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Re: dam my horns image - me three !

Hi

"According to Earl's papers, the larger the waveguide angle for a given throat size, the lower in frequency the higher order modes will propagate. Also, the larger the throat diameter for a given waveguide angle, the lower in frequency the higher order modes will propagate. So I assume you are saying that once the throat or angle gets too large, the more the phase plug comes into play."

This is correct, as the horn angle becomes smaller a source / throat with greater directivity (larger acoustic size) can be used. Conversely, a wider angle requires a source with less directivity (acoustically smaller).
As the horn angle becomes larger and frequency increased there is a point where the driver exit diameter and the drivers internal horn angle also governs the radiation pattern.
If one removes the bug screen from a typical compression driver (not that you should), one normally sees a horn continues into the interior where at some point there is a small diameter. At that point the slots from the phase plug sum into the final output and it is at this summation where one must make the each section of the radiator add as coherently as possible, in a dimension that is acoustically small enough to permit that if all of it arrives there properly. The art of the phase plug is in bringing all of the rings to one spot with the right path length and size and so on and so on (which the so on’s continue for a while).

"However, as I understand Earl's paper, if the throat, angle, and waveguide length are below critical dimensions, the higher order modes will not propagate in the horn. It's like a low pass filter. If I recall from memory, in a 1-inch throat and 60-degree waveguide, I believe the first high order mode is around 20kHz, but goes down to 16kHz at a 90-degree angle, etc."

This is consistent with what I measure and most of my products but I would put it more like that once the acoustic dimension is small enough, the higher modes simply cannot be produced.
Fwiw, Don Keele’s thumb rule for a horn’s directivity break point applied to this situation would suggest a 1 inch dia horn (exit) dimension would begin to become directive (narrower than the horn wall of the external horn) if the external horn is greater than about 50 some degrees wall angle (at 20KHz).


"In this scenerio, I'm not clear what problems the phase plug causes for Earl to use foam to absorb higher order modes, if they cannot propagate below 16-20kHz for a 1-inch driver. Perhaps there are other issues in the driver/phase plug that are not discussed in his papers. Hope he can clear this up."

I don’t know what driver he is using but if one had a 2" exit, the change (if too much, too abrupt) in angle between the internal horn and external horn could well produce significant modes "up high".
The phase plug wouldn’t be part of this unless at the summation point inside the horn, the acoustic size was large enough to support a directive wave structure. As in the external horn, the narrower the internal angle of the horn, the larger the summation point can be acoustically.

Cheers,

Tom Danley


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