Idea for Helmholz resonator/diffusor

Hello Thomas, me again. Ok, I only had a minute to look at the diagram, and then look at the equasions. HOLY COW! YOU explained some missing pieces of puzzle that I have been asking about for 2 decades. This concept is similar to some I saw on the web at an acoustics lab in England 2 yrs ago. They had some of the slots covered with a piece of 703(thats what it looked like. They also had hollow slats, with the rear open to a reflection panel, which was the bottom of the well on one side. Damn thats hard to explain. Inside the slats were 703 or some kind of absorber. Interesting configuration but totally experimental and not proven they worked yet. This was the basis for the Stylmark extrusion concept. So you can seperate the slats with a slot, and some of the slats were open to a reflection panel. Ha! Totally guessing at the quadratic sequence, but the shape of some extrusions lend themself to the concept. Even flanges for 703 in the wells! Whether or not it would work is exactly why I contacted you in the first place. I'll see if I can look the site up again and post the what do you call it, URL? :D If thats allowed here? Anyway, I know I'm in the wrong league here, and hate to show my ignorance in this field, so if things I'm referring to here are totally stupid, please say so, so I don't make a fool out of myself any more than necessary to find it out. :? fitZ

Yeah, let us see what you're talking about. Posting URLs is most definitely allowed here. :D Turns out the design methodology I described was both wrong and right. The depth is indeed tied to the width.... but it's more complicated than what I outlined. I'll post another detailed design method - only a completely CORRECT only one this time.;):D Thomas

Ok Thomas, thats cool. I'm still moving in and just got this connection up and running. Lot's to do but I'll get it up as soon as I get my other computer hooked up. I'll definetly be following your forum Thomas. From a total laymens viewpoint though :D I'm finally in a place to build my studio, but over time. I would love to throw some CAD stuff on here to illustrate my questions regarding the acoustics, but I have to get the software you, John and Steve were talking about first. Doggone, you know what they say about "teaching old dogs new tricks". I'm used to Autocad but I'll use the others for conversion. Although, I dig the way Johns drawings come out in Smartdraw, so I guess I'll have to woodshed this winter!. :D First hour, autocad(drawing), second, my guitar practice, third, Nuendo, fourth, Smartdraw, ffth, building the studio, ah, breaktime. :D Doggone I'm tired already! Let's see, we still have ......work on the shop, work on the house, work on the polebarn and oh yea.....bla bla bla here!! Oh , record a track of the tune I was writing before I moved :shock: Hmmmm, is it bedtime yet? fitZ

Yeah, I'm certainly up for "warm milk and cookies" - only, make it a cold beer, and those cookies better be shaped like a pretzel, and don't EVEN forget those little lumps of salt on 'em, and... :roll: :roll:

one question on the calc in the diagram. Is it important to tie W to m in calculating D? If the aim is to use say 2x6 lumber. W=approx 2, so the largest D would be 2x9 = 18 - which would be impractical.

wouldnt W + about 1.5" with a piece of 2 x material? that would make the deepest bit around 16" if you made "d" around 2-3" deep, right?

I think if you want to use 2-by lumber, it might be better to go with a narrower bandwidth diffusor. So, instead of 11, we use 7 as our prime number. The sequence is then: m = 4,3,0,2,2,0,3 A good value for s is 3/16" so: W = 1.5" + 3/16" = 1.6875" And if we make d = 1.5" the sequence becomes: D = 8.25", 6.5", 1.5", 4.875", 4.875", 1.5", 6.5" The diffusor will be effective from 1 kHz to 4 kHz. Thomas

Interesting design! Do you have any ideas on how to 'tune' the diffusion part of this unit? In Everest's Master Handbook of Acoustics, he goes thru QRD and other math-based diffusion calculations, and tells how to engineer the range of the diffusor by altering the sizes of the cavities. Do those same figures apply to the formula you proposed? Kase www.minemusic.net

Two birds with one stone huh? How bout three? :lol: Here is my brainfart. It'll take 5 replys to illustrate it. Here is the elevation. fitZ

Well, I don't know how to post multiple .gif's in one post, and I don't know why the bbs. won't accept more than one from me in 24 hrs. So here is the next installment. Plan section. Like I said though, brain fart. Drawing exercise only, but it was fun. 3 more to go, so hang in there. fitZ

barefoot wrote:

Below I've attached a generic design for a 10 slat primitive root sequence (prime root = 11). The depth D of each slat in the sequence is simply calculated by multiplying the unit width W by the sequence multiplier m then adding in a minimum depth d. The slot width s should be relatively small compared to the unit width. A slot width of a 10th to an 8th of W is probably a good choice. No need to vary s since the varying slot depths already make this a broadband absorber! :D Thomas

Hello Thomas, hello all, I'm wandering - after so many years - if I understand correctly this design. Suppose d= 0, W = 30 mm, s = 3 mm Each slat width would be 27 mm and : D1 = 240 D2 = 180 D3 = 60 D4 = 150 D5 = 0 D6 = 30 D7 = 90 D8 = 210 D9 = 120 D10 = 270 A full 10 slat sequence of the DSR would look like this (top view) : Dimension of a full sequence : Period width = 330 mm Deepest well = 270 mm That makes a squarish design, is that correct ? I've seen rectangular designs more often. When I enter this data in the QRDude software it says : "Period width of 330 mm is less than the design waveleng of 848 mm. Diffused energy will not be evenly distributed. Minimum well width to fix this is 78 mm. Any ideas ?

