Yes, I suspected that was the problem with the H91 - the mouth is too small to support 500 Hz. One of the nice things about a tractrix horn is that it has a defined mouth that cannot be exceeded whereas an exponential horn can be extended to any size mouth and is still expanding.- FWIW, many horns that have large aspect ratio mouths ( in comparison ) to the H91 , show a reduction of those impedance ripples ( or impedance reflections ). It's worth looking at Widgets' impedance plots of his H4003 (?) horn clones ( I think that plot is posted in the public forums ) .
The directivity issue is the main reason I decided to "finish" the project before finalizing the XO. These speakers will be firing 'into' a curved reflector panel ala paragon/metrogon. This will obviously help dispersion, but I may be more concerned with power response than on axis response and may need to leave in a fair amount of on axis HF rise to get good HF response in the listening area. I now know how to 'get' and how to 'tame' the HF response with quite a lot of options.- FWIW ; I use a round mouthed exponential horn on a Altec 288-8K ( that supports 800 hz fairly well ). The horns rising Directivity Index ( DI factor ) means I need to use a series LCR ( strapped across the driver ) to flatten a quite symetrical peak ( centered around 6200 hz ) . I also use a pretty simple HF bypass circuit that does ( somewhat ) overdo the HF content ( and therefore, "contaminates" the octave below /as you have run into / & forces the need for a bigger notch )
Thanks to everyone - especially Zilch and EarlK - for all the help. When I finally get things finished, I will post picutres and response curves. In the meantime, I may come back to the well for help as things progress.
Again, thanks to all!
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