Geek alert!
The Fountek ribbon tweeter used in the Axis VoiceBox S has the tweeter foil mounted and tensioned in a way that dramatically reduces resonances in the tweeter. The mounting also avoids the distortion-causing stretching that other corrugated foil tweeters undergo as they age. What you get is high-frequency response that is smooth and clean way past 20kHz. The mounting technique is patented, and can be used in all kinds of electro-acoustical transducers that use a diaphragm to pressurise the air.
Fountek Patent for Loudspeaker Diaphragm Mounting (PDF, 1.4MB).
audiorev.net 
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