A new center channel for the Octagon - Page 2 - AVS Forum
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post #31 of 47 Old 09-18-2012, 07:26 PM
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Coctostan,

Sorry for not replying earlier, but I went through a stretch of 42hrs of work with only 3 hrs of sleep intermingled in there.

The non-dispersive paraline (as I understand it) relies on the refelctive property of a parabola to have equal pathlengths and have a coherent wavefront at the exit. Because a parabolic surface is used as a reflector, the center region will be 'illuminated' more strongly than the ends. I wish I could cobble up a drawing, but can't right now. It would be a good integration problem to figure out the shading curve the parabolic paraline gives....

The dispersive paraline would uses a hyperbolic reflecting surface, and an elliptical reflective surface would make a 'focusing' paraline. All are possible.

JSS
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post #32 of 47 Old 09-19-2012, 08:11 AM - Thread Starter
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Max,
The "coaxial" pic makes me wish I hadn't sold my BMS 4540s! Would have been easy to try with those ultra compact drivers.

So you will have a baffle wall curving away from the listening area? How much width do you have to work with? In hindsight, the Octagon LCRs should have been a full 4' wide...didn't realize taking directivity lower in frequency would be so interesting and beneficial.

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post #33 of 47 Old 09-19-2012, 12:42 PM
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Quote:
Originally Posted by maxmercy View Post

Coctostan,
Sorry for not replying earlier, but I went through a stretch of 42hrs of work with only 3 hrs of sleep intermingled in there.
The non-dispersive paraline (as I understand it) relies on the refelctive property of a parabola to have equal pathlengths and have a coherent wavefront at the exit. Because a parabolic surface is used as a reflector, the center region will be 'illuminated' more strongly than the ends. I wish I could cobble up a drawing, but can't right now. It would be a good integration problem to figure out the shading curve the parabolic paraline gives....
The dispersive paraline would uses a hyperbolic reflecting surface, and an elliptical reflective surface would make a 'focusing' paraline. All are possible.
JSS

Thanks for the explanation. I'll have to do some thinking about how it will affects things. I'm going to re-read the patent to see if I can uncover anything. If the shading is minimal, my CBT plan might still work.
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Originally Posted by pnw View Post

Max,
The "coaxial" pic makes me wish I hadn't sold my BMS 4540s! Would have been easy to try with those ultra compact drivers.
So you will have a baffle wall curving away from the listening area? How much width do you have to work with? In hindsight, the Octagon LCRs should have been a full 4' wide...didn't realize taking directivity lower in frequency would be so interesting and beneficial.

My plan would be to use a straight array and delay it to an approximate 36deg arc. It would use a combination of DSP delay, paraline curve delay and paraline "tilting" delay. The tilting part being the most important but I'm having trouble explaining it. I need to make a quick SketchUp.They all physically exit (or enter from the perspective of the conical horn) on the same plane.

I should be able to make a mock up using just three compression drivers and some pretty simple woodworking.

Even if my CBT plan doesn't work, this could make Synergy horn construction far simpler and a Synergy horn with a tall shaded and delayed paraline could work quite well in the vertical domain. It certainly wouldn't be tough to build and test.
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post #34 of 47 Old 09-19-2012, 08:51 PM
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I may have been making things too complicated. If a paraline both shades and delays the further you get from the center of the exit, a single paraline might actually work similar to a CBT as it stands. It would mainly be a question of how much it shades and delays and whether that can be made to fit the CBT curve or at least some approximation. Like I said earlier, it is not vital to have constant directivity, just controlled and consistent narrow vertical directivity as low as possible.

I'm not sure how a ground plane version would work though. Maybe there would be a way to fold it so both the bottom and top parabolas combine and exit only above the driver entry.

Another possibility would be a paraline with two different parabola profiles for the upper and lower portions. The lower portion would maintain equal path lengths and provide minimal shading. The upper portion would be "stretched" to provide more delay at the top and greater shading. I wonder if Danley is employing something like this with his GH60 and the "Shaded Amplitude Lens". I should probably just PM Ivan Beaver or Danley himself.

Anyway, I've attached a screen shot of the asymmetric "eye" I'm referring to. I just guessed on some dimensions. It would likely be taller and skinnier with the driver entry a little lower. This one is 140cm tall and 50cm wide. The zero-delayed section would cover the bottom 50cm or about 1/3 the height which is about right for a CBT. I'm still working on calculating the shading, but the delay should be about 40cm which is a pretty good ballpark.

