Hey guys...we need a little rallying here... - Page 7

You know the drill; gotta look up the tech manual:

http://www.jblproservice.com/pdf/AE%...ls/AC26-WH.pdf

SFG, yada, yada, it's one heck of a little woofer for $80 MSRP.

[We are WAY beaucoup OT here.... ]

The roundover could extend up and over but still lay flat on the baffle. Actually, if it kept it's same general square shape, it could be used as the upper part of the entire front baffle.

I called the other guy this evening, he was busy so he wants to talk to me about it tomorrow. He said he'd help, but he doesn't really know what we want done yet. So we'll see.

CNC time listed on ebay for as low as $30/hr

http://shop.ebay.com/i.html?_nkw=cnc....c0.m270.l1313

I'll refer everyone again to the early OS papers, which used an elliptical OS, and yes, there was an on-axis suckout in that, as well.

I agree with your analysis with respect to JBL. Geddes was working with them back then, and there was an agreement that JBL would cover the cost of patenting in exchange for a license to the technology. That process revealed that OS was already in prior art, so the deal ended.

I'd surmise that during the course of that, JBL engineering sussed it out thoroughly with actual prototypes. They have since made EOS products, but only with dome tweeters -- LSR series being the prime example, but there are others in the consumer product lines. An on-axis suckout is a deal-breaker in SR, of course.

It's not as if, being in the public domain, OS technology hasn't been available for the entire industry to exploit, and it's likely a good guess other big hitters have looked at it, as well. I'm not suggesting that it doesn't or can't work, merely that only Geddes himself has found means to convince others it does, and is also big whoop, thus far....

The only thing I'd add to that is that JBL and other pro speaker manu's have very different design considerations than Geddes or any of us. OS has certain design considerations such as the suckout and the lack of horn loading.

Although I haven't tested it, I hypothesize that the closer to true elliptical (as opposed to super-elliptical or squared) and the more axes of symmetry present (ie circular being the greatest), the deeper and most concentrated the notch will be. The greater the asymmetry and less "round" the horn is, the wider and shallower the notch will be.

I don't have the research, data (or even scientific explanation as I'm a layperson in this field) to prove my hypothesize, but this is what I think is occuring in layman's terms:

1. As a horn's mouth terminates, some diffraction effect occurs (I'm not sure how to explain this).
2. The diffraction effect, occurs at a specific time in the wave propagation depending on the path distance from the source.
3. If a point at the mouth termination is the same distance from the source as some other point on the mouth termination, the same diffraction effect will occur.
4. These two identical diffraction effects spaced apart will causing cancellation at a frequency based on the spacing of those effects.

This could be completely crackpot, but it is a guess to what is happening.

Here is how I see it relating to different shapes:

Any point on a circular horn mouth would have an infinite number of points on the mouth with a diffraction effect at the same time and spacing causing a specific null frequency to occur.

If you eliminate the roundness of a circle, you basically get a square. For a square, each point on the square will have 1 opposing point with the same path distance and spacing. Those two points will have a matching set of diffraction points in the opposite axis for a total of 4 points at a given distance. The distances for each set of equidistant mouth point are also quite spread out. The nulls will occur over a spread of frequencies and will not be very noticeable.

If you take away one symmetry of the square you get a rectangle where each point will have an opposing equidistant point but won't have a matching pair in the opposite axis like a square. The cancellations would be spread out over greater frequencies and would have less strength showing as a broadband notch.

Moving back to a more circular arrangement from the rectangle you get an ellipse. An ellipse would act like a rectangle, but with a tighter spacing of frequencies effected. The cancellations would be somewhere between the null on the circular horn and the broadband notch of the rectangle.

The closer the ellipse is to being circular the narrower and stronger the notch becomes. Also, the closer the ellipse is to being rectangular (aka a wide super-ellipse) the wider and weaker the notch becomes.

I think it also holds up that this only occurs on-axis since the horn path lengths to the mouth quickly lose their equidistant nature once the horn is rotated.

Anybody think that this holds water? Don't be afraid to shoot it full of holes. I'm just thinking aloud.

If this is legit (or is close to the target), I think it might push us to using a super-ellipse. I also think it explains why JBL uses the funky looking horns in their PT series and many manufacturers offset the throat. Could it be eliminated by using a horn that is so large as to have this phenomenon occur out of the passband (like the JBL mega-PT)?

Yes but sadly, making a waveguide prototype is very time consuming to get right, even with that machine. The CNC won't make it 100% perfect. My guess is that they would charge for about 15-20 hours and then charge material fees, setup fees, and then shipping. Probably get close to $900 - $1000 when all was said and done.

Doing the aluminum plate molds is very expensive.