Everything you need to know about QRD's is explained IN DEPTH here: http://www.subwoofer-builder.com/qrd.htm fitZ

msikio wrote:

Hello Thomas, hello all, I'm wandering - after so many years - if I understand correctly this design. ...snip.... Any ideas ?

Hi msikio, I think you're missing the point of my design with regard to setting d=0. In this thread, prior to my design formula post, you can see I also posted a drawing describing the overall concept: You see the design is both a diffuser and a resonator. There is a cavity behind the slats. So it doesn't make sense to set d=0. You want d to have some minimum depth so that you have a full array of slots for the resonator. D=30 might be a good value in your design. Make sense? Hope this helps! Thomas

you are also not restricted to just a primitive root sequence. you could use a quadratic residue or maximal length sequence as well. using normal lumber sizes as your largest (or rounding as well) you can simplify the construction. conceptually, you could also (assuming you like cutting lots of wood) vary the width as well as depth.

Beautiful drawings Glenn! I assume you're using sketchup? How to you get those dark corner hatches?

thanks! you can take any style and edit it to increase the corners - the one that comes with SU8 is the architectural style. this one is a customized one based on the "technical endpoints" style found on sketchucation. on the edit tab, select the endpoints and extension checkboxes and set the size.

and if you want to experiment further... using plywood or 2x lumber cut to form the shape and kerfed plywood or mdf, you can create a set of wave panels (somewhat akin to what fitZ had posted earlier). you can download this model off the Google 3D warehouse. basically a single wave form based on QRD p=7 and just inverted or flipped (total of 4 combinations) so cutting it out of some 3/4" plywood for the form should be pretty easy. glue and tack on the 5'x7" strips of kerfed material. attached to 3/4" plywood side panels - spaced about 1-2" between them, and back with 2-4" absorber covered in cloth.

Wow! Long time since I've seen you post Thomas. How are you? Nice models Glenn. To the OP, you can also use fractals...this is a "diffractal" or you could use a 50/50 Skyline/absorption or you "could" use binary absorption. or then again you "could" do the opposite..a binary skyline type diffuser with absorption or yet..how bout something like this.. or this... you could save yourself a lot of trouble and time...just build a plain ole Poly :mrgreen:

Hi Thomas - congrats on your speaker success. I've completed 4 studios recently with your speakers as mains. :D http://www.johnlsayers.com/Pages/Ellamy.htm http://www.johnlsayers.com/Pages/Gorini.htm there's one nearly finished for Pete in Perth Australia and 112F in Taipei, Taiwan for Zen who intends to send me the finished pics but he's been booked out since he opened and hasn't had time to scratch himself. Everyone loves their Barefoots :thu:

cadesignr wrote:

Wow! Long time since I've seen you post Thomas. How are you?

Hi Fitz, I'm doing well. Hope you are too! Yeah, I don't have much time to post anymore. Even on gearslutz I'm pretty much limited to just fielding specific questions about our speakers.

John Sayers wrote:

Hi Thomas - congrats on your speaker success. I've completed 4 studios recently with your speakers as mains. :D …. there's one nearly finished for Pete in Perth Australia and 112F in Taipei, Taiwan for Zen who intends to send me the finished pics but he's been booked out since he opened and hasn't had time to scratch himself. Everyone loves their Barefoots :thu:

Hi John, Thanks! Looks like you've been doing well too. Some of our customers have sent us similar pictures or posted images of their studios that you designed. They look fantastic! It's great to find that our work can meet up even half a world away! Thomas

yeah, but can you soffit mount them? :wink:

wow Glenn - care to share your model of the barefoots :)

check you pm... i'm awaiting on some details from Thomas and once i have them i'll re-adjust as needed and re-send them.

yeah - there's also a set of legs that sit them horizontal over your desk.

yeah, i just got the info so i should have something shortly... i figured out how to soffit mount the barefoots... you need to have the MiniMain 12's :twisted:

haha - you are just showing off your sketchup skills coupled with a fancy render program. :) I never got into the fancy render programs as I don't need to dress up my designs to convince the clients. 8)