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Old 6th October 2019, 08:59 PM   #451
DonVK is offline DonVK  Canada
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Quote:
Originally Posted by Dmitrij_S View Post
O course, I forgotten to mention the maximum mesh size I set was 10mm.

Performing BEM simulations I was not sure how to set correctly an interface splitting the BEM object into internal an external sub-domains. I set the interface as depicted by semi-transparent mesh in the picture below. Now I doubt, wouldn't it be better to choose the shape of the interface so that it would be tangent to the mouth . Interestingly, is there any rules how to split an object into internal and external sub-domains to get correct results in BEM external radiation problem ? Perhaps this is where additional error comes from ?

The 10mm inner horn mesh size is fine.

The inner/outer split should be something like this. I moved the mouth interface subdomain boundary so you can see it, but it actually sits at the horn apex even if there is rollback.
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File Type: jpg example_mouth_intf.jpg (72.5 KB, 385 views)
File Type: jpg example_mouth_intf2.jpg (107.0 KB, 403 views)
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Old 7th October 2019, 06:02 AM   #452
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Quote:
Originally Posted by Dmitrij_S View Post
...As regards the "impedance hole", I'm not sure if this is a peculiarity of the horn or is it some kind of numerical simulation error.
Did you change the inner horn wall impedance in any way? It is rendered green which means it hasn't a default setting (100% reflective), IIRC. Or is it green because it's in the interior subdomain? Now I'm not sure. I first saw a green boundary after I changed the wall impedance.

I have not found much differences between different interface shapes, some may be slightly better than another in minor numerical aberations but nothing like what you show. From the manual it's also clear that it shouldn't make any difference in principle. Only avoid very sharp angles, but that holds everywhere. I don't think the interface boundary itself could be the cause here.

Last edited by mabat; 7th October 2019 at 06:16 AM.
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Old 7th October 2019, 08:48 AM   #453
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- Anyway, if I was to choose an interface boundary to your horn, I would make a "bubble" heading from the mouth apex outward.
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Old 7th October 2019, 08:58 AM   #454
Dmitrij_S is online now Dmitrij_S  Europe
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Quote:
Originally Posted by mabat View Post
Did you change the inner horn wall impedance in any way? It is rendered green which means it hasn't a default setting (100% reflective), IIRC. Or is it green because it's in the interior subdomain? Now I'm not sure. I first saw a green boundary after I changed the wall impedance.

I have not found much differences between different interface shapes, some may be slightly better than another in minor numerical aberations but nothing like what you show. From the manual it's also clear that it shouldn't make any difference in principle. Only avoid very sharp angles, but that holds everywhere. I don't think the interface boundary itself could be the cause here.
I didn't change the wall impedance, it is set by default. Only the color of the walls was changed. It seems that dr. Gedddes was right, the problem was too coarse mesh (10mm). With a finer mesh (5mm) the "impedance hole" disappeared.
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Old 7th October 2019, 09:23 AM   #455
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Thanks for reporting, good to know.
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Old 7th October 2019, 12:35 PM   #456
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Is it a difference between this:
Tech Focus: Variable Intensity (VI) – ProSoundWeb

and straight waveguide tilted?
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File Type: jpg EV-VI-Dual-Boxes-Black.jpg (236.7 KB, 335 views)
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Old 10th October 2019, 05:10 PM   #457
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Quote:
Originally Posted by mabat View Post
... You can try to convert my OS formula to the first form - you'll see where the center is and what the semi-axes are.
I came across my older notes so here it is:
Eq (2) is a rewriting of (1) and from that follows that to take a throat angle (tau) into account, the contour must be shifted in x coordinate by (3) and its scale multiplied by (4). It is all incorporated in the "modified" OS formula.

- If one wanted to have a throat angle the same as a coverage angle, it would (surprisingly) collapse into a cone
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Last edited by mabat; 10th October 2019 at 05:20 PM.
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Old 10th October 2019, 05:35 PM   #458
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Also note that the maths doesn't prevent you from a throat angle even bigger than the coverage - see what happens

Link to the online graphing of this example: Desmos | Graphing Calculator
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File Type: png OS-1.png (63.8 KB, 231 views)
File Type: png OS-2.png (65.1 KB, 47 views)
File Type: png OS-3.png (64.1 KB, 49 views)

Last edited by mabat; 10th October 2019 at 05:44 PM.
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Old 10th October 2019, 06:26 PM   #459
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BTW, for those who would like to fiddle interactively with the last OSWG-SE formula, this is a great tool (angles in degrees): OSWG-SE / Desmos | Graphing Calculator
- Note that x is supposed to be limited to [0, L].
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Last edited by mabat; 10th October 2019 at 06:39 PM.
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Old 10th October 2019, 07:08 PM   #460
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I guess it should be more like this: Desmos | Graphing Calculator
For s = 0 you have a pure OS. Increase it to add more of a termination.

- Together with a cone of the same coverage angle: Desmos | Graphing Calculator
(You can observre the effect of a different throat radius (r) on the contour.)

Last edited by mabat; 10th October 2019 at 07:37 PM.
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