Poly diffusers, effectiveness and anomalies?
OpusOfTrolls
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5th March 2013
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Poly diffusers, effectiveness and anomalies?

I am thinking about using polycylindrical diffusers, but I haven't found enough information on them. How big do they have to be, in wall space and depth, to have impact at lower frequencies? Also, do they create lobing all by themselves, or is their reflection clear?
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I cant speak to this issue in a technical sense, but QRD diffusers seem the overwhelming choice when diffusion is used now a days.

What particular reason brings you to look at poly's ?
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The lack of a HF limit. Basically to redirect reflection paths and create a stronger point source without temporal modification.
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The first studio I went into as a kid had polys covering about 50% of the live and control room surfaces, all built by the owner. He had some that were horizontal, ~10m long x 2m tall that he said trapped significant LF. I'd guess that they were ~25cm deep. The ceiling of the live room was totally covered with polys, probably 10m x 1m. The place had a great vintage vibe.
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There is a local shop with a listening room covered about 40% with fiberglass panels and small deep polys, actually gave quite a clear tone to the room. Polys work well to extend the reverb tail because they are not very absorptive in the scattering region. They were using these: http://www.acousticgeometry.com/curv...diffusors.html
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Those look nice. Pretty different than the old wood designs.
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Quote:
Originally Posted by OpusOfTrolls View Post
The lack of a HF limit. Basically to redirect reflection paths and create a stronger point source without temporal modification.
I suppose I am not the one to try to help since the part i bolded I dont even understand
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Well this is why I asked about the lobing, and with some googling I found a paper linked by our Andre that shows polys do create lobed reflections. As a curved surface, the sound to the poly is spread (with lobing) compared to a flat surface, which may not have a reflection to the listening position. Having the poly with surface area incident (for reflections) to the listening location creates a source or reflection path from there.
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I've seen photos of studios with an array of parallel polys of varying widths. I seem to remember that design element being brought in to decrease lobing, or rather to vary the distribution. Another variation proposed by a local designer is to build a sine-type of surface with a larger sine feature covered by smaller sine features. I thought that sounded like a total nightmare to build.
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Quote:
Originally Posted by OpusOfTrolls View Post
Well this is why I asked about the lobing, and with some googling I found a paper linked by our Andre that shows polys do create lobed reflections. As a curved surface, the sound to the poly is spread (with lobing) compared to a flat surface, which may not have a reflection to the listening position. Having the poly with surface area incident (for reflections) to the listening location creates a source or reflection path from there.
You have been active here long enough to know that it helps immensely to provide links to times of interest that you have found.

ANY repetitive pattern will exhibit lobing. This is not a unique attribute of polys.

Andre
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Quote:
Originally Posted by avare View Post
You have been active here long enough to know that it helps immensely to provide links to times of interest that you have found.

ANY repetitive pattern will exhibit lobing. This is not a unique attribute of polys.

Andre
But let's say there is a (2d) circular wavefront reflecting off of one poly. Let's also say it was a transient event, impulse, or favorite way to describe a wave that won't cancel itself because of an interference reflection. How is the return reflection from the poly looking?
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if you like the aesthetic, mixed polys are one of the best ways to get a highly diffusive and balanced live room. large polys across corners, mixed sizes scattered around the walls and ceiling (some can even be the slat types) will generally get you to a very nice place... sometimes it's easy to over-think a simple device which works...
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Quote:
Originally Posted by OpusOfTrolls View Post
How is the return reflection from the poly looking?
At frequencies where the size of the device is effective as a diffuser, the same as the original. The level will be reduced. For discussion purposes, using a plane wave, the reflection will be reduced in level as if the reflection originated from the centre of the radius of the poly. For non-planar waves, optical wave modeling is the way to predict level reduction.

Thank you for posting the link to the paper.

Reflectively,
Andre
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Oh, thank you for the research! Is there a design guide in any of the well known books? Cox?
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Quote:
Originally Posted by OpusOfTrolls View Post
I am thinking about using polycylindrical diffusers, but I haven't found enough information on them. How big do they have to be, in wall space and depth, to have impact at lower frequencies? Also, do they create lobing all by themselves, or is their reflection clear?
I had the same problem. No info whatsoever on depthm width versus cutoff freq etc. I even asked some specialists on a forum. Apparantly there are almost no data. So I went for it and just constructed a few. I will try to post some pics later, I could link to the studiotips forum, but it is down for the moment.

I can assure you they work very well, even in the back couch, way too close. This would never be possible with a qrd or similar.
The problem is, nobody wants them to work, because they cost me about 25 dollar per square meter, instead of 300 ...

