrofoam and the silk cloth plus the design of these 7 long rows spaced 2 1/2" apart in varying depths. A picture and further description of this product can be found at www.furutech.com/magic1_1page.htm Regards, Robin

[Rob Babcock]

If it'll stick to a smooth painted truck bed, I'd think it'd stick to foam.  But I've never used the stuff, nor do I know how many layers it would take to build it up to where it's able to hold it's shape.  Has anyone here worked with the stuff before?

BTW, how do they make the Skyline panels?  I'm not sure if they're hollow molded plastic or if they have a foam core.

[BrunoB]

BTW, how do they make the Skyline panels?  I'm not sure if they're hollow molded plastic or if they have a foam core.

I just would like to let you guys know that the Skyline panels have a particular shape that follows some prime number theory for optimum sound diffusion. I don't understand the details but the theory is described in this book: Master Handbook of Acoustics by F. Alton Everest

The depths of the wells follow a prime-7 quadratic residue sequence. These moldings may be affixed to a wall butted together or spaced, hor- izontally or vertically. If ..."

Hope this helps.

[Rob Babcock]

I'm just talking hypotheically- would it be "immoral" to "reverse engineer" their panel?  By buying one you could measure it and make your own.  Of course, I have no idea how well they work in the first place.

I've got The Master Handbook of Acoustics, and while they purport to utilize "prime number diffusion", the basic principles should be possible to figure out on ones own.  Or at least by using a random pattern you should still get good diffusion.  The great concert halls of the world, the old classics, were all built without root number diffusion.

[nathanm]

Maybe if you reverse engineered their product, mass produced it, marketed and sold it maybe some lawyers would ring you up, but it'd be no more "immoral" than any other DIY project.  Hell, what if you had a bookcase that just happened to be arranged in the correct mathematical sequence?  Would lawyers bust your door down?  Heh!

I've seen pictures of some guy out there who already did it with the pink styrofoam stuff.  It wasn't much to look at, but it was simply foam blocks arranged and glued together in the same manner as the Skyline.  RPG's product is probably molded, but it's just styrofoam at its core.  Granted, with a few gazillion R&D dollars backing it up but hey, that ain't our concern!  I think you will find a number of folks who have DIYed this thing (let's face it, it looks darn cool) out there.  For instance I found one dude's setup on the 'ol athens audiophile club thingy page.  I am sure one could argue that getting the EXACT right heights for all the blocks is imperative, but I wouldn't be surprised if one's own random pattern got close enough for government work.

Actually what I thought would be really cool would be to make a giant-sized Skyline only with 12" album covers on the tops of all the blocks, taking up an entire wall.

I am still curious about the hard vs. absorbent issue if anyone's got an angle on that.  Thanks!

[Rob Babcock]

I guess I picked the wrong way to say that!   You're right, it wouldn't be any different than any other DIY project (eg the DIY mdf coatrack/candellabra looking diffusor doohickey some guy made).  

They may have had huge $ in R&D or they mighta read the same book and thought "Root diffusor" sounded good.  The way you describe making it with the pink foam is just what I propose- only you then coat it w/bedliner.  There may be some other plastic "magic shell" you could use but I don't know what it is.

The Big Brains of acoustics can definately chime in here anytime, but my gut feeling is you'd want it to be hard vs absorbant.  It strikes me that if it's absorbant (and styrofoam probably would be somewhat at some freqs) it wouldn't be uniformly absorbant.  I think maybe a LEDE setup, with a huge wall size diffusor in the rear might work pretty good.

[BrunoB]

Here is a PDF document from the BBC explaining the math and showing how to build a 2D diffusor.

[sch]

Hi!!! I went here : www.decware.com and bought the drawings for US$20,00 to make their wood diffusor...it is simply great !! I'll post pictures one of these days...
Santiago

[nathanm]

Yeah I really like Steve's design too - it would probably be the most visually appealing being wood and all.  But it's not like plans are 100% necessary as he's got a side view right there on the site.  All one crafty woodworker needs do is save off that pic and extrapolate the measurements from there.  Still, for $120 I'd much rather get the kit.  They look like they'd be rather weighty, though.  Not sure.  A whole wall of those with a nice stain or oil finish would be beautiful, I can't imagine a W having any AF qualms with that!

[JLM]

Seems like at higher frequencies (say 1,000 Hz on up) the soundwaves are smaller, more directional, and have less energy so that absorption (foam) and the typical diffusion products can work.  Catching them on the first bounce from side walls, floor, and ceiling seems to make sense.

Lower frequencies have large soundwaves, are less directional, and have more energy.  Overall room size, shape, and absorbency would seem to be the primary factors in controlling lower frequencies.  I still can't understand how the pressures created from these frequencies "collect" in the corners or how small pillows can help much.

[8thnerve]

 I still can't understand how the pressures created from these frequencies "collect" in the corners ...

Think of it this way.  The way that the sound wave moves is by equalizing with air molecules next to itself (itself being just the same molecules but with a higher or lower pressure (density) as compared to atmospheric pressure in that specific location).  So as it gets to a wall surface, it cannot continue to equalize in the direction it was going, and must now pressurize back in the direction that it was coming from more or less.  But now this pressurization is happening with molecules that are already pressurized with incoming sound, hence more overall sound pressure.  Now don't point out the inaccuracies, this is a very simplified view of what happens.  Think of a bunch of runners running to tag a wall, then sprint back in the direction they came from.  You will inevitably have a build up of runners at the wall given a fixed observed time frame.

Does this make any sense at all and is it helpful?

[Rob Babcock]

Bill Duddleston (of Legacy) once had an interesting observation:  he pointed out that people tend to think of sound waves as bouncing like the balls on a pool table or a laser hitting a mirror, and it does behave like that at times.  But he says it can also be viewed like a fluid, expanding and flowing dynamically.  That makes a lot of sense and kinda dovetails into what Nathan was saying.

[Ethan Winer]

Rob,

> it [sound] can also be viewed like a fluid, expanding and flowing dynamically. <

I'm not a physicist by any stretch, but I believe this is frequency dependant. That is, at high frequencies sound behaves more like a beam, and at low frequencies it bends around objects. Maybe someone better versed in physics can confirm or deny.

--Ethan