I have owned and used both the Sequerra and Apature super tweeters. The biggest benefit was with electostatics that rolled off in the top octave, but even with conventional speakers there was a noticible difference, though it would be a value judgement to say that the difference was an improvement. There are several possible explanations. First the built in crossovers have finite slopes so even a crossover with a 15K 3db down point, which is quite higher than my experience, is down only 15db at 7K if the slope is 12db per octave. This is clearly within the audible range. Second, beats, or difference frequencies can be generated through wave interferance in air and on the diaphragm. IE a 22K and a 23K tone will have a difference frequency of 1K, again within the audible range. Finally, I cannot site the source but I recently read an article indicating that there may be a response to frequencies as high as 50K in humans. I might also point out that in conventionally decoded redbook CD there ain't anything up there. In LP, analog tape and the new digital formats there is the capability of recording ultrasonic sounds.