The bigger ones are made out of 1220 mmx1220 mm panels, curved to 980 mm width, giving a depth of around 320mm. I filled them completely with fluffy rockwool - no resonances allowed.

I am 100 percent against time scattering devices which are actually absorbers instead of diffusers, and 100 percent against resonators of any sort. I treated the entire roowm with polys and superduperchunk style basstraps. Great result, no cost.
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Well, I don't think any acoustic device doesn't have it's place, but for what I am doing a poly is the only option. I am making one large poly, about 6ft wide and maximum 3ft deep, for the rear wall. Whether it is a true poly or ribbed, circle or sine based, is unsure, but I need a multiangled surface to have specular return from lateral directions and specular redirection from the center. Since the listening location is about 6ft from the rear wall, a max depth of 3ft seems correct. A QRD would not be the best choice obviously.
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Quote:
Originally Posted by OpusOfTrolls View Post
I am making one large poly, about 6ft wide and maximum 3ft deep, for the rear wall.
That is a 180° arc. That seems very large. Sounds impinging on the egdes will be barely shifted. With out knowing all your criteria, have you considered 90° or even 60° arcs? Remenber that the angle of reflection equals twice the angle of incidence.

Andre
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I hadn't envisioned a 180º arc.

My previous drafting has a triangular deflector, but this isn't looking so good as reflections are not coming back well. It does deflect the center signal, but ends up holding the return away. I can go for a max width of 8ft, 4ft from the centerline, but no larger.
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Originally Posted by Yannick View Post
The problem is, nobody wants them to work, because they cost me about 25 dollar per square meter, instead of 300 ...
That's only if you take the simplest approach. Cost really has no upper bound when you consider the variety of materials available.
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I would think two feet deep would already be plenty.
In my search for data, the consensus was: bigger&deeper means lower cutoff frequency, more curvature means more hf diffusion. A poly will diffuse down to a quarter wavelength corresponding to the depth, but I found nothing to substantiate that.

I originally wanted to go for the one big poly as well, but practical considerations had me make a small array of three, with different sizes. I needed something 6.5 feet wide as well.
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Appearantly the polycylindrical is completely in phase with itself.
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Jens,

that paper is so flawed it is unclear to me if you posted the link to confirm or to disagree ?

1. what is the date ?
2. the author imposes the fact that a true diffuser needs temporal dispersion. That is his definition, not the standard definition.
3. then he goes on to prove semicilinders do not work by posting an impulse response of the reflection. He consciously does NOT post the same for a temporal device.
4. Also, the author insists on comparing to semicilinders, which are way inferior to polys.
5. the main objective (by the author) against non-temporal devices is the comb-filtering (which in fact is dramatically reduced compared to a flat reflector which he does not bother to mention), while using temporal devices will actually introduce comb filtering on such a massive scale you cannot even measure it anymore.

- here you can say: hence it does not exist. But this is seriously flawed, since our ear is not a measuring device, and is not listening to steady tones only. It responds much quicker.

6. on page six the author writes 5 critiques on Schroeder diffusers, which are all valid for any kind of temporal device. Even if they are better designed and measure better, they are still subject to the same problems.
7. a temporal device will almost always be a resonator.
8. if you look at the commercial website of RPG you can see the last decade they are moving firmly towards poly-like objects. Their newest top-of-the-range diffusion is a project based calculated waveform, without the slightest hint of temporal diffusion.
9. The comment in the paper about the "best" concert halls is also completely subjective. Here in Europe, the "best" halls are mainly the older shoebox type halls, with massive plain reflective surfaces. Actually, the "worst" modern halls I have heard in Europe are those with a lot of diffusion, hence no stage effect/reflections, hence NO sound/early reflections helping the musicians. Just perfect diffusive reverb. They sound as if the players are standing far far away and playing softly.
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Quote:
Originally Posted by OpusOfTrolls View Post
Appearantly the polycylindrical is completely in phase with itself.
That is the main strength of the poly. At the same time, for some critics, that is the main weakness...

I am searching for intergrity of phase and impulse response from microphone to playback - so why would I introduce phase problems in the acoustic treatment ?
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If you are aiming to preserve the source et al, you would be using absorption! Temporal effects in diffusers aren't 'bad', they are different. But one thing a phase grating will never have is an unadulterated impulse response. I am pretty sure the intent with phase grating is to modify the impulse with focused asymmetric delay to create a 'color' that helps simulate a particular acoustic enviornment.
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OT: as far as conspiracy theories go, the studiotips website is down.
Also google can find the following document:

https://www.google.be/url?sa=t&rct=j...43287494,d.d2k

But the Gearslutz search function cannot.
Those are the only interesting real measurements I ever saw, poly versus temporal device. (I really like to write temporal device, makes me think of Star Trek)
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