Gobel and the Bending Wave

Yes that's it.  

That’s fine theoretically but in practice the bottom ported Ohm Walsh driver produces sound both pistonically and via wave bending at the same time.  

It would seem apparent that for wave  bending the cone cannot be a "rigid" piston.  

That is my understanding. I have two pair of Ohm Walsh in my home for many years that demonstrate it. It was discussed to some degree a few years back on the Ohm Microwalsh who’s heard them thread. Should still be on the record. One of the contributors was Dale Harder who designs Walsh style speakers and is probably  about as much of an expert on Walsh drivers these days as there is.

No contradiction. The driver modulation is pistonic with sound produced via wave bending as described. The current Walsh model speakers produce lower frequencies pistonically and the higher frequencies through the midrange via wave bending.

Hmm well bending wave Walsh style drivers are essentially specially designed cones that operate pistonically just like traditional dynamic drivers.


"
Bending wave loudspeakers

Bending wave transducers use a diaphragm that is intentionally flexible. The rigidity of the material increases from the center to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside back into the center, long waves are absorbed by a surrounding damper. Such transducers can cover a wide frequency range (80 Hz to 35,000 Hz) and have been promoted as being close to an ideal point sound source.[62] This uncommon approach is being taken by only a very few manufacturers, in very different arrangements.

The Ohm Walsh loudspeakers use a unique driver designed by Lincoln Walsh, who had been a radar development engineer in WWII. He became interested in audio equipment design and his last project was a unique, one-way speaker using a single driver. The cone faced down into a sealed, airtight enclosure. Rather than move back-and-forth as conventional speakers do, the cone rippled and created sound in a manner known in RF electronics as a "transmission line". The new speaker created a cylindrical sound field. Lincoln Walsh died before his speaker was released to the public. The Ohm Acoustics firm has produced several loudspeaker models using the Walsh driver design since then. German Physiks, an audio equipment firm in Germany, also produces speakers using this approach.

The German firm, Manger, has designed and produced a bending wave driver that at first glance appears conventional. In fact, the round panel attached to the voice coil bends in a carefully controlled way to produce full range sound.[63] Josef W. Manger was awarded with the "Diesel Medal" for extraordinary developments and inventions by the German institute of inventions."

The model shown here ( Epoque Aeon ):

https://www.youtube.com/watch?v=Ipjahbc6QRE

looks like the seemingly hidden driver at the top could use the advertised bending wave principle similar to Manger speaker drivers. Pistonic motion would be radial with sound emitted at an angle to that.

The original and somewhat well known "bending wave" configuration that has been around for some time as mentioned is of course that of the Lincoln Walsh-style transmission line drivers where driver is mounted vertically with vertical pistonic motion. Original OHM A’s and F’s from years back did this fairly full range with a single physical driver, newer OHM Walsh model speakers do it similarly but just for the key mid frequencies using much more conventional (and robust) drivers. German Physiks DDD drivers do it for the higher frequencies. Dale Harder’s Exotic Speakers line (very hard to find) take the original OHM Walsh full range design and refine it with more modern technology.

BTW My understanding is that many otherwise conventional dynamic (pistonic) drivers can also function as bending wave drivers for certain frequencies, most perhaps just not very well unless designed to do so. They also do have to be mounted vertically though so the sound emanating at an angle from the rear of the cone is primarily what reaches your ears.

Are AMTs/folded ribbon drivers bending wave? I’ve never thought them to be as I understand it, though perhaps more unconventional in operation than most high frequency drivers, but in a different way. Same true of MBL speakers in general, unique configuration but technically not "bending wave" TTBOMK.