How to make Diffuser + Absorber?

[mtl777]

Hi, I'd like to find some detailed plans for building a combined diffuser + absorber in one. I'd like it to be diffusive for frequencies at or above 610 Hz and absorptive below that. Due to the small size of my room, I can't afford to lose too much wall, so the total thickness of the build must not exceed 8 inches.

Any help would be greatly appreciated. Thanks in advance!

Mitch

[knightfly]

Don't have exact calculations, still trying to find some helpful info - have you seen this?

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=1884

Closest I can come til I do bunches more study/research... Steve

[mtl777]

Wow, that looks pretty cool! Combined with OC 703, looks like it would work. Though with all those curves, it may be a huge effort to build. I'm eager to see what else you might come up with.

Thanks a bunch!

Mitch

[mtl777]

What do you think of this "Flatffusor" from RPG:

http://www.rpginc.com/products/flatffusor/index.htm

I could make a DIY version of this, with the thickness of the template and absorber modified to satisfy the requirement (diffusive above 610 Hz and absorptive below). Looks easy to build as the only major hurdle is drilling holes on a board. Any suggestions for the following?

1. Algorithm/formula to generate the 31x33 binary array of absorptive and reflective elements? (Or I could just copy the picture from the RPG site if that's too hard.)

2. Diameter of the holes?

3. What material and thickness for the template? (I think RPG uses 1/8" high density fiberboard. Is that readily available at Home Depot? Can I use plywood or hardboard instead?)

4. What material and thickness for the absorber? (3 pcf OC 703 or 6 pcf OC 705?)

Thanks!

Mitch

[mtl777]

I decided to do some guesswork and came up with the following plan:

For the binary amplitude template, it will be made of textured FRP wall liner panel ( see http://www.sdplastics.com/frpcolors.html ) glued on top of 1/4" fine grain MDF. The holes will be a little less than 1/2" diameter. The wall liner panel is a tough, non-porous material. I think that its textured finish as the exposed surface will help to randomize the scattering of sound waves.

Beneath the template will be 2" rigid fiberglass insulation 1.5 pcf (OC 701 or equivalent). Then put spacers to mount the structure with 2" air space from the wall.

This is all guesswork, of course. I have no idea if it will work and satisfy the desired performance criteria. This kind of thing requires lab testing that I don't have the luxury of.

Any comments before I proceed to build this? Do you think the textured panel is a good idea?

Thanks!

[knightfly]

Not sure if this is a good idea for a smaller room or not - seems like limited LF absorption, so I hope you have other plans for that part - smaller rooms have more bass mode problems because the modal density is less at low end of audible range than in a larger room, where lowest modes are sub-audible and within the audible range there are more modes so less variations in level. RPG sells these things, so obviously they want you to buy them - whether they are practical for most home sized studios I'm still not convinced.

This would definitely not be my FIRST priority for a small room, but if you have the time it could be a fun experiment... Steve

[barefoot]

Here's a design that John and I came up with called the Diffusive Slot Resonator.
http://johnlsayers.com/phpBB2/viewtopic ... 3&start=15

It's a combination of a slot resonator and a Schroeder type diffusor.

Thomas

[mtl777]

The DSR is a great idea. However, it will require more than 8" depth which exceeds the maximum I could possibly allow for my small room. Too bad, it would have been nice.

Even 8" is already stretching it too much. I really wanted less than 8", the lesser the better as long as it still works. That is why the RPG Flatffusor appeals to me--it is so thin. I just hope it works as RPG advertises. Then, with my DIY version of it which is a lot thicker, I hope to get deeper LF absorption. Anyway, I will also be putting broadband absorbers--the "Super Chunks" type--on all four corners of my room, which would hopefully take care of the LF just in case the DIY Flatffusor is not really effective in that region. I will also be augmenting that with 2' x 2' x 4" broadband absorbers scattered in a chessboard pattern on other wall areas of the room.

Mitch

[Bassist_]

This is a fantastic page that provides you with the ability to design your QRD with minimal math... heck the math is explained and then a nice little java applet does it for you. Play around with it!

http://www.mhsoft.nl/diffusor2.asp#calcul

There is also (one the same gents page) great design information of Panel Absorbers, Helmholtz Panel Absorber, Slot Absorber, Placing the Traps, and the (RPGish) Skyline Diffusor.

I hope this helps others. It's mana from heaven for me

Cheers
Steve

[AVare]

Great site Steve (Bassist_).

Thanks
Andre

[knightfly]

Guys, I've not checked the other calculators on that site, but I have emailed the guy 2-3 times in the last 8 months or so about the fact that his slat resonator calculator (look familiar?) is using the WRONG formula, as were the majority of them online last I checked. The correct one is the one I posted in the acoustics forum in February. At higher frequencies, the incorrect one isn't too far off; but at lower frequencies, it's off by an octave or so.

You may want to check the other calculators against a known correct source before blindly using them... Steve

[Bassist_]

Thanks Knighfly... I just downloaded a number of research articles (BBC and NRC) and was about to dig into those. Thanks for the heads up on those formula's.

cheers
Steve