Matching horns to compression drivers, a discussion

Radian 950PB, plot from Radian themselves.
Not the BEPB.
It's the 951 with a factory fitted 1,4" to 2" expansion.

Whereas the 475BE measures worse then the standard 475.

Vance is doing this all over the place in his reviews. Like he just picks up whatever horn is on his desk, then writes up a review about a CD.
Take the Eighteensound ND4015BE review from voice coil mag as an excellent example.
1,5" exit compression driver, 4"dia. BE diaphragm.
Mated to the XR2064 2" horn.

Noted in the article the ND4015 2" version was tested, which does not exist and never did.
What exists is a couple optional external throat adapters, that was not designed to fit the ND4015.
It is very easy to see the issue resulting from the mismatch between throat adapter and driver from 10K and up.

Compare that to Docali's review" found here:

So Voice coil mag. should be taken with a grain of salt at times.
Not sure what Vance Dickason is up to, clearly there is huge room for improvement in some of his testing and declarations of what he does.
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Pano, there is nothing in the thread to suggest or support empirically that it is the CD exit angle to horn throat angle mismatch that is causing some differences on the Yuichi horn.
The Yuichi initial expansion is very similar to the Smith DSH ("distributed source horn"), an approximation of an exponential expansion.
The Yuichi used a hypex (hyperbolic-exponential) in the throat, very little difference.

Smith, Yuichi.png

Putting a 60 degree angle exit driver on a horn designed for drivers using ~1/3 that angle both increases the Fc and causes a throat reflection, bad for response.
The Radian spec sheet says as much:
60 degree exit angle.png

Doing the opposite, long "old school" ~15 degree driver exit coupled to a wider throat expansion does not make such gross problems appear in the response, especially if the transition area is smooth.
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OS is not a smooth continuation to a cone. Cone has zero curvature, whereas OS has the highest curvature right at its start.

For a driver like the 2" Radian above, it should be the best simply to start the horn/waveguide at the exit of the phase plug and continue with a smooth curve from there, all the way to mouth. Or, make an intentional curvature change and add a resonance that helps to increase the efficiency.
Perhaps something like OS.
Screen Shot 2024-04-02 at 12.12.32 PM.png

A "fast expanding" waveguide as Mabat described ;)
In other words, don't use the Radian 760NeoPB on any typical exponential-type horn, which have a low expansion rate commensurate with a low Fc.

And it's recommended "minimum crossover frequency of 800Hz using a 24dB/octave crossover, higher with lower filter rates" is another indication that it is not designed for horns like the 290Hz Fc Yuichi.
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There's very old published research showing the wave pressure front propagation in a conical horn is a dog's breakfast of disorderly modes not consistent with orderly progression of a plane, spherical or other fancy shape down the length of the horn.
Conical is the perfect continuation of spherical of the same angle.

The graph you mention shows a plane wave into conical.. These days we would use OS instead. In between, as you model OS with intermediate angles of driving wavefront, it becomes closer to conical.
Berd said:
I have had a pair of Radian 950PB (AL diaphragm) for about a year now and have been using them on a pair of Azura 340 horns.
I liked what I was hearing and so I decided to upgrade to the Be version.

The new drivers arrived today and the throats have a big step in them which is not present in my older 950's.

I have a bunch of compression drivers and I have never seen a big step like this in a throat.
All the other drivers I have are smoothly tapered and I always thought discontinuities in the throat/high pressure region of horns was something to be avoided.

Radian customer service says they have not changed the throat designs but the two examples I have are clearly and significantly different.

Is a step like this normal ?
- Lifted from this thread Radian 950 throat, Is This Right???

Allow me to circle back to this fact.

I'm pretty confident that "kevinkr" would still be using his Radian 950's along with his Yuichi's if his Radian 950's were of the original design type.

FWIW, the designer of that driver is a member here ( though hasn't been around for a number of years ). Seems he did the design work about 25 years ago.

It's pretty clear to me that Radian has been changing up ( cheapening ) their product line ( all the while denying it ).

That's really the "Ground-Zero" of Kevin getting hosed and then wondering about his un-expected results.


Radian_950pb throat image.png
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I think Earl has nailed it. When I have some time I will pull one of them out of my storage cabinet and take a good picture, but I believe mine have the step as shown.
Yes, it's the 950 factory throat adapter.
It's a 951 with a adapter mounted to it.
Means one can produce 1 driver, fit a adapter, then you have 2 'separate' products while having to manufacture 1, and a adapter. Makes for easy profit.

Radian cutting corners is nothing new, you will find posts on lack of internal fit/finish and assembly around the web.
Like they're 5 slit phasing plug, has not always been very well made and finished.

It is also one of the only "premium priced" compression drivers that does not have any form of copper in the gap.
Visible from the impedance traces.
Lets you get away with a less powerful magnet etc.

Does make sense in the form of saving money on manufacturing, and boosting profits.

Not really makes sense on a "premium" BE diaphragm driver supposed to cover the top octave.