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Old 30th January 2011, 03:03 PM   #201
tb46 is offline tb46  United States
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Default Model v. Fold

Hi soho54,

I think I got it now, it's the difference between designing a fold from a model, and iterratively changing both until they meet, and deriving a model from an already existing fold, where the fold cannot be changed. Naturally, now I'll have a lot of stuff to look at again.

Thanks again, I also like your folding in sketch-up thread, its pretty much what I do in ACAD only nicer. :-)

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Old 30th January 2011, 07:54 PM   #202
soho54 is offline soho54  United States
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tb46, you seem to be tracking now.



I have been getting some PMs, so I guess I should add something a little easier to understand.

Here is the THAM as straightened out by Xco1. I have made the five S positions needed for a four segment horn simulation stand out more, and have added a light blue line connecting the points. The idea is to get the blue line to follow the edges of the horn as closely as possible.

The first shows his default choice, and it tracks very well with the true horns edge. There is a little area unaccounted for in the corners, but there is not enough there to be a real problem in section 2. Section 3 seems a little iffy here, but there is an error in the length there, and it should be ~4cm shorter. The puts let horn out of bounds, and deepens the FR dip some more.

The second shows what happens when you move the S3 position closer to the throat. The extra blue area is volume the simulation has that the real horn does not. This will significantly alter the FR of the simulation from the model.

The third has the S3 position moved even closer to the throat.

Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

This last one is what happens when you opt for a three section horn, instead of the four section simulation.
Click the image to open in full size.

Last edited by soho54; 30th January 2011 at 08:17 PM.
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Old 30th January 2011, 08:34 PM   #203
tb46 is offline tb46  United States
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Hi soho54,

Yes, that makes it even more obvious. On the THAM15 I knew that the 3-section was off. Thanks.

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Old 30th January 2011, 10:27 PM   #204
Xoc1 is offline Xoc1  United Kingdom
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Thanks for the positive comments on my drawings!
I have tried to take on board Soho54,s comments on the way I plotted around the last bend. So I adjusted my model accordingly so that the middle section line in the corner bisects the corner. I think that this bend is problematical to plot due to the rapid change of section.
This shortened L34 a little but not as much as Soho 54 predicted. Please let me know if there is a geometrical way of getting this closer to the truth.
Revised Drawing and Sim attached
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File Type: jpg Tham 15 Schematic Revised.jpg (300.9 KB, 1295 views)
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Old 30th January 2011, 11:20 PM   #205
soho54 is offline soho54  United States
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I'm not sure of the exact number, it just seemed like it was ~4cm in my mind at the time. It has been awhile though. To be honest, even if it was 4cm off it isn't enough to worry about.

I was mainly trying to get across to everyone the need to check everything out for yourself. Don't trust anyone's numbers that aren't your own, after making sure you have double checked everything to make sure it is as exact as possible. This is part of the reason I try not to post hard numbers, and just things people should keep in mind while doing there own checks on models. I can goof things up too.

You did a very good job there. If you hadn't I wouldn't have used it.

Last edited by soho54; 30th January 2011 at 11:23 PM.
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Old 31st January 2011, 02:59 AM   #206
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Hello Soho54 - I've been following Danley since 2003, and I thought I knew how to fold a corner.

Tom Danley said: "A word about folding, one mistake made in horn folding is to make the horn passage significantly less volume
that it was when straight. The air in a bend has a rotary motion and so momentarily has rotational properties, the effect of this is
that the air has slightly more mass (inertia? DS) than in a straight section. This in turn allows a small reduction in acoustic length
for that section but it is small. When I lay out a bend I usually try to keep the total volume close to that of a straight horn."

Danley is suggesting that fig. 3 is a little long, but he doesn't say how long. Fig. 2 is the radii @ .707, which is even a little longer,
but preferred by many people. Figure 1 is way too short.

I haven't seen your method (see fig. 4). Can you tell us how you came up with it
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Old 31st January 2011, 03:14 AM   #207
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Quote:
Originally Posted by Don Snyder View Post
Hello Soho54 - I've been following Danley since 2003, and I thought I knew how to fold a corner.

