QRD "Skyline" diffuser build underway

Discussion in 'Listening Spaces' started by tomlinmgt, Mar 6, 2017.

  1. tomlinmgt

    tomlinmgt Lunatic Member

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    Decided to build another Skyline style QRD diffuser. The first one I built about a year ago was made from dense foam because I figured it would be easier and a lot less expensive than one built from wood. Turns out it was only a little easier and only a little less expensive. I think I have about $40-45 tied up in this latest wood unit with the foam unit coming in at a little more than half of that and build time, at around 4-5 hours, was about the same for both units. The wood unit is undoubtedly more attractive. And if you like the smell of cedar, well, there's that.

    The base is inexpensive pine plywood and pegs are 2x2 cedar which was around $4.5 for an 8' stick. The only thing with these cedar 2x2's (which I figured out AFTER I'd started making my cuts) is they can vary by as much as 1/4", so it's more important to get sticks that are square and of equal dimension rather than straight.

    This 13-root design was cooked up using the QRDude software and offers true diffusion from 900 to 4500Hz and scatters down to 450Hz...so a nice broad operating range with this rascal.

    Pegs sorted...

    image.jpg



    Populating the board for a mock-up..

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    Completed mock up...

    image.jpg image.jpg

    I'll start gluing them down next and then will trim the excess base material on a table saw. Finished dimension will be around 22"x22"x 7" (height of tallest pegs).

    - Michael
     
  2. bob06

    bob06 Addicted Member

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    Looks great. Does it matter what size room that would go in?
     
  3. Crestwood23

    Crestwood23 keep on chooglin' Subscriber

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    Very cool, I really want to make one of these! Where's a good place to hang it? On the front wall between the speakers or on the back wall behind your head?
     
  4. wgb113

    wgb113 Well-Known Member

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    Nice work! How much does it weigh and how will you affix it to the wall?
     
  5. tomlinmgt

    tomlinmgt Lunatic Member

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    Thanks for the compliments, guys. Answers to your questions kind of overlap since there's a bit of info one should be aware of regrading the implementation of diffuser devices in the listening space, so I'll just start spewing what I know and somewhere in all of it you'll find your answers. The size of the room isn't so critical, but you do want to avoid locating the unit closer than about three feet away from the listening position because inside that range you can experience some weird sonic artifacts and anomalies. Also, if the device is located a good distance away from the listening position, say twice as far or more as the distance between the speakers and listening position, you likely won't hear much of its effect...especially if it's just a single, smallish unit. Diffusers work best when several units are clustered or arrayed, so one of the dirty little secrets about diffusion is that it takes a lot to start having a noticeable effect..unlike absorption which shows immediate and apparent benefits when located at early reflection locations. Finding the best location for a diffuser (or a cluster of several) depends on several factors, as a general rule you want to try to locate them on any flat, hard surface areas that are within close proximity to the early reflection locations, but not necessarily at an early reflection location (where absorption is usually the better choice). Rear walls are a great location for them if the seated listening position isn't too close to the wall and the early reflections on that surface are treated with absorption. Along side walls at about head height when standing (or higher) is also good. On the front wall directly behind dipole or bipole speakers is a very effective location for them, and on the ceiling in an area that's generally above the listening position is also good.
    The important thing to understand when addressing room acoustics for hi fidelity playback is that you want the listening position (LP) to be in a reflection fee zone (RFZ), which is an area that's free of early reflections (the bad kind). An early reflection is sonic energy that's reflected directly toward to the listening position from hard, flat surfaces and arrives there too quickly after the arrival of the primary sound wave coming directly from the speaker. Early reflection locations can be found using the mirror method,

    http://www.audiokarma.org/forums/in...means-why-its-importat-how-to-find-it.446798/

    ...and the general rule in determining if it's an early reflection is to take the distance from the speaker to the listening position and subtract that from the sum of the distance between the speaker and the early reflection location and the early reflection location and the listening position. If that distance is less than around 17' it's an early reflection. For instance, lets say the distance between the speakers and LP is 10'. Then lets say using the mirror method you found an early reflection location on one of the sidewalls and it's 7' from the speaker and 9' from the LP....so 7+9=16
    So you take 16 (distance from speaker to early reflection location + early reflection location to LP) and subtract 10 (distance from speaker to LP) which = 6.
    6 is less than 17 so that spot would be generating an early reflection. If the spot you found with the mirror method is say 14' from the speaker and 15' from the LP and the speaker is 10' from the listening position then you'd have 14 + 15 = 29. 29 - 10 = 19 and 19 > 17 so that would be a late reflection...which is ok. I like to put diffusion anywhere in an area that's in close proximity to where the distance delta is near 17 (but always greater than 17)...so filling in the gaps between absorption a little ahead of, even with or in areas behind the LP are good.

