Regarding supertweeters

Read: http://www.cco.caltech.edu/~boyk/spectra/spectra.htm

Your own conclusions will be better than mine.

I believe in supertweeters, because I know, from personal experience, that auditory ability exists well above the frequency at which you can detect the pure sine wave that is used for testing.

However, I think you may be on to a point about CDs, and their 20KHz limitation. Some LPs can go a bit higher, but there is some argument about how high, and it certainly varies a lot between different LPs. If there is no signal at 30KHz, a supertweeter good to 50KHz is just an expensive paperweight. Probably the best application for a supertweeter (apart from some very exceptional vinyl systems) would be a SACD or DVDA system, where there is no doubt about the ability to have signal above 20KHz. Of course, the recording equipment must be good enough to include this signal.

Hifimannen, in order for the supertweet to contribute (musical) supersonic sound this content (sound) must be on the storage medium in the first place. It rarely is, as Eldartford notes.

Further, no cdp plays over 20k. What is there is often spurious hi frequency noise (unless you throw a sine at it).

Even further, if yr amp specifies 20kHz then its amplitude response will be much reduced beyond that --limiting the usefulness of a supertweet.

To make things worse, our ears' sensitivity typically goes down as the frequencies go up over 3kHz (i.e. to our ears, a louder signal at say 10kHz will sound as loud as a soft signal at 3kHz).

Ultimately, a supertweet usually functions as a "supratweet", intensifying (i.e.playing in conjunction with the nromal tweet) ordinary audio frequency signals over 13kHz or so -- thereby creating the effect of "spaceousness".