That is about 55" x 20" which means this could be built from about 2 sheets of 1/4" baltic birch. I've never worked with 1/4" so I don't know how much bracing would be needed. smile.gif

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post #35 of 47 Old 09-21-2012, 08:27 AM - Thread Starter
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This will be an extremely interesting project....looking forward to it. BTW, page 4 of the Paraline patent mentions using 3/4" material. McMaster has a good selection of small diameter spacers you could use to ensure the central plate is immobilized.

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post #36 of 47 Old 09-21-2012, 09:44 AM
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Quote:
Originally Posted by pnw View Post

This will be an extremely interesting project....looking forward to it. BTW, page 4 of the Paraline patent mentions using 3/4" material. McMaster has a good selection of small diameter spacers you could use to ensure the central plate is immobilized.

I'll look at that, but from what I've read you want to maintain a spacing that is less than a 1/4 wl of the highest frequency you want to reproduce. 1/4" will get you up to as high as any 1" compression driver will reproduce. I'm not sure what downside there is when passing a 3-500hz signal through though. There is presumably some lower limit.

It is made of 5 layers.

1. The driver mounting outer layer.
2. The outer path layer.
3. The middle redirection layer.
4. The inner path layer.
5. The exit layer.

For the size above, I could do 4 layers per sheet of 60"x60" 1/4" BB ply. That means 4 sheets for a L/C/R. If I made it a little smaller I could do a full paraline on a single sheet. The nice thing is that this should be pretty portable and measuring outside won't be tough. I figure I could use thin strips of 1/4" ply to brace the layers. If they are thin enough and place along the expected path I don't imagine they will have a large impact.

I'm still working on figuring out the amount of shading that will occur. I believe it is a function of the angle of incidence. If instead of a parabola it was a circle, and the exit was circle, both pathlength and amplitude would be equal, but obviously a line-shaped exit wouldn't be possible. I actually just had an idea. Using a circle profile exiting to a circle could make for a very easy to build synergy horn. It could be done coaxially like in my drawing above. In fact, it could even mount to an existing horn.
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post #37 of 47 Old 09-21-2012, 10:13 AM - Thread Starter
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I hear you on the 1/4 WL, 1/4". The patent (excellent reading BTW) says 1/3 WL & 3/4" so there may be a bit of fudging. Very short lengths of dowel should also work for spacers.

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post #38 of 47 Old 09-21-2012, 01:31 PM
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I just posted about using a paraline to couple multiple drivers to a SEOS on the DIY Sound group forum early this AM. If I had been thinking I would have posted it here. As you say, use a centered 1" dia hole in the exit plate and change the "eye" shapes in the inner layers to circles.
Coupling to a SEOS gives you a point source down into the midrange but not the pattern control unless you use a large SEOS. It certainly would be a lot easier to build than a Synergy, although the line array version doesn't look that hard. The approach with a single large mid coaxially covering the CD looks attractive but I can only guess at how to couple its output into the input chamber with the CD.

Jack
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post #39 of 47 Old 09-21-2012, 06:58 PM
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Quote:
Originally Posted by coctostan View Post

I'm still working on figuring out the amount of shading that will occur. I believe it is a function of the angle of incidence.
I've been thinking on this, and I believe you're correct here. It sure seems like the curve of a standard equal-path-length paraline "eye" approaches having a 45 degree tangent line at the top and bottom. So the length of arc segment illuminated by the sound approaches 1.414 (the square root of 2) * the length if it were a 90 degree straight hit, and the resultant shading factor would approach the inverse of that ,or a factor of .707.

Just an awkward thought experiment, I'm too many years removed from even basic trig to work out how that might translate into a general formula for calculating the end shading based on different dimensions that might be employed. redface.gif Let alone drawing the curve in its entirety.
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post #40 of 47 Old 09-21-2012, 08:21 PM
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Quote:
Originally Posted by pnw View Post

I hear you on the 1/4 WL, 1/4". The patent (excellent reading BTW) says 1/3 WL & 3/4" so there may be a bit of fudging. Very short lengths of dowel should also work for spacers.