Check these cheesy ones out:

http://cgi.ebay.com/CHECKERS-INJECTI...#ht_500wt_1128

http://cgi.ebay.com/INJECTION-MOLD-P...#ht_500wt_1128

http://cgi.ebay.com/Plastic-Injectio...#ht_500wt_1128

I believe your analysis is sound. What I don't quite get myself is why there is a specific-frequency notch in the elliptical, as the distance from the source to the edge varies continuously across the profile, and for any given wavelength, only "matches" at four points around the perimeter.

If I have time later tonight, I'll look up the paper and post a link; just have to remember the primary author's name to find it....

Edit:

Bauman & Adamson, "Acoustic Waveguides - In Practice," JAES V41 #6, 1993 June

Fig. 7a. -- the notch is broadband, an octave wide, 7 kHz - 14 kHz and about 2-3 dB deep at low res, 0° - 50° shown:

Quoting myself :

"Moving back to a more circular arrangement from the rectangle you get an ellipse. An ellipse would act like a rectangle, but with a tighter spacing of frequencies effected. The cancellations would be somewhere between the null on the circular horn and the broadband notch of the rectangle."

It makes sense to me that the path lengths and spacings in this case would be more tightly packed than a rectangle and not as deep as a rectangle.

I think we could basically mitigate it entirely if the horn is big enough for the effect be outside of the passband or if move to a superellipse.

The questions I have:

1. What are the positive and negative effects of a elliptical vs circular horn (besides the obviously smaller CTC and preferably vertical dispersion)?
2. What, if anything, would be lost going to a superellipse vs an ellipse?

I wonder how we could model this. I've never used Akabak but maybe an akabak expert could give it a shot.

Bigger would move it lower (longer wavelengths). Small tweeter EOS pushes it toward the top. This waveguide was pretty big, as I recall.

360mm W, 233mm H, 235mm D (QSC 152i size)

Pattern (from polars): 53° H x 45° V

Rectangular superellipse, maybe, otherwise you lose the CTC advantage....

I meant a comparison of a 90x50 superellipse, somewhere between an ellipse and a rectangle vs a 90x50 true ellipse (and even versus a rectangular OS). What is the effect of essentially opening the mouth in the corners?

No clue. I'm not a waveguide theorist or designer. I read this stuff, remember it, and try to make sense of it, is all.

[Sometimes, I also even remember where I read it.... ]

The dip has more to do with the area expansion than the shape of the mouth. There is a very large discontinuity at the mouth for straight-walled horns and it'll even show up in the classic horn equations.

The reason the QSC horn works well is because the mouth has a better termination at the mouth.

Btw, I find the whole HOM propoganda thing to be quite humorous because the same thing creating the dip is what the prophets of HOM are defining HOM as....

I also wanted to add that the reason the pro audio horn designers give for not pursuing straight-walled circular horns is that it's the least efficient use of space. I suppose the ironfist DIY'ers out there wouldn't mind oversizing the horn, but it is going to compete with other issues, like polars at the xover frequency.

Could we just build onto the QSC? I realize it wouldn't be the easiest thing, but maybe.

I was searching around for the superellipse and found this, so I'm just putting it here as a 'bookmark'.

http://local.wasp.uwa.edu.au/~pbourk.../superellipse/

What causes the large increase in output when for example a dome tweeter is used with a WG, if not horn loading?

Can it be fully accounted for by the directivity, which focuses the energy into a smaller solid angle?


I forgot to mention earlier that I don't think there'd be demand for anywhere near enough to justify injection molding.

Ok here's a wild idea - get Earl involved.

IIRC the only thing holding him back from doing an EOS G is the tooling cost, which sounds like it might be handled here.\\

He supplies a good design, and both he and DIY'ers get an EOS WG source.

The paper says the dip is caused by "finite aperture effects" as predicted by waveguide theory.

**Click**

[I'm done reading this stuff for now.... ]

Making a mold of the QSC horn would not be tough at all. Modifying that mold accurately would be challenging, but do-able.

For the $$ some of these places would charge for tooling, you could seriously make the initial plug yourself. It would just take time.

How accurate does it need to be? Usually accuracy is listed in fractions of a wavelength, in this case I would think that 20kHz would be the upper max, a little bigger than 5/8ths of an inch. Making a waveguide to 1/10th lambda accuracy would mean it only has to be accurate to 1/16th-1/32nd of an inch RMS/peak to valley across the surface.....this is easily do-able with DIY techniques...diy telescope mirror makers make surfaces BY HAND to better than 2-3 millionths of an inch accuracy, without machine tools....

JSS

JSS

Jacek Zagaja From DIYaudio.com gratiously created this for me.




He also sent me files with .IGS and .STEP extensions, I have emailed them to EricH to see if he can work with them.


