Geddes on Waveguides

454Casull said:
If you happened to have a prototype phase plug which allowed for a perfect plane wave output behind a perfect OS waveguide, you wouldn't need the foam plug at all, right?


No geometry of waveguide can have NO HOM. So the foam plug will attenuate them in all cases. With a poor driver, the need is greater and with a poor horn I doubt even the foam can make the sound of the diffractions go away. It's all additive, lower is better no matter how you get there. An OS waveguide with foam will have the lowest levels of HOM possible with existing drivers - period.
 
soongsc said:
[B
This wave guide is 60deg, slight compression from the diaphragm to the throat. Note that I used a concaved diaphragm driver. The concept is to have a spherical wave front before and after the thoat which was illustrated in a picture I had posted a while back.

[/B]
And some rough measurements 0~45 deg equalized.

An externally hosted image should be here but it was not working when we last tested it.


If the wave guide becomes wider, then the directivity will be better. I'm still debating against myself which way to go.

The diaphragm breakup mode is really amplified here which gives some harshness in the sound unders some conditions.

Note that the notch originally at 8KHz is now gone. I also will be doing measurement of the first wave guide with a modified throat to see what the response looks like.

I'll also put the small wave guide on a baffle to see whether the curves become smoother or not.
 
Hello,
Originally posted by soongscAnd some rough measurements 0~45 deg equalized.
Really? That would be a quite smooth beaming, except the first 7.5(?) degrees.
Otherwise the directivity looks good. It wood be nice, if you could convert it to a directivity pattern diagramm similar to the Nathan ones: the lower one.

It seems to be hard to achieve a constant SPL inside the coverage angle and a smooth attenuation outside simultaneously. I miss the attention to the first point sometimes.

Regards, Timo
 
Recently I got aware of a newer driver from Celestion, the CDX1-1430.

According to their AES-Paper which seems to describe exactly that driver (or something pretty close), they have managed to get a true spherical wavefront (they call it "spherical cap") into a 25deg exit angle, by virtue of a specific time aligning phase plug design.

This seems to be a perfect candidate for an OS-Waveguide, since the expansion of the profile into a, say, 90deg WG is softer than with a driver with 6deg exit angle. This, together with the claimed wavefront properties, should make for an almost ideal driver/WG, producing almost zero HOM (into a 25deg conical WG the HOMs would be zero, wouldn't they? Disregarding reflections from the mouth's edge/expansion into the baffle).

Only problem I see the rather high resonance point at 1.7kHz... but the driver has been used down 1.2kHz with success, IIRC. And user comments about it -- sound-wise -- are very encouraging.

- Klaus
 
I've read that paper and it seems to be contradictory. At one point the phase plug appears to be design to produce a plane wave at the exit and at another point its said that it should produce a spherical wave. The examples clearly show a panes wave. How a spherical wave is produced is not decribed, and from what I see that claim is not accurate.
 
tiki said:
Hello,

Really? That would be a quite smooth beaming, except the first 7.5(?) degrees.
Otherwise the directivity looks good. It wood be nice, if you could convert it to a directivity pattern diagramm similar to the Nathan ones: the lower one.

It seems to be hard to achieve a constant SPL inside the coverage angle and a smooth attenuation outside simultaneously. I miss the attention to the first point sometimes.

Regards, Timo
Hi Timo,

I am not good at that kind of data processing, and relay on whatever existing tools can provide. But I do think the on axis response will be better when the guide is widened. The thing I would like to do is see if the notch above 10KHz and the peak above 20KHz can be smoothed out. I think it may take about 3 more wave guides till I nail it. But the curve shape will probably look simple but be complicated when it's done. I even wonder how surface finish will effect the results.
 
Hello,

In a previous message Earl Geddes published a smooth soundfield map simulation for his OSwaveguide.

http://www.diyaudio.com/forums/attachment.php?s=&postid=1754191&stamp=1235402217

We don't know at which frequency - a probably low frequency- it has been obtained and as Tiki wrote in his message:

http://www.diyaudio.com/forums/showthread.php?postid=1753250#post1753250

"several horns look good up to 1kHz in Hornresponse. One should try higher frequencies to separate the wheat from the chaff. "

Recently, long term very precise BEM simulations of OS waveguide and Le Cléac'h horn has been perfomed by Bjørn Kolbrek at different frequencies.

I assembly 4 soundfield maps obtained by Bjørn Kolbrek for the OS waveguide (mounted on a modelized compression driver) in a single graph. To be attached to this message the resolution of the maps has been largely reduced but I think the essential of them has been preserved.

We can see that the smoothness of Earl geddes simulation is verified at 1000Hz but when the frequency rises the sound field show important perturbation.

Best regards from Paris, France.

Jean-Michel Le Cléac'h

(thanks to Bjørn Kolbrek to allow me to publish the results of his BEM simulations)

gedlee said:
Turns out that the sim can do a sort of plane wave and when correctly done here is what I get:

Note that there are no HOMs. The wavefront are as smooth as could ever be expected.
 

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hello,

You'll find in attached files soundfield maps obtained by Bjørn Kolbrek on a Le Cléac'h horn (Azura horn AH425 mounted on a modelized Altec 288 compression driver) at different frequencies, in order to be compared to the soundfield maps obtained for the OS waveguide.

As for the OSwaveguide BEM sound field maps, the resolution of the maps for the AH425 horn has been reduced but I think the essential of them has been preserved.

