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Old 23rd January 2020, 05:19 AM   #861
mabat is offline mabat  Czech Republic
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Quote:
Originally Posted by Dave Zan View Post
... Flaws in the standard are of more interest now that Marcel has provided us with tools to manipulate not just vertical and horizontal polars.
Guys, I only give you a primitive tool that generates STL files based on some numbers. Be careful with it
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Old 23rd January 2020, 05:50 AM   #862
Patrick Bateman is offline Patrick Bateman  United States
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Originally Posted by Dave Zan View Post
This points out a weakness in the "spinorama" (CTA 2034 standard) that it only measures on the horizontal and vertical axis so "dirt can be swept under the carpet" i.e just pushed into the unmeasured diagonals.
For instance it seems possible that the JBL M2 beaky horn may do this.
Are you still in contact with the people involved with speaker measurement standards?
Have you discussed this or considered it yourself?
Any idea of how I obtain a copy of the new standard update (CTA 2034B) to see if it's been addressed?

Best wishes
David
Whenever I've tried to make large waveguides with wide beamwidth, I find that the wavefront has a habit of "detaching." Basically the waveguide expands so fast, the wavefront ceases to "see" the walls of the waveguide.

So you wind up with a big waveguide that performs no better than a small waveguide.

Click the image to open in full size.

I think that's why you never see these ultra wide waveguides in sizes that are much bigger than about 5-7" in diameter.

Click the image to open in full size.

One solution to that problem, of the wavefront "detaching", is to shrink the other axis dramatically. For instance, the vertical axis of the JBL 4367 is just 40% of the horizontal axis.

Click the image to open in full size.

Another solution, is the "beaks." These waveguides are basically TWO narrow waveguides, stacked on top of each other, and nearly perpendicular. Then blended together.

So... Yes: this is partly cosmetic. But it's also a way to make a waveguide that has wide directivity on BOTH the X AND the Y axis.

Greg Timbers, designer of the JBL Synthesis Array, wrote the following:

"The design goal of the 4367 was to equal or surpass the performance of the 4365 in a smaller enclosure and for 1/3 less money! Done and Done. The system will thrive on LF EQ. There is plenty of headroom in the woofer so 4 - 6 dB of boost around 32 Hz will really spice up the mix.

The system has very nice imaging but it cannot touch any Array in that area because the horn orientation is the wrong direction and the horn is in a wide boxy enclosure. The M2 does better in this regard because the horn is symmetrical in pattern and designed to have a significantly wider coverage pattern. That just can't be done in such a compact horn as those in the 4367 and 4365. BTW, those two horns behave very similarly with neither having a major edge on the other in measured performance.
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Old 23rd January 2020, 07:12 AM   #863
Dave Zan is offline Dave Zan  Australia
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Originally Posted by Patrick Bateman View Post
...with wide beamwidth, I find that the wavefront [detaches]
To some extent this is understandable but the exact details are not clear to me.
There are claims that an "OS waveguide" can "pull out" the wavefront but when I have asked about this I have never received a clear answer or data.

Quote:
...it cannot touch any Array in that area because the horn orientation...
Nice to see this quote, it supports my own back of envelope calculations, wide but vertically short horns aren't optimal.
It relates to a problem here that has bothered me for years -
As you have pointed out, for the "diffraction acoustics" zone we need to make the source narrower to make the pattern wider.
And yet for the "ray acoustics" zone we need the opposite.
How do we do the transition?
I.e. the mouth of a rectangular horn will have essentially the aspect ratio we need to cover but the throat needs to be far from circular to diffract into a rectangle.
The answer at one frequency looks like it should be the inversion in a circle Inversive geometry - Wikipedia.
But how best to cover a frequency band has defeated me so far.
Maybe now I can simulate it with Marcel's software, or perhaps he will have some mathematical idea

