THAM15 - a compact 15" tapped horn

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. :)

Regards,
 
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.

xman1.gif

xman2.gif

xman3.gif


This last one is what happens when you opt for a three section horn, instead of the four section simulation.
xman4.gif
 
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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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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.
 
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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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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.
 
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.
 
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I came up with it through a lot of research, and trial and error.

In addition air don't scale. What works in small ducts doesn't work in large ducts, and what works with large air movement doesn't work with
small air movements. It's called the Reynolds Number. You can use any of the popular folding techniques, and most of the time, the SPL curves will
lay on top of each other, but don't count on it.

Tom Danley gets away with murder ... How does he do it???

Just watchin & learnin

Don
 
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Yeah, there are a lot of concepts, and math to absorb.

I could give you a simple generic formula to get you in the ball park if you wanted. I say simple, you need to do an equation of the throat size, the ideal mouth size, then another set of equations to shorten it from ideal to 1/4WL size, and then another set to allow you to know the cross sectional area at whatever points you need for your section ends. With HR you can skip that last part.

This would just be a simple exponential horn. Anything else would require different calculations. :eek:

Wait, there is a simpler way. Input the driver into HR, and go Tools>System Design> with driver. Now look at Tools> View Schematic, and then go File>export>Horn Data. Set all the Height boxes to the internal width you want your horn to be (yeah, I know, ) change all the Increment boxes to 1.0, and set the Width Flare to Uni. Click OK, and save it. Now open the TXT file up, and to find the mouth size take 34400 and divide it by the Lower rolloff corner frequency you used when you Designed>with Driver before. That will give the the Length of the horn, and when you check it in the TXT file you can get the area for your mouth.

It reads a lot worse than it is. :D
 
In addition air don't scale. What works in small ducts doesn't work in large ducts, and what works with large air movement doesn't work with
small air movements. It's called the Reynolds Number. You can use any of the popular folding techniques, and most of the time, the SPL curves will
lay on top of each other, but don't count on it.
I seem to remember specifically saying that most techniques don't scale right as the volume gets larger around the bends.

There is also a lot more to a bend that the Reynolds Number, as the pressure differences create different layers of resistance, and that doesn't even touch on different frequency issues.

Are you talking about folding a horn from a sim, or building a simulation of an existing horn? Either way most people get it wrong.

What is so mystical about Tom's horns? They sim exactly the same as any other horn, and spit out the correct FR and Impedance graphs. :confused:
 
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What is so mystical about Tom's horns? They sim exactly the same as any other horn, and spit out the correct FR and
Impedance graphs. :confused:

Goes back to the LabHorn, where the expansion of the first 10 inches was all over the map. I would never believe that would work, but it did. Then the
dts-20 Tower of Power, where the sound was injected thru a pie-shaped wedge, then expanded, and lastly just ducted. And, of course, the Unity and
synergy stuff, injecting sound into a cone thru ducts.

He (Tom D.) knew how much to push the envelope, and I find that "Mystical."

Any sufficiently advanced technology is indistinguishable from Magic (at least to the uninformed).
 
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OK, gotcha. ;)

He does dream up some very interesting toys, without a doubt. What I find most interesting about them is that once you get a handle on them they are very simple concepts, and you have to wonder why no one ever put two and two together before. That just makes them even more remarkable.
 
Here's Tom's LabHorn with a "conservation of volume" construct that defines the fold in the back two corners.
While this proves nothing, it suggests that my methods agree with Tom's thoughts. I appreciate your efforts
to put this on a more formal footing, but remain convinced that my methods are close enough until your
research is complete.

It will be interesting to compare HornResp sims using these two methods.
 

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I have been thinking about it, and using a single board should make it easier to check for compression in AkAbak.

Just resurecting this - I did some rough high-SPL tests today, basically to the output limits of my car amplifier. It was difficult to get any tests that conclusively measured compression across the pass band of the horn, but @40 Hz, the compression effect seemed to be minor to the limits of the amplifier (it did start showing up, but could be due to several factors).
 
Just resurecting this - I did some rough high-SPL tests today, basically to the output limits of my car amplifier. It was difficult to get any tests that conclusively measured compression across the pass band of the horn, but @40 Hz, the compression effect seemed to be minor to the limits of the amplifier (it did start showing up, but could be due to several factors).
Are you writing about thermal compression (voice coil heating raising impedance) , or something else?
What is the power rating of the amp and speaker?

Most music has between 1/3 to 1/20th the average power of sine waves, 1/3 power would be approaching worst case dubstep or heavy clipping or compression.
Normal pink noise has 12 dB crest factor, so an unclipped 1000 watt amp is delivering less than 100 watts average.
It would take a very powerful alternator and lots of battery to deliver appreciable sine wave power for more than a short burst.