What's With the Ring Radiator Tweeter?

Are you talking about the true (?Vifa or Scan Speak I think) Revelator ring radiator? If so you are refering to a very highly regarded and well made tweeter which will respond differently to the electronics and enclosures used. They are very clean and can be extraordinarily musical. I have heard the real thing many times. Ring radiator simpy refers to to design which has been cloned and imitated of course more cheaply so be careful about the tweeter you are listening to before making any judgements.
BTW The first time that expression was used goes back to some 50s designs I know because I have compression tweeters with aluminum "ring radiator" housings from 1959.

I'm not sure which one(s) are used by Sonus Faber but I'm curious as to the design, how they are constructed, etc. Also curious as to any experience with them in specific speakers.

Yea I know that tweeter it is used in may hi endspeaker assembly companys like Wilson,Krell,Von Schweikert the list goes on & on. Mostly companys that cant make there own speakers. never the less it is a good tweeter.

Von Schweikert also uses it. From their website...

Due to new developments in transducer design, VSA has announced a highly improved version of their classic VR-4 Gen III model. The new Special Edition model utilizes a new dual-ring radiator, an extraordinary breakthrough in tweeter technology from Denmark. This new "dual-ring" design eliminates the tradeoff between low mass and rigidity, i.e., the problematical diaphragm flex causing distortion. This new tweeter operates from 500Hz to 40,000 cycles per second, with negligible distortion. In fact, it sounds more like a ribbon than a dome tweeter, but power handling and dispersion characteristics are state-of-the-art, correcting the problems found in flimsy ribbons. Although it is trite to say that this new tweeter will allow you to hear things in your recordings that you have never noticed, that is the only way to properly describe the sensation of hearing the Special Edition for the first time! Clarity is achieved not by excess treble energy or amplification of harmonics, but by dramatic reduction in distortion and phase shift.

That certainly provides a lot of info, but I wonder if anyone knows what material(s) are used in making them. I've always been a big believer that materials always inpart their own sound in speakers - metal tends to sound like metal, plastic to sound like plastic, etc. That's why I've liked silk tweeters and carbon fiber woofers. What exactly is the sound of silk or carbon fiber?