This worked out well ( BMS tweeters / DDS Waveguides )

[Mark Seaton]

Interesting pics. I see what was done, and understand why. I'm not sure I'd call the throat a diffraction device, but it isn't my first pick of how such a horn would be executed. It appears the circle to oval throat just has a shallow expansion to allow the different horizontal and vertical angles to meet while allowing a closer ratio of H to W for the 60 vs 80 degree angles. This is actually responsible for the smoother polar map you see published where pattern is lost at similar frequency range. The only question is really how the transition is handled between the narrower throat and the wider outer angle.

I would want to evaluate it carefully, but I wouldn't write it off based on this alone, especially given the polar maps published.

[Paul W]

To me, the XT 1086 throat looks like a family member from Earl Geddes US patent #7068805 for a "Bi-Spheroidal waveguide".

[Dennis H]

To me, the XT 1086 throat looks like a family member from Earl Geddes US patent #7068805 for a "Bi-Spheroidal waveguide".

I'm not so sure. If I understand what Earl is proposing, the waveguide follows an oblate spheroid profile around the elliptical mouth shape -- the throat is wider where the mouth is wider. I wrote a spreadsheet that does that, transitions from a circle to an ellipse following the oblate spheroid formula. The 18sound is the opposite -- the throat is narrow horizontally while the mouth is wide horizontally -- so it's not following the oblate spheroid formula. I'm not saying it doesn't work just fine, it's just not following Geddes's theories.

[Paul W]

You may be right...between my monitor and John's photo, I can't tell whether the narrow throat is oriented vertically or horizontally.

Ludvig or John?

[rc white]

If you look at the specs they only specify up to 12.5Khz. and state that the horizontal response is 80 degrees -6db here.
A typical 1 inch exit compression driver has a -6db. output of around 60 degrees because at these frequencies we are well into the far field kr >>1 region and the stretching pressure effect that allows the wavefront to follow the wall has dropped to a very small value, in effect if the waveguide wall has greater than a 60 degree included angle, the wavefront leaves the wall, and the waveguide cannot control the directivity.
It is necessary to reduce the source width to around 20mm. for it to stick to the 80 degree wall, this is the probable reason for the narrowing Denis H mentioned.
rcw

[Ludvig]

You may be right...between my monitor and John's photo, I can't tell whether the narrow throat is oriented vertically or horizontally.

Ludvig or John?

The throat is vertical, while the mouth is horisontal. The transition between the throat and the 80 degree wall is somewhat sharp while the transition between the throat and the 60 degree wall is smooth.

[Mark Seaton]

If you look at the specs they only specify up to 12.5Khz. and state that the horizontal response is 80 degrees -6db here.
A typical 1 inch exit compression driver has a -6db. output of around 60 degrees because at these frequencies we are well into the far field kr >>1 region and the stretching pressure effect that allows the wavefront to follow the wall has dropped to a very small value, in effect if the waveguide wall has greater than a 60 degree included angle, the wavefront leaves the wall, and the waveguide cannot control the directivity.
It is necessary to reduce the source width to around 20mm. for it to stick to the 80 degree wall, this is the probable reason for the narrowing Denis H mentioned.
rcw

While I'm not certain, it appears the Directivity Maps (color shaded charts) extend to 20kHz. You can see the very slight hotspotting of the vertical directivity in the 7-9kHz range, although the chosen compression driver can start to play a part in this range, and the -6dB point really doesn't move much.

Do remember that we almost never see this level of information on tweeters, horns and especially multi-way speakers. For some perspective, this would be considered an exceptional response for a 2 way speaker. My own priorities in looking at such plots or related data (polar patterns) is to try and eliminate or greatly minimize the off axis secondary lobes (resurgence in level), while maintaining a wide enough main listening window for the given application without sharp transitions in directivity. Gradual trends are both easier on the ears and acoustics, while also being easier to shift the response 1-3dB either way to keep the power response and on axis response complimentary as possible.

[rc white]

A year 2000 AES paper by Cola and Doldi showed that at above the 8-10kHz. region the driver has as much to do with the directivity as the horn does, they tested a B&C sourced one and one from JBL, the later was generally smoother.

The 1993 paper by Bauman et al., tested an exponential OS and diffraction slot horn and the measured directivity above 10kHz. of the diffraction slot horn is noticeably better than that of the other two.

I haven't seen tests of it but I suspect that differences in compression drivers are lessened by incorporating a diffraction slot to extend the 60degree plus directivity to above10kHz., in the end this is about the only way you can get a one inch throat device to do this, and if you want to avoid using a diffraction slot what you really need is a additional tweeter with a smaller throat.
rcw

[Mark Seaton]

In line with what rcw posted, I would fully agree that in my testing and that of others I have seen, there are significant enough differences internally to some of the compression drivers where you will find that a different driver can have a better or worse transition or match with the flare of the horn at or near the throat as well as affecting what goes on above 8-12kHz.

Tom Danley always liked to remind me that the first part of the horn is in fact inside the compression driver. In the few cases I've either disassembled or acquired the internal profile, the differences observed make much more sense. There are of course other factors related to the path length differences and the resulting pressure front at the exit or inside the compression driver, but the end result we observe is that there will be some interface differences between drivers.