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Rudy81

Poly-Cylindrical Diffuser and LF Absorber

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Time to dive into improving my room acoustic treatments.  Previously, I had built a large Quadratic diffuser which is on the back wall of my HT.  I also built six deep absorbers which currently hang at the first reflection points in the room.  That discussion can be found here:

 

Recently, I have been delving into the use of Poly diffuser/absorber devices to be used at the first reflection points rather than just high frequency absorbers.  The theory is that such poly devices will help keep the liveness of the room, maintain a wide soundstage and also absorb those very difficult to control low frequencies.  So, I have built two large polys to see if they actually work. 

 

The build is a constant radius cylindrical surface that has absorbent ultratouch insulation on the interior surface ( I don't like using fiberglass).  The back of the frame has a heavy limp mass (1lb. per sq. ft. vinyl) that will work to absorb the LF. Based on everything I have learned about diffuers and absorbers, this should work well.  I spent quite a bit of time re-reading the Master Handbook of Acoustics and this device seems to fit with the theory presented in the book. Initially, I plan on just using my ears to listen to any changes.  If they seem positive, I will take some room response readings.  Should the devices work, the plan is to build poly diffusers of different diameter sizes for the back wall as well as the first reflection points. 

 

I also plan on re-building the deep absorbers I have and implementing the limp mass concept to improve LF absorption. This should be fun.

 

 

UltraTouchFill_SM.jpg

Frame_SM.jpg

PolyInterior_SM.jpg

LargePolyRaw_SM.jpg

LimpMas_SM..jpg

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Very interesting project. I have a few questions, but they will probably get answered as you post more pictures and descriptions as your work progresses, so I will be paying attention.

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Feel free to shoot questions.  You might help me with ideas!

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Your well-built diffuser/absorbers look to be about 3 feet tall in your pictures. Will they sit on the floor, or is it recommended to place them at mid-wall height? How far will the mass loaded vinyl back side be spaced away from the wall?

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The height is actually 4'.  I plan on sitting them on something for testing so the mid point is the same as my Oris drivers.  Should this work well, I will set them up as a column, along the wall height, two stacked one on top the other.  Probably overkill, but never been know to do things less than that.

 

The MLV will end up about 1.5" from the wall surface once hung on a french cleat.

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3 hours ago, Rudy81 said:

The MLV will end up about 1.5" from the wall surface once hung on a french cleat.

Is that a suggested dimension for a certain frequency range?  Your radius cuts look perfect, did you CNC those end pieces?

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A poly isn't really considered a proper diffuser anymore. It focuses the sound at certain directions at certain frequencies strongly, thus doesn't evenly spread out the sound like a good diffuser will do. It can diffuse spatially well however, if several units with a weird dimension are placed next to each other. But it will not offer any temporal diffusion either way and is IMO an outdated product that's been replaced by something better.

 

Obviously I can't argue with you if you like the effect of it.

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2 hours ago, Khornukopia said:

 

Is that a suggested dimension for a certain frequency range?  Your radius cuts look perfect, did you CNC those end pieces?

 

I did not use a CNC.  For each of the three sizes, I drew out a semicircle and then took a 130 degree angle.  Set the depth go give me the chord length I wanted.  The LF frequency range seems dependent on the weight of the MLV.   The heavier the MLV, the lower the bass absorbed from all my research.  I chose the 1lb. per sq. ft. to get to the lower frequencies. 

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2 hours ago, Bjorn said:

A poly isn't really considered a proper diffuser anymore. It focuses the sound at certain directions at certain frequencies strongly, thus doesn't evenly spread out the sound like a good diffuser will do. It can diffuse spatially well however, if several units with a weird dimension are placed next to each other. But it will not offer any temporal diffusion either way and is IMO an outdated product that's been replaced by something better.

 

Obviously I can't argue with you if you like the effect of it.

 

Considered by whom?  I have read many current and old articles and books on the subject.  Yes, some folks claim these diffusers are out of vogue.  I don't care if things are in vouge or not....they just need to work. Many now lean toward the quadratic diffuser theory.  Interestingly, I originally planned on making several QRD unit.  I have the plans ready to go and purchased them from a commercial vendor who also builds them.  After really researching the issue, however,  I found many current commercial vendors also offer the poly design with the added benefit of the LF absorption. I researched current articles on these poly designs and was persuaded that this is a better design.  At least as far as building a couple and trying them out.

 

Arguments are made that the quadratic diffusers actually do 'focus' frequencies due to the cavities tuned to different frequencies.  The QRD also changes phase alignments due to the varying cavity depth.  I have never seen any article claiming the that the poly focuses any given frequencies.  The only argument I heard on that subject had to do with equal sized polys next to each other focusing sound at the point where the polys meet, sort of like a concave structure. Again, that will not be the case here since I will be using various radius polys if this test proves positive.

 

One of the down sides of the poly is their size. That is not a consideration in my room, so not an issue for me.  We shall see if there is any audible changes that are noticeable.