Yep, there has been some discussion on Diya about this. You can actually see this affect in the CBT papers. Keele uses 3/4" tweeters and he gets comb filtering above about 14khz I think. 1/4 wl is playing it safe. Of course, there could be a tradeoff I'm not accounting for.
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Originally Posted by JackNC View Post

I just posted about using a paraline to couple multiple drivers to a SEOS on the DIY Sound group forum early this AM. If I had been thinking I would have posted it here. As you say, use a centered 1" dia hole in the exit plate and change the "eye" shapes in the inner layers to circles.
Coupling to a SEOS gives you a point source down into the midrange but not the pattern control unless you use a large SEOS. It certainly would be a lot easier to build than a Synergy, although the line array version doesn't look that hard. The approach with a single large mid coaxially covering the CD looks attractive but I can only guess at how to couple its output into the input chamber with the CD.
Jack

I'll look at your thread. I'm not sure a SEOS is necessarily needed with a paraline. You should be able to shape the wave front of the paraline and possibility transition neatly into a conical horn. I haven't given this approach a lot of thought to be honest. You are right though that it would only be worth it with a large SEOS like the SEOS-24 that was proposed at one time.

Quote:
Originally Posted by antisuck View Post

I've been thinking on this, and I believe you're correct here. It sure seems like the curve of a standard equal-path-length paraline "eye" approaches having a 45 degree tangent line at the top and bottom. So the length of arc segment illuminated by the sound approaches 1.414 (the square root of 2) * the length if it were a 90 degree straight hit, and the resultant shading factor would approach the inverse of that ,or a factor of .707.
Just an awkward thought experiment, I'm too many years removed from even basic trig to work out how that might translate into a general formula for calculating the end shading based on different dimensions that might be employed. redface.gif Let alone drawing the curve in its entirety.

I think the 45 degree tangent is right. If it were a circle, it would be a 90deg tangent at all points and would be equal amplitude. Stretching the "eye" would create a tangent angle less than 45deg and a more obtuse reflection.

I'm going to put together some equations in my excel sheet to see if I can calculate the shading.
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post #41 of 47 Old 09-26-2014, 06:08 AM
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Hi, pnw!

How did this project turn out? Unfortunatly your web site is down.

Looking forward to learn more about your theater.

Best regards,
Jan
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post #42 of 47 Old 09-26-2014, 09:04 AM - Thread Starter
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Hi Jan,

The system has never sounded better, so no complaints here.

I never pursued the Paraline...still haven't seen a DIY implementation with smooth response.

Sorry about the web site. Another project has gotten in the way of bringing it back on line...but I'm happy to answer any questions you have.

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post #43 of 47 Old 09-26-2014, 10:47 AM
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It's been a while since I read Paul's webpage also, here's the last wayback machine snapshot
http://web.archive.org/web/201301230...%20Octagon.htm
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post #44 of 47 Old 09-27-2014, 05:10 AM
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Quote:
Originally Posted by pnw View Post
Hi Jan,

The system has never sounded better, so no complaints here.

I never pursued the Paraline...still haven't seen a DIY implementation with smooth response.

Sorry about the web site. Another project has gotten in the way of bringing it back on line...but I'm happy to answer any questions you have.

Hey pnw, what ever became of those Aurum Cantus tweeters and Seas W26's that you had proposed using in this project? Are you currently using them, and if so, would you mind sharing some details on the design?
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post #45 of 47 Old 09-27-2014, 05:54 AM
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A couple of questions:

Did you build L and R channels similar to your new center channel? Any other changes?
Thanks for sharing. Your ideas are great inspiration for me.
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post #46 of 47 Old 09-27-2014, 07:34 AM - Thread Starter
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Marty,
The Aurum Cantus AMTs and Seas W26's are in the photos and measurements on page 1...so they are in use and sound great. In addition to what's on page 1, what details would you like?

Jan,
Thanks!
No, I didn't replicate the center in the L&R channels. So far, no other changes...had a better idea I'm sloooowly working on, but that will be several months before completion.

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post #47 of 47 Old 09-28-2014, 02:46 PM
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Quote:
Originally Posted by pnw View Post
Jan,
Thanks!
No, I didn't replicate the center in the L&R channels. So far, no other changes...had a better idea I'm sloooowly working on, but that will be several months before completion.
Thanks for the update! I'm looking forward to see what you are cooking.

Jan
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