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. . . What would the tweeter's horizontal response look like if the coaxial MW cone is a 6x9 or similar aspect ratio?
[Tweeter mounted flush with voice coil and not pillar mounted].
With the horns I was experimenting with the 10K dip was more a function of the tweeter than the horn.
What 6x9 woofer do you have?
Suppose a cone is not circular, but ellipsoidal. Then the radiation is slightly wider along on the long side (9")
And narrower on the short edge (6"). But don't forget that the baffle dimensions affect this too.
If the tweeter is mounted in the middle of the ellipsoidal cone, then the radiation would follow- wider along the long edge and narrower on the short edge. Because the cone of the mid acts as a waveguide fort the tweeter.
The 10Khz dip that we often see often a function of the apex of the dome tweeter interfering with a nearby object. The exact spot is fairly close (high frequency ~ low wavelength) interface. If the dome "sees" a nearby discontinuity, the reflection will cause a destructive interference that will be seen as a dip.
There is a key to eliminating any destructive interference that cause a dip. Drivers where the engineers have paid a lot of attention to this, like KEF and Genelec, have managed to eliminate the dip. From a DIY point of view, 5th element has done this too, perhaps there are others. But it needs heavy modification, and it's a one-way street if you mess it up.
Now for drivers where this dip is present, well in reality it looks a lot worse than it sounds, because what he hear is not just the on-axis, but how the driver projects sound in 3D space.
But, of course, as we strive for (the illusion that is) perfection, no dip is better than an ugly dip.