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On ‎6‎/‎19‎/‎2019 at 9:54 AM, Bjorn said:

focuses the sound at certain directions at certain frequencies strongly

 

22 hours ago, Rudy81 said:

The only argument I heard on that subject had to do with equal sized polys next to each other focusing sound at the point where the polys meet, sort of like a concave structure.

 

Both of your comments got me wondering about these 3-D cellulose wall tiles I planned to attach onto the inside of my garage door. The speakers are in the corners flanking the garage door. Do you think the small concave surfaces on these tiles will have a negative effect on the music in the room, which is currently sounding very fine?

 

IMG_3221.JPG.05376bb7b82311e93f1fa4f0203f4625.JPG

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It would be interesting to see how the sound would change if you lined the garage door with those tiles.  Any concave surface should focus frequencies.  Hence the design of parabolic microphones and your dish antenna.  The opposite is true for convex surfaces, which disperses sound in many directions depending on angle of incidence. Mind you, in no way am I an expert on the subject.  I have been gathering knowledge from many sources on the subject of poly use in an acoustic environment.

 

I do know a parabolic mic is amazing in how it focuses sound from long distances. 

 

I am about to start my listening tests on the two polys I built....more to follow.

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Had a couple of hours to kill and started trying to compare the differences, if any, with and without the polys.

 

Listening impressions:

- Imaging was excellent, as it always has been with the Oris.  No change noted.

- Overall liveness of music was more apparent, if I dare say better.  Not too sure and will need more time to evaluate that aspect.

- Biggest and most obvious change was a larger soundstage overall.  The main speakers were harder to pinpoint and they seemed to more evenly fill the front room space.  Very nice change and yet provided crisp detail and excellent instrument placement.  I thought highs would get 'blurry', but that was not the case. 

- I did not detect any LF changes, but was curious enough to break out the REW measurement gear.  I tested each main and the subs separately to see what effect the polys had in the room.  Mind you, there are only two polys in the room at the reflection points. Glad I set up the testing gear as it shows LF absorption at different low frequencies. 

 

So far, I am intrigued enough to spend more time on poly construction.  Plan is to place 3 different sized polys at the main reflection points from floor to ceiling. Total of 12 polys to do that. Also, add polys in the back of the room and replace the Quadratic diffuser I built previously. I would also like to add poly structures to the ceiling-wall corners. Those I plan to build in the room similar to what the Master Handbook of Acoustics shows, with the addition of a MLV behind the curved surface.

 

I will follow this up with the REW results.

 

Left-side.jpg

Right-side.jpg

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Posting waterfall results for the subs and the center channel to show the difference with the polys and without. I am no expert on REW, so if there is a better way to show the changes, please let me know.  It seems the polys do absorb at various frequencies, but primarily around 100hz and above.  I also noticed the LF response evens out somewhat when looking at the subs' graph.  Decay times seem better with the polys for all 3 main channels and the subs. The differences are easiest to appreciate by going back and forth between graphs in REW, but here are the graphs.

 

 

Subs Original.jpg

center with poly.jpg

center original.jpg

Subs with Polys.jpg

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Looks good. Should get better as you add more of your high quality Poly-Cylindrical Diffuser and LF Absorbers.

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On 6/27/2019 at 11:26 AM, Khornukopia said:

Looks good. Should get better as you add more of your high quality Poly-Cylindrical Diffuser and LF Absorbers.

 

I sure hope so. I am currently mid build on a set of 12 polys.  Plan is to place 6 on each of the side reflection points.  It will take 6 to go floor to ceiling with each of the 3 different sizes. 

 

Plan is to take room readings again with the current absorbers, then with nothing on the walls, and finally with the new poly setup.  We shall see how effective they are.

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I’d love to see the build plans for these
That’s some great work and ideas you have and doing. Thanks for sharing this with us


Dollar for dollar Klipsch has no equals
Name one other speaker company that can build a speaker and keep working like new after 45 plus years of service. Answer NO ONE !!!!!!

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1 hour ago, carlthess40 said:

I’d love to see the build plans for these
That’s some great work and ideas you have and doing. Thanks for sharing this with us


Dollar for dollar Klipsch has no equals
Name one other speaker company that can build a speaker and keep working like new after 45 plus years of service. Answer NO ONE !!!!!!

 

This build is of my own design and I would be more than glad to share what I did with any of you who are interested.  I will be taking more pics in the next few days since there may be some interest.  I currently have built 4 different sizes.  Details to follow.

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Can’t wait for the photos


Dollar for dollar Klipsch has no equals
Name one other speaker company that can build a speaker and keep working like new after 45 plus years of service. Answer NO ONE !!!!!!

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I am interested in how these are made too.

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This is intriguing, but I've yet to see an attractive absorber I can use in my den that is used for more than audio.  My only long wall has a large giclee print I don't want to give up.  That happens to the the edge of the opposite La Scala's squawker pattern. 

 

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