Best regards from Paris, France

Jean-Michel Le Cléac'h

(thanks to Bjørn Kolbrek to allow me to publish the results of his BEM simulations)

Jmmlc said:
Recently, long term very precise BEM simulations of OS waveguide and Le Cléac'h horn has been perfomed by Bjørn Kolbrek at different frequencies.

 

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Jean-Michel,

thanks for this data.

But, to me it looks like the plots for the OS-waveguide don't use a suitable profile at the throat. May we ask you or Mr. Kolbrek to give more detailed geometric information on how the driver was modelled, geometrically (throat exit angle, throat length etc)?

From the picture it looks that a driver design was assumed that is a pretty bad mate for a wide dispersion OS-WG (too long throat, small exit angle).

- Klaus
 
Hello Klaus,

You'll find a magnification of the throat part of the OS waveguide mounted on a compression driver.

For what I can judge, in the limits due to the precision of the drawing, the compression driver ensures a plane wavefront at the throat of the waveguide.

So we can admit that the OS waveguide should operate at its best in those BEM simulations.

Best regards from Paris, France

Jean-Michel Le Cléac'h
 

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Thanks Björn & Jean-Michel for the clarification.

My thinking was that it seems the industry is at last slowly responding to the demands of OS-type waveguides by offering drivers with a short and wide, or even no throat (like one of those expensive TADs, and the Celestion driver I brought up). So these might be better used instead of classical drivers.

It'll be interesting what Earl's opinion on these simulations is...

- Klaus
 
My opinion is that these simulations clearly show the superior directivity control for the OS device. As I have said over and over, Jean Michel keeps trying to compare a beaming horn with a CD waveguide. That doesn't work for me. When he has a CD horn with comparable directivity then I'll be interested. His contour fails the single most important aspect of a waveguide and that is directivity control. It doesn't meet the objectives and as such isn't worth considering.

But I do think that the throat in those models does not look right. Why isn't the driver throat on the Le Cléac'h horn?
And why is the Le Cléac'h horn terminated in a baffle when that is not what is done in practice?

All in all I find this a pointless study which is designed to be misleading and not instructive. Quite typical. Nearfield efects are not whats important, its the far field polar pattern that counts - the Le Cléac'h horn will fail that test. And there is no foam in the OS simulation.

Pretty pictures, but not very useful for a real world device.
 
I wonder if Bjørn Kolbrek would be interested in doing a sim using the same small wave guide I have posted data on. I could provide the curves in a DXF file format with the diaphram. It would be interesting to compare with measurements and see if directivity and smoothness correlate with measured data in any way. I understand that it will not be exactly the same due to driver performance, but certain trends should be comparible.
 
gedlee said:
But I do think that the throat in those models does not look right.

Earl,

Could you please show an example of what it should look like? I don't have the intuition to see when a waveguide throat looks right or not, I just draw the contour from the equations and expect it to be correct.

gedlee said:
And why is the Le Cléac'h horn terminated in a baffle when that is not what is done in practice?

The AH-425 is NOT mounted in a baffle. If it had been, the amplitude contours would have met the baffle at right angles. The grey lines are isophase contours spaced 90 degrees apart, btw. The blue horn contour is the entire boundary of the simulation, which is done by the Shell Element Method, the code can be found here. I have compared it to other BEM implementations, and it works very well.

gedlee said:
All in all I find this a pointless study which is designed to be misleading and not instructive. Quite typical.

It was not, at least not on my part. The simulations were run at different times, out of curiosity on how the field inside these two devices would look like. I tried to make them as realistic as possible.

gedlee said:
Nearfield efects are not whats important, its the far field polar pattern that counts - the Le Cléac'h horn will fail that test.

Doesn't the nearfield determines what the far field look like?

gedlee said:
And there is no foam in the OS simulation.

And as I have said before, I have no idea on how to implement that in BEM. Suggestions?

Best regards,

Bjørn
 
Bjorn

From your perspective of trying to do models and learning how to simulate horns and waveguides I find your work highly respectable. But for Jean-Michel to compare a directional horn to a CD waveguide is pointless - he did that not you, I hope that you know better.

For the throat, I would need a far more detailed look at the curves down into the throat. It was not possible to tell from what was posted what those details were.

As to the nearfield to farfield, the nearfield does determine the farfield, but not everything seen in the nearfield will propagate to the farfield. For example do a run of a piston or cone in a baffle in the near field and then compare it to the farfield. You will see that many details seen in the nearfield do not propagate to the far field. The farfield tends to be an average of the nearfield where the details as washed out. Much of the fine structure that you show for the OS waveguide will NOT propagate to the farfield. These effects are called Evanescent Waves and they are "complex" (in the mathematical sense) and have wavefunctions that are real exponentials - they dampen out exponentially with propagation.

All this said I do not doubt for a minute that the fine structure that you show is real or close to real. Constant Directivity (CD) does not come without its costs. There is a price to be paid to achieve it and that price is a certain level of diffraction and HOM. Did you get these exactly right? I don't know, to me it doesn't really matter. I have always said that they were there just as I have said that you can't get CD without them. But CD is the more important (indeed most important) aspect of a waveguide. If people haven't gotten that from reading my work then they have missed the whole point. If a fine structure and some HOM are the price to be paid for true CD then I will gladly pay it!!

What Jean-Michel has shown is not new. Smooth contour horns can have smooth results as shown, BUT THEY ARE NOT CD. That and that alone is the whole point.
 
Bjorn

In BEM I believe that you would have to have a two stage model. The first model would be the foam where the interior medium was not air but had a complex wave speed - this may not be possible with an existing code. Then this would give you the boundary conditions at the exterior surface of the foam which would be put into the far field model.