Tom Danley said: "A word about folding, one mistake made in horn folding is to make the horn passage significantly less volume
that it was when straight. The air in a bend has a rotary motion and so momentarily has rotational properties, the effect of this is
that the air has slightly more mass (inertia? DS) than in a straight section. This in turn allows a small reduction in acoustic length
for that section but it is small. When I lay out a bend I usually try to keep the total volume close to that of a straight horn."

Danley is suggesting that fig. 3 is a little long, but he doesn't say how long. Fig. 2 is the radii @ .707, which is even a little longer,
but preferred by many people. Figure 1 is way too short.

I haven't seen your method (see fig. 4). Can you tell us how you came up with it

My own emperical measurements suggest that Fig. 4 gives the closest results between predicated and measured acoustic length. Fig. 3 keeps the volume constant, but results in a length that's a few percentage points too short (an error I had to build POC#1 and POC#2 to find out). Fig. 1 is even worse. I've heard about Fig. 2, and for my two POCs, the calculated length works out quite close to the real length. However, the method given in Fig. 4 is easier to implement via spreadsheet, and it's the one I've chosen to use for my next TH - POC#3.
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Old 31st January 2011, 03:17 AM   #208
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Quote:
This last one is what happens when you opt for a three section horn, instead of the four section simulation.
So what is the preferred rate of expansion in a taped horn?
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Old 31st January 2011, 08:41 PM   #209
soho54 is offline soho54  United States
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Quote:
Originally Posted by Don Snyder View Post
I haven't seen your method (see fig. 4). Can you tell us how you came up with it
I came up with it through a lot of research, and trial and error. About two years ago I got feed up with the "common knowledge" surrounding horns, so I threw it all out and started over from scratch. I read everything I could by Keele, Edgar, Leach, and Danley looking for common threads. I have pages and pages of excel work exploring their different papers, and ideas from posts. I then went back to Rayleigh, found measured data on acoustic pressure around bends, and sourced fluid dynamic, FEA, and BEM simulations.

I then spent a lot of time trying different ideas out, coming from the view point of getting the simulation to match reality. I would work until I found something that worked for a certain folded horn, and then tried it on another. You can find different ways to sim a single horn, but most of them fail when you try it on another. I kept redoing things until I found a way that works on all the horns I have tried it on.

In truth, every method you have there along with the 45deg, and SQRT of the product of the inside and outside path lengths works well enough on smaller bends in horn with only one to three corners. It is when you start to add more corners, or larger flares that they start falling away. The .707 one is more of a mid horn up thing.

Up until ~6months ago I actually used different methods for different corner setups, but in the end after dozens of more tests I have found that using the single method produces the most consistent results no matter the corner number, or geometry.

The problem with most of them is that they are inflexible, don't scale well, and do not account for the extra volume in a non-squared corner that will add path length. The bass horn system I came up with does.

The Danley quote is actually a key piece to the puzzle. If the volume around the corner is exactly the same as the straight line distance, then the acoustic path will be shorter than the straight line distance. That means a corner should have slightly more volume than it's straight sectioned counterpart, and adding even more volume will increase it's acoustic length beyond the straight sections length. It also means that if you know the volume is the same, and you path length sims longer than the real deal, you haven't measured the path through the corner correctly.

The nice thing about this method I found is that you can take a measured horn, create a sim from the plans that is equivalent. Then use the simulation output to refold the horn, and the results will be within a tolerance of mm to the original horn. It works in reverse as well. The only trick is learning where/when reflectors are needed, but it isn't hard to figure that part out. It is just a continuation of the line of thought stemming from the quote, and applying the same solutions.

I don't think it is the only way to do it, or even the best way. It just seems to work for me, and a few others.

Last edited by soho54; 31st January 2011 at 08:44 PM.
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Old 31st January 2011, 09:15 PM   #210
soho54 is offline soho54  United States
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Quote:
Originally Posted by 4pyros View Post
So what is the preferred rate of expansion in a taped horn?
It varies with the cutoff frequency needed. The driver also plays a part, as sometimes the best flare rate for a certain combo is not at the "ideal" position.
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