I definitely know that I cannot hear even AT 20kHz, much less above it. Maybe I could hit 20kHz when I was younger, but I'm turning 40 this year and can guarantee that my threshold is well below that. I am also a staunch supporter of scientific knowledge and logic and have had many an argument with hard-core audiophiles that insist that they can hear above 20kHz, but refuse to test that in an reasonable way.
But here's the thing -- there is some suggestion that it's possible to at least perceive ultrasonic frequencies. LTD02 linked to a study that indicates the possibility of precisely that. Mark Waldrep, one of the pioneers of "high definition" sound (as he calls it) also freely acknowledges that he cannot hear even above 15kHz, but claims that he can absolutely tell the difference between a high-res mix and a CD-quality mix (both that he did, so no tricks) and doesn't know how that's possible.
The idea of perception is interesting because we freely acknowledge it on the lower frequencies. I believe that the lowest frequency measured in a laboratory as being audible was 12Hz (20Hz, typically). Yet some of you folks have subwoofers that can reproduce frequencies lower than that. I am fairly certain that if you played a 10Hz tone at reasonable volume, that you wouldn't be able to HEAR it at all, but you'd absolutely PERCEIVE it.
Thing is, we do know why that's possible. The rather massive amounts of energy (relatively speaking) embodied in the lower frequencies puts physical pressure on our bodies, even if our ear cannot place it as a sound. Higher frequencies have far less energy and so there's no indication that we "feel" the waves in any similar manner.
Yet, the low frequency example does show us that perception can extend beyond hearing and there are logically minded people who claim to be able to detect "something" when audio goes ultrasonic.
So I want to see for myself. I have representative sample of high-res audio that is also remixed into CD-quality and I simply cannot tell any difference at all between them in an ABX test. But my equipment absolutely cannot reproduce those frequencies, anyway, and so I wouldn't be able to, even if my body physically could.
That brings me back to my original request. Are there DIY designs that can handle these kind of frequencies? If there are, but they are notably more expensive, then I'm just going to pass since I'm not willing to spend (much) extra money on something that I honestly don't think is possible (but want to prove it to myself). If there are and they have a comparable price, then why not?