    Not sure what the unit will weigh yet. I'll definitely get that answered once it's completed.

    My last listening space was a mix of A LOT of absorption and some diffusion in a nearly square room. My new listening space is larger and without as many parallel surfaces so it doesn't generate quite as many early reflections as the prior space. In addition to that, it's my living room (last space was out in the garage), so I'm conscious of the visual aesthetic of the treatment devices I'll be using. I think diffusers have a great deal of visual appeal, so that combined with the fact that I don't have a lot of early reflections has me wanting to try a lot more diffusion for this current room I'm using. I'll be building more of these as well as some other different diffuser types.
     
    Last edited: Mar 8, 2017
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  6. Crestwood23

    Crestwood23 keep on chooglin' Subscriber

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    Very informative - thanks!
     
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  7. unfairlane

    unfairlane AK Subscriber Subscriber

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    A lot of work. I use some bigger wood-blinders, makes adjustable & decorative diffusors, wery efficient too ;)
     
  8. tomlinmgt

    tomlinmgt Lunatic Member

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    LOL. Yeah, it is work. Not sure I'd call it A LOT of work, but that's subjective, I suppose. And for me it's a creative endeavor that results in something acoustically useful and aesthetically pleasing (I suppose that could be considered subjective as well), so there's an element a fulfillment I attach to my diffuser builds. I tend to see diffusers as a statement piece, as well. You see properly constructed diffusion devices in a room and you know there's probably something fairly serious with regard to sonic quality going on there...could be recording or playback, doesn't really matter either way...somebody knows what's up and one can probably safely bet a positive sonic result is at hand. No, diffusion isn't for everybody...and is probably viewed as over the top by most in the hobby. But it's earned its place as legitimate science and is a powerful and useful tool for those who elect to optimize acoustics in the name of better fidelity.

    I see this comment a lot..."my blinds are used as diffusers"...which is technically incorrect. One thing I think is worth mentioning is that there is a distinction that should be understood between true diffusion and simply scattering sonic content. For example, irregularly shaped knick knacks/decor items, bookcases with (staggered) books/cd's/album covers, vertical or horizontal blinds, etc are not "diffusing" sound/sonic energy in the true sense of the term...at least as far as acoustic science is concerned. Those types of items are "scattering" sound energy...which lies somewhere between a flat reflective surface (not good) and diffusion (very good). I'm generally not so hung up on it, but if you truly want to realize the sonic benefits of actual diffusion...which is defined as controlled, even and predictable diffusion of sonic energy in the time domain over a given spectral range determined by the design of the diffuser device...it takes a carefully constructed device (could be any of several types) placed in the appropriate location. With scattering it can be more difficult, if not impossible, to predict exactly how the reflected energy will arrive at the listening position. One could inadvertently scatter spectral content in such a way that a good bit of mid range energy is scattered away from the listening position while a good bit of high frequency energy is directed right at the listening position, which would probably not produce a positive result. Also, if all the surfaces are exactly the same dimension and in a repeating pattern, like blinds, a situation called periodicity is created and this is a sonic artifact that can have a negative effect onr fidelity. If all the blind slats are positioned in way that directs sound away from the listening position one is probably ok. But if they direct sound energy into another reflective surface that then directs it to the listening position it could be creating a rogue early reflection.
     
  9. unfairlane

    unfairlane AK Subscriber Subscriber

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    tomlinmgt ; I did some research on this some decades back and was surprised to see that a simple profile looking like a half-closed/open blinder was the most effective way to cancel standing waves. A lot moore efficient than the type of profile you`re building and a lot better than any known absorber.
     
