Anybody know anything about spheroidal, oblate spheroidal, or bispheroidal waveguides

[454Casull]

Apparently they have much better dispersion control than do standard horns, though I think they don't load as well.

[phase_accurate]

There was once an article in the JAES about them. IIRC it was written by Earl Geddes.

Regards

Charles

[brummkreisel]

Hi 454Casull,


as I understood it, the idea behind the Geddes approach is the following: the dispersion characteristics of a sound-radiating aperture like a horn are determined by the velocity distribution (vector field) at the mouth of this horn. Standard horn "theories" use highly (over)simplified models and wrong assumptions on how sound propagates through a horn and how the wavefront is shaped (remains plane, or: remains curved etc.). Also, the theories' main concern is loading to maximize output.

Now, when you let aside for a moment the loading thing, and instead focus on the dispersion issue, you will have to think about a horn curvature that yields an analytically defined (and hence: known) velocity field at every point in the horn. From that, you can "easily" compute the velocity field at the horn mouth and hence its dispersion characteristics. (Note that the latter are also influenced by diffraction effects on the edge of the horn and by the possible presence of a baffle into hich the horn is mounted.) Solving the involved equations results into those "waveguide" shapes you mentioned. However, a lot of people claim to use "waveguides" (which is a technical term in acoustics), so it might be helpful in discussions to specify "Geddes Waveguides".

The main advantage of the Geddes approach is hence the possibility to know beforehand what comes out of your appropriately shaped horn. With that, you have a hand on directivity. Oh, and the loading issue is not really an issue, at least when it comes to home hifi applications. I mean, who cares whether you get 110dB out of your compression tweeter or 108dB at one watt?

Cheers from Montreal,
BK


PS; As a matter of fact, Geddes published a bunch of papers on (or related to) this topic. If you need'em, I can give you the AES references.

[454Casull]

I checked out some of his papers and caught a few tidbits of information, but the math was too much for me and I glossed over it.

Thanks for your post!