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-   -   Acoustic Horn Design – The Easy Way (Ath4) (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4.html)

turk 182 3rd October 2019 08:27 PM

am i reading the graphs correctly...the amplitude shading corrects the off axis response?

mabat 3rd October 2019 08:35 PM

I assume it could. It's a polar map for the interior of a infinite OS waveguide (or close to that). What influence it has on a far field performance of a finite horn is still to be found out. That takes another simulation.

gedlee 3rd October 2019 09:29 PM

Quote:

Originally Posted by mabat (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4-post5933433.html#post5933433)
Step-tapered throat amplitude, 2" throat, 75 deg coverage (left: flat; right: four steps from the axis: 1 - 0.9 - 0.8 - 0.75):

This throat shading has to be done by the phase plug, which is were my patent comes from. In it I show how the phase plug can be designed such that amplitude shading would occur at the throat. I never actually made drivers, so I could never try it. Maybe today with 3D printing it's a piece of cake, back then nothing like that existed (for reasonable amounts of money.)

turk 182 3rd October 2019 10:05 PM

any drawings of what this would look like?

gedlee 3rd October 2019 10:17 PM

Quote:

Originally Posted by turk 182 (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4-post5933536.html#post5933536)
any drawings of what this would look like?

Acoustic Horn Design – The Easy Way (Ath4)

Dave Zan 4th October 2019 03:49 AM

Quote:

Originally Posted by Dmitrij_S (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4-post5933143.html#post5933143)
Dave, could you recommend a book describing the math of the evanescent wave effect ? To me it is not evident the nature of the effect in acoustics.

Hi Dmitriy
The animation that Marcel linked is pretty nice as a starter, thanks Marcel.
The standard book for this aspect seems to be "Fourier Acoustics" by E. Williams.
I must admit I haven't had the chance to read it much yet so I don't know how it compares other classic works like P.M. Morse on this subject.
Morse would be the other place I would start.

Best wishes

Dave Zan 4th October 2019 04:29 AM

Quote:

Originally Posted by gedlee (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4-post5933292.html#post5933292)
Your in luck!

Thanks, I will study it this afternoon.
I think your comments essentially reflect my concerns.
My first impression is that the measurements will be insensitive to the different modes, except in the evanescent wave area close to the throat.
So the data we want will be contained in relatively small differences between measurements, in comparison to the size of the measurements themselves.
That will be mathematically evident as a near-singular matrix, just as you observed.
You wrote that the work was curtailed at B&C, how far did it actually proceed?
You worked out the maths, was there a prototype built, tested?
What approximate mic distances did you have in mind (or test if it went that far)?
Is there any test data or estimates of what sort of accuracy would be needed?

I have a lathe and mill - a PWT is a project I have considered, could actually build in less than a human lifetime.
Would need a pressure mic...hmmm

Best wishes
David

orjan 4th October 2019 03:29 PM

Hi,

Further info from the link Mabat posted.

YouTube

YouTube

Seemed appropriate to discussion.

/örjan

gedlee 4th October 2019 04:44 PM

Quote:

Originally Posted by Dave Zan (https://www.diyaudio.com/forums/multi-way/338806-acoustic-horn-design-easy-ath4-post5933772.html#post5933772)
I think your comments essentially reflect my concerns.
My first impression is that the measurements will be insensitive to the different modes, except in the evanescent wave area close to the throat.
So the data we want will be contained in relatively small differences between measurements, in comparison to the size of the measurements themselves.
That will be mathematically evident as a near-singular matrix, just as you observed.
You wrote that the work was curtailed at B&C, how far did it actually proceed?
You worked out the maths, was there a prototype built, tested?
What approximate mic distances did you have in mind (or test if it went that far)?
Is there any test data or estimates of what sort of accuracy would be needed?

Best wishes
David

My role at B&C was to lay down the theoretical foundations of projects that they would move forward. We did several and some of them were published. But I left just as I delivered the document that I attached. I do not know what, if any, steps were taken to proceed with this project.

B&C had done some testing that indicated that the wavefront from the drivers was certainly not flat, but their procedure was limited and could not differentiate the different modes. My approach could.

As to the evanescent waves, please remember that this is only an issue at LFs, and that HOMs are not at issue at these frequencies. So in the region where HOMs are a concern, there are not evanescent waves because at those frequencies they propagate. This does mean that the wave shape at LFs may be hard to obtain, but that's because only the plane wave will propagate anyways, making it a non-issue as far as what reaches the listener. Wavefront curvature is primarily an issue when the HOMs can actually propagate to the mouth, and if they can do that then measuring them is not a problem.

As to the mic spacing, I would think that a regular spacing would not be good, or at least not what I would try at first. But, the mathematics should be able to handle any spacing as the singularities that would occur with a regular spacing would just get resolved by the SVD calculations.

Evanescent waves in a tube is not discussed in any of the texts books by Morse or Williams. Williams book has a small section on them, but not as they apply to our situation. One can see they mathematically by looking at the PDF that I linked. If Kr > K then Kz become imaginary and the normal complex exponential defining wave propagation becomes an exponential decay with Z.

danibosn 4th October 2019 06:40 PM

Is this gold


https://www.diyaudio.com/forums/atta...h4-oswg-se-png

already implemented in ath4?


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