I read quite a few “experts” here with the absolute, unequivocal statement that nobody can hear about 20KHz. First, the human ear is an analog system. There is no 20KHz “cutoff” beyond which nobody can hear. Everyone has a curve of decreasing perception and some point where the curve falls below the level of perception. The 20Khz frequency cited was chosen for two reasons:
- the vast majority of people cannot hear anything above that frequency (note that I did not say all) and
- it’s a nice round number. If they really searched for the 99th or 99.9th percentile among thousands of listeners, it might be 19,585 or 20,116 for that matter. Mostly, they tended to test with fixed frequencies. But, it is a statistical cutoff, not a brick wall. Some people clearly hear to higher frequencies, many as high as 22KHz.
That said, there is also growing evidence that many people “perceive” frequencies above 20KHz. There are many theories that support the idea that this is an alternative hearing mechanism, and it is yet to be proven. But, some scientific evidence has shown perception of content around 26KHz by people with apparently much lower cutoffs. This work remains controversial.