As a physicist, I would like to continue my earlier comment by stating that uncontrolled tests with equipment of unknown characteristics may be leading many people to think they have hearing that extends into the inaudible. This is bad science. There is the burden of proof using real lab gear, and that, as far as I can tell, has not been demonstrated on this forum. The physics of hearing and sound can't be fooled. Regarding my one student who claimed hearing just over 20K, age and gender may allow it, but I did not have a chance to verify the physics of what was happening to my satisfaction. There was some research way back about defeating the low pass filter of the middle ear by using bone conduction, however, I was never able to test this with my students. Put a 19KHz ultrasonic transducer on the skull - see if there is a difference in perception. There was a limit to what I was willing to send through students without signed releases. Oddly, the lab had an x-ray tube from around 1960, and the included experiment was to allow students to view hand bones using the included fluoroscope. How times change...soon all actual experiments will be eliminated as being too scary, and far fewer kids will ever develop an interest in science. Kids need to touch the real thing. I remember my first time alone with an electron microscope - didn't come out for days, changed my life.
Anyway, using uncalibrated computer tone generators through low-bidder (even what we consider high-end) sound cards into unknown output electronics into transducers of dubious pedigree needs to be replaced with lab-quality, calibrated function generators and transducers. For those who have never used such equipment, I can assure you that there is a difference. Calibration is a key to getting this verified. Also, it is very easy to verify what was said above about energies and frequencies - just get two generators, a scope, and minimal components. Mix, watch. Oh, as long as you have two frequency sources, put one to X and one to Y on the scope and watch the lissajous patterns for hours. Mix in other waveforms (triangle, square) for hypnotic patterns. Hours of fun for those of us who dislike everything about computer games. I will post a few pics if I get a chance to set it all up on a bench. For anyone who wants to see a video of a sound experiment lead by one of the coolest physicists ever, google the TED talk by Cliff Stoll. You'll enjoy his talk, and you can do it at home. Experiment - enjoy!
Thank you. That's what I was saying a little back. Good luck with our "consumer grade" hi-fi equipment being accurate enough. Even if everything you own is lab quality, 95% of IEMs drop incredibly fast after 10-15khz. So no matter how good your equipment is, it is only as good as the weakest link.