28 kHz is in fact correct, although only in lab conditions:
> The commonly stated range of human hearing is 20 Hz to 20 kHz. Under ideal laboratory conditions, humans can hear sound as low as 12 Hz and as high as 28 kHz, though the threshold increases sharply at 15 kHz in adults, corresponding to the last auditory channel of the cochlea.
(ref: https://en.wikipedia.org/wiki/Hearing_range)
