suspending a small tweeter in front of woofer for "coaxial" effect?

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long story short, i'm building a set of 15" Karlsons for myself. my non audiophile brother wants a set of speakers - if my Karlsons work out nicely i'll build him a 12" set.

for my own, i was going to take a 3-way approach.

for a smaller set, though, i thought it would be interesting to use a coaxial speaker. unfortunatley the Eminence Beta12CX has too high of a Qts to be ideally used in a Karlson so i had for a while scrapped the idea because any other coaxial i came across was way expensive. then earlier today i was thinking about a possible solution when i remembered seeing pictures of vintage coaxial speakers in which a tweeter was suspended centrally over the woofer cone as part of the frame. this got me thinking...

in most Karlsons the speaker is mounted to the back of the baffle. if i were to attach a minimal crossbar from the front side of the baffle, with a disc in the middle to attach a 3/4" or 1" tweeter(like a Morel MDT12 or Peerless 811435) to, i could come up with what would seem to be a reasonable approximation of a coaxial of that style. this would allow me to maintain a reasonably point-sourced setup and grillecloth it like an old skool Karlson so that neophytes would be none the wiser

has anyone tried this sort of thing before? is it doomed to failure? Karlson or no Karlson.
 
ONe of the worst speakers I've ever heard was a coaxial KEF selling for C$1500 about 15 years ago. WIth any complicated signal it just fell apart. Hard to describe, but psychotic would be a good descriptor.

:xeye:

The salesmen had just given a big spiel on how great this coaxial point source thing was.
 
Reviving this

I'm considering this arrangement for an upcoming build and was curious as to what may affect quality of the final arrangement.
Obviously anything on axis will create diffusion issues but I imagine particular shapes and treatments may alleviate some of the issues. Secondly, considering crossover network order, distance from woofer to tweeter (wave originating points) should accommodate phase alignment shifts. Likewise phase alignment should be considered in general. I imagine the frequency at which to tune to would be the crossover frequency. Also as mentioned earlier the size of this suspended tweeter should not exceed wavelength of any frequency reproduced by the woofer behind it? Assuming limits to that are crossover frequency. I'm thinking curves are better than flat surfaces? Sound absorbing surface materials on rear surfaces?

I'm wondering if this would not behave like a phase plug?

I also imagine a previous posters experience with "psychotic" performance was most likely do to phase alignment and crossover point issues as I asked about earlier.

Anything else I may be missing?
 
This worked really well. The waveguide made it such that the tweeter didn't see the woofer behind (increased directivity). Conversely, the tweeter and waveguide had almost no effect on the woofer response.

Better than any post mounted tweeter I have ever seen, by far. Plus smoother overall response than Uni-Q, which tends to have reflection problems with the neck blend and cone surround.

David S.
 

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I have had great success with it, best when I added a wool waveguide to the tweeter. The midrange doesn't care about the wool but it cleans up the off axis response of the tweeter a lot. I'd be suggest using a cone tweeter though, dome tweeters need large waveguides to control directivity where cone tweeters by themselves have pretty high directivity even without waveguides.

If you want to use a dome tweeter then I'd suggest getting a normal coaxial where the midrange cone can act as a waveguide.

I did try mounting a wool waveguide around the tweeter too, worked even better than without =) Sadly I don't have any pictures of the build, it was of the prototype and building the final speakers is still on my TODO list.
 
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