The sense of hearing is remarkable. I don't know what is my present upper frequency range, but at one point although I could not hear a 15Kc tone (pure sine wave), I could very plainly hear the effect of a 15Kc HF filter on broad spectrum sound. I think that I could have sensed an even higher frequency filter, but that was the highest one available in my electronics. I found this interesting, and formulated the following theory.
The difference between a 15Kc sine wave and a 15Kc square wave is the presence of higher harmonics. The sinewave vs squarewave difference in sound can be sensed even if the harmonics responsible for the difference would, by themselves, be inaudible. I think that the ear senses the rate of change of pressure, which can be high for a low frequency signal if the signal is not sinusoidal.
So, to address the question, I don't think most people can hear a supertweeter (higher than 20Kc) but I do think that they can sense its effect.
