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Well, the reason for the extension idea is twofold

Thanks for your answer, I Think I have a better understanding now.

Actually, I hade two things on my mind.
1. The eliminiation of the cavity-error caused by an 10 degree 2" driver exit meeting and a 4 degree 2" horn throat.
2. Since I am doing an extension of the horn to solve the first problem, Is there reason to extend the horn even further?

No, The Radian 750Pb is a 2" exit driver and the horn has a 2" throat.
But the driver exit-angle is 10.4 degrees and the horn-throat-ending is about 4 degrees. This make a smal (but not negligable) cavity inside the driver exit (an anomaly in the horn curvature).

By removing the dust protection cloth at the driver, I could extend the horn going into the driver, all the way down to the air-vents, which in my case is approx. 1,4" in diameter. This means that the problem with driver-exit-angle and horn-throat angle will dissapear.

Since, the preasure alteration in the horn throat is high, this extension must be mechanically firm and sturdy. Still it needs to be quite thin walled to go inside the driver exit bottom. The last inch lip will be coneshaped with a 10,4 degree outside angle, and a 3-4 degree inside angle. I have made some sloppy experiments using carbon fibre/epoxi for a bent extension, which would meet this requirements (se pix).

By making a 107mm extension of the 200Hz Tractrix horn curvature, the diameter goes from 2" to 1,4" (everything else the same). The horn is still a 200Hz cut off horn and the frequency bandweidth is unchanged.

Now, to clear my question (which I Think you have partly answerd).
Whould it of any benefit to make this extension even longer (than 107mm), going to a even flatter curvature than the 4 degree exit discussed above. Or is the tractrix curvature opitimized as it is?

Note, the pix showes one of my experiment with a bent extension pice which is 2" in one end, and 1,4" in the other. In this case, the length is extended from 107mm to 200mm (so it´s curvature does not follow the tractrix curvature by the book). But as I said, this was more of an experimnent using carbon fibre, than evaluating sonic gains.

By making the extension piece bent the extension could be extended arbitrarily ceeping the WAF-factor low ;o)

Finally, I am not sure if I should make this straight or bent when it came to the real extension. I just wanted to show this piece as an alternative.

Finally, I understand that the WE555 is a unique driver. And that I will never come close to its properites with modern PA-drivers. Still the questions must be asked if there is something to learn from those days in modern designs.

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