  10. tomlinmgt

    tomlinmgt Lunatic Member

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    You should probably share this finding with acoustic engineers and recording studio designers around the world that they're doing it all wrong. ;)

    And just to be clear, a diffuser does not cancel standing waves and isn't designed to do so. That's simply not its intended function.
     
  11. unfairlane

    unfairlane AK Subscriber Subscriber

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    Does not? Based on what experienses? This has been (a part of) my profession for quite some time and have came to learn a quite bit of what really does it and does not.
    Anyway, good luck with your woodwork.
     
  12. tomlinmgt

    tomlinmgt Lunatic Member

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    The most troublesome standing wave issues generally occur in the lower octaves of the spectral range, which is out of the operating range of most all diffuser designs. Low frequency standing wave issues are best dealt with by some form of absorption. That's all well documented information. I've looked long and hard for short cuts to effective acoustic treatment and you're the first person I've ever heard suggest that blinds can kill standing waves. We are talking about the same thing...right? You keep using the term "blinders" but I assume you mean "blinds"...such as would go over a window.
     
  13. unfairlane

    unfairlane AK Subscriber Subscriber

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    Absorbers is absolutely the last way out in my book. Some buildings might not be wery "playable" thogh, that leaves you with few choises.

    Different frequences will need different acoustic solutions. When I talk of blinders (yes I meand blinds, big ones) & standing waves it is primarely as a speakerdesigner but I do use them as a part of my room-threatment too, for everything but lowest frequences. Like the ones you are making up.
     
  14. tomlinmgt

    tomlinmgt Lunatic Member

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    Agreed on your first point, which is why I took issue with your statement that blinds will cure standing wave issues. I'm not sure what you meant when you said "like the ones you are making up". Standing waves at the lower frequencies are notoriously problematic when it comes to achieving balanced reproduction across the spectral range, so when you mentioned "standing waves" I assumed that's what you were talking about (since you didn't specify otherwise). Low frequency standing waves tend to be very problematic in most domestic spaces, so when "standing waves" are mentioned in a discussion about room acoustic it's generally regarding low frequency energy. I agree that blinds could be effective at redirecting mid range and high frequency energy and breaking up interaction between two parallel reflective surfaces, but don't agree that they will have much of any effect in the lower frequencies. So it appears we're really not in disagreement, we're just not effectively communicating our thoughts.
     
  15. Fitero

    Fitero Active Member

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    Very gracefully dealt with tomlinmgt.
     
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  16. djnagle

    djnagle Lunatic Member

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    Hey T. Looking good. When I owned the studio we had a handful of those for filming on location. We just moved from Michigan to new Mexico and leasing for a year. My system is in boxes so I am an ear bud guy for a while.
     
  17. tomlinmgt

    tomlinmgt Lunatic Member

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    Thanks D. I finished it up but haven't mounted it yet. I'd like to build a few more, but have just too much goin on right now. I saw the pics of your new place...looks great and welcome to the Southwest! Hope you and your wife are enjoying the change...there's some really pretty country out your way...and a good community of artist from what I recollect from my visits. Good luck in the hunt for something to call your own!

    I really enjoy building diffusers...all the different types. There's just something about them. Love workin with the wood, the creative process, execution, the aesthetic ..all of it. It's
    therapeutic and fulfilling. Once I get goin on one it becomes an obsession.

    image.jpeg
     
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  18. ateal

    ateal Well-Known Member

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    That's a beautiful piece of art right there!!! I hope you keep it natural cedar and don't paint it. How much does it weigh and what will be your method of fixing.
     
  19. tomlinmgt

    tomlinmgt Lunatic Member

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    I
    Thanks! There's no way I'd molest the natural look (and smell) of the cedar. Besides, my living room has natural cedar beams on the ceiling and a cedar fireplace mantle, so it will be appropriate in the context of the interior design scheme if left natural. It weighs about twenty pounds and I'll mount it using drywall anchors.
     
    Last edited: Apr 21, 2017
  20. dosmalo

    dosmalo T-Totaled Subscriber

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    Michael, all I've found at Lowes is either pine or treated 2x2s where do you find your 2x2 cedar?
     

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