Also Eric PMed me this
http://cgi.ebay.com/ws/eBayISAPI.dll...02_304652#shId

Wharfedale Pro SVP-15 2-Way Passive 15" Speakers Pair, its waveguide is an elliptical one. Its local pick up only and surpisingly in the same city as me.

I will buy them if people want and get the waveguide out of them for modelling.

Geddes says most honk like horns, Im not sure about the high end K2 series but most common horns honk, too much diffraction, he said he researched a long time for what shape had the least amount of diffractions that causes HOMs because he believes HOMs are what causes the honking sound. "Honking" is really subjective, I have never pinpointed it to an exact sound there is glaring sounds, there is sort of distorted low pitch stuff like a low duck honk but its still not very well defined.

Why not just post that question for Geddes to answer on his Geddes waveguide thread or do a search, it can not be the first time someone has wondered about this.

Here is the thing, we are either designing speakers to match what JBL does or match with Geddes does. Again for 2+ years we have been just chasing the ideas Geddes put forth. If we want to change directions then great lets put the measurements side by side and choose. NO one care much about horns/waveguides until Geddes started on the Waveguide stuff and he has spent years discussing it and provide data/research for others at no cost on DIYAudio.com. I find it humorous that some posts would suggest its just all sort of a con job

btw, I knew at some point you would post something about JBL.

Im not interested in JBL, they neither support or post in DIY communities and for that I will try to never buy anything that supports them as a company. 3rd party sold for a really low price, sure

Yeh, I have no problem making the plug, I can do it fairly easy. I mentioned that they would charge $500 for a mold of the QSC but that I could do it much cheaper. The problem is that no one has really said WHAT to modify on the QSC.

I don't know why the tooling costs would be holding him back. It's not THAT much when you look at it as an expense from a business perspective. Especially when we're just talking about the prototype costs.

Its expensive to do it right according to Geddes.

As for the suggestion to ask him, why would he help at all? Im sure he is more then happy to help a group of individuals that have posted negative comments about him and his work

And practically speaking that means?

It really doesn't matter because the impedance mismatch is still there creating a reflection at the mouth.

I really wish there wasn't so much emotional hopeful belief about "waveguides" because people aren't stepping back and looking at this scientifically and are getting way too caught up in the minutia when there's a lot more performance sitting on the table.

Btw, the honk of horns is created by horn throat distortion and is mostly the result of a high compression ratio (small throat aperature) and a slow expansion in the throat (what I like to call the small straw effect).

Having a fast expansion at the throat is good for reducing overload in the throat, but it needs to be carefully balanced with matching the acoustic impedance to prevent reflections.

It has already been proven that a tractrix expansion has perfect impedance matching for a plane wave at the throat, but it doesn't necesarily provide the most ideal polars. Taking into account that you're not perfectly planar at the throat and that tweaking the area expansion later in the path has minor compromises, you can have a mostly tractrix expansion that offers way better polars than anyone here has posted yet.

So you disagree with most of Geddes research about diffraction beyond the throat and HOMs??

All I know is that Geddes Abbeys and summas measure the best and from all subjective listening tests they sound nothing like common horn designs. No other design has said this so either the measurements are fabricated and people are lying to protect the interests of a product or there is something to those many years of search out what the best design was.

Honestly, Geddes has never been in this to sell a lot of speakers (atleast his actions tell my business brain he isnt) so Im thinking this has little to do with marketing. I also suspect he will stop selling speakers in the next couple of years and just do the big money consulting gigs in Asia.

His theories will exist and will still be followed by some, others can choose what they want to use and do. The only way to know for sure is to have them all and do the right testing.

If we are getting close, he has no option but to involve himself; otherwise, he's out of business in DIY. I expect that to occur, anyway, and the current "kits" to be discontinued soon. Building and selling these out of his garage to DIYers established a market position for him that he had failed to achieve in multiple prior attempts directly promoting his technology via more conventional business models. He's now moving on to a different plan, the "DIYer morphs into manufacturer" scenario common in this industry with the cachet of "Gedlee Design" now in his portfolio. It's easily seen that his participation in DIY forums is diminishing in favor of a more controlled "World's Best" direct promotion, sufficient quantities of "Kool Aid" having been consumed by DIY recruits....

+1, I have seen the same pattern over the past year and a half.

Does anyone know anything about the Wharfdales I posted from ebay on the last page?? I do not mind spend $200 to findout about its waveguide.

I am not sure I follow. Can you expound? Do you have any diagrams or measurements?

Buy it and measure it. If nothing else, learn more about the dynamics of the on-axis notch. There are plenty of elliptical waveguides on the market; key is establishing that this one is OS or PS, which may be determined via accurately measuring the flare profile. Nothing else is worth pursuing at this point, in my view....