Attn. Dr. Geddes: What about acoustic lenses?

[NV&H]

I recently did a little searching on the internet for devices that could be used to control the dispersion of HF devices, other than highly engineered horns. I came across acoustic lenses that JBL developed back in the late fifties, early sixties. They looked like louvers mounted in front of the horns. They look funny, but once I figured out how they worked, they struck me as very elegant solutions to the targeted problem. By providing ever longer pathways for sound sound emanating further away from the centerline of the horn, thus delaying its launch from the device to varying degrees, it diffracts the sound much like a glass lens diffracts light. But they only control horizontal dispersion given their 2-dimensional form.

I was blown away by this.

Seems like metal foam with open cells of the right size could be used to create acoustic lenses that could control both vertical AND horizontal dispersion. Since you use a foam plug in your horn designs, I was wondering if this approach ever occured to you and if so, what you thought about it.

[BHTX]

Those JBL lenses are a very outdated concept. There's a reason why they haven't been used for so long. They sucked.
I remember Geddes responding to this exact topic here in the past. I don't remember his exact words, except that they didn't work very well.

edit:

http://www.diyaudio.com/forums/showt...95#post1541195

Originally posted by graaf
Hello Dr Geddes!

I have a question concerning an "acoustical lens"
something like here: http://www.jblpro.com/pub/obsolete/A...ns_Family1.pdf

They can be found in some "hi-end" or even "statement" speakers from the 70-ties, JBL and others (for example Yamaha FX-1 and Onkyo Scepter 10)

Why are they under "obsolete" now?
What was wrong with them?

best regards,
graaf

Originally posted by gedlee
I did my MS thesis on acoustic lenses. They work very poorly. They are resonant and don't get the job done. I'm surprised that they lasted as long as they did.

What's with the nostalgia craze!? Those old designs were basically not very good. If something hasn't survived there is usually a reason for it. Thats why the things that did work are still arround - compression drivers, shorting rings, pleated suspensions with ribbed cones, in paper. Those things have not changed very much at all and for good reason - they work! Things like short horns, acoustic lenses, diffraction horns, ring radiators - they all went away or are going away because they don't work very well. A good set of modern speakers is head and shoulders better than this older stuff.

[NV&H]

Perhaps the advent of metal foam would make a better lens.