How Everyone can hear at least 30 kHz via Square Waves
Amplify a 10 kHz sine wave. Switch sine to square wave. Stark difference is immediate.
Sine-wave fundamental remains. Added are odd-order harmonics of fundamental, first and loudest of which is 30 kHz.
The change from sine to square is accomplished by adding onto the sine wave its odd-order harmonics. The 10 kHz sine wave becomes square due to the summing of 10 kHz sine wave, plus 30 kHz sine wave, plus 50 kHz sine wave, plus 70 kHz sine wave, plus 90 kHz sine wave, etc. with each successive odd-order harmonic decreasing in amