Super tweeters are one of the stranger product groups to hit high-end in the last few years. See, human hearing rates between 20Hz and 20kHz. Super tweeters, on the other hand, only tackle the upper frequencies, shooting all the way up to 45kHz and even 100kHz. Craziness? Everyone who has listed to a super tweeter says they can "hear" the difference. And when the super tweeter is removed, said reviewers feel the music loses something significant.
The story goes that the tiny bones in your ear resonate above 20kHz and your brain processes the information, though not in the typical way. Live music goes above 20kHz, so why not make speakers that do as well? (don't even ask about the limited range of CD mastering...) Companies like Tannoy and others have been hip to the highs for years.
Pictured is the Extended Response Tweeter from AV123, makers of giant-slaying Rocket loudspeakers, priced at an extraordinarily low $349. So popular, the ERT is backordered until the end of November, having sold out their first few runs already. The way it works is you stick the ERT on the top of your speaker (as shown), attach the cables to your speaker's binding posts and, presto, instant super dog whistle.