Best wishes
David
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Old 23rd January 2020, 07:13 AM   #864
mabat is offline mabat  Czech Republic
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Can't wait to see some superformula-based horn that performs better than what we have seen so far.
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Old 23rd January 2020, 07:16 AM   #865
mabat is offline mabat  Czech Republic
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Originally Posted by Dave Zan View Post
There are claims that an "OS waveguide" can "pull out" the wavefront but when I have asked about this I have never received a clear answer or data.
That could be tested in ABEC quite easily, couldn't it?
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Old 23rd January 2020, 07:27 AM   #866
mabat is offline mabat  Czech Republic
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BTW, the tool already has the functionality of morphing an arbitrary shape into a rectangle. You can start with whatever you like and tell the software to e.g. "start at half the depth and gradually deform it so that it becomes rectangle at the mouth". You would get this by setting the parameters "Shape=raw2rect" and "Shape.FixedPart=0.5". Maybe some interesting creations could be made this way. You might also discover you would need some more parameters to be more flexible - just tell me.


...And after all of this is tried and tested, I think it is quite likely that we return back to a good old big round OS WG, heavily rounded (still possible, BTW). And I think the reason you don't see these big devices much around, is because they are BIG.
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Last edited by mabat; 23rd January 2020 at 07:52 AM.
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Old 23rd January 2020, 08:50 AM   #867
mabat is offline mabat  Czech Republic
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Oh, I see the limitation now - the "raw2rect" function currently takes only the bounding rectangle of the existing mouth. I should add parameters for an arbitrary target width and height.
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Old 23rd January 2020, 09:43 AM   #868
Dave Zan is offline Dave Zan  Australia
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Originally Posted by mabat View Post
That could be tested in ABEC quite easily, couldn't it?
I expect so, I just haven't done it yet.
I would also like to see some experimental data.

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...And after all of this is tried and tested, I think it is quite likely that we return back to a...round OS WG, heavily rounded...
I concur we need heavily rounded to reduce mouth diffraction, for sure.
Also makes sense to me to have the horn blend into a baffle.
I am less convinced that round is ideal, we typically want around 90 for horizontal but I see little point to spray sound at the floor or too far upwards.
I personally am not too bothered by size up to around 750 mm horn width, to fit diagonally in a 600 mm module.
How tall is constrained by the requirements to keep the woofer centre close to the horn axis, rather than space limitations.

Best wishes
David

Last edited by Dave Zan; 23rd January 2020 at 09:55 AM.
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Old 23rd January 2020, 10:21 AM   #869
mabat is offline mabat  Czech Republic
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I am less convinced that round is ideal, we typically want around 90 for horizontal but I see little point to spray sound at the floor or too far upwards.
There may be some merit in that. The point is that it is not so easy to achieve without deteriorating the power response over the whole band. Such asymmetrical horn would have to be so big and the driver crossed so low that the blend to a woofer is in a frequency range where the two directivities are already close to each other, i.e. where they are of low DI and it doesn't matter anymore. You just can't "blend" a round woofer say at 800 Hz with a rectangular horn and expect nothing bad happens. It does - in fact then there are no less reflections, they are only more colored. From this perspective, there is no point in trying to make the center-center distance smaller by reducing the height of the horn. That just doesn't work. If not round, then I would go for a squarish shape, for sure not anything considerably asymmetrical.
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Last edited by mabat; 23rd January 2020 at 10:39 AM.
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Old 23rd January 2020, 12:01 PM   #870
Dave Zan is offline Dave Zan  Australia
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There may be some merit in that. The point is that it is not so easy to achieve...
That just doesn't work. If not round, then I would go for a squarish shape, for sure not...considerably asymmetrical.
Yes, I have also come to the conclusion that even for a moderately asymmetric pattern, say 80 by 50, the optimum horn mouth is close to square.
Not what I initially expected but it turned out better to make the mouth a little taller rather than try to make the mouth aspect ratio match the pattern ratio.
That means some fiddles to make the pattern control, flattish parts of the horn horn all meet correctly in the throat.
The vertical apex is not the same depth as the horizontal apex, but not as extreme as some of the old PA horns.
And a main reason it was a problem for PA was that such horns didn't array well, not an issue here.
Also possible to make the roundovers of the vertical transitions a little different to the horizontal.

Best wishes
David

Last edited by Dave Zan; 23rd January 2020 at 12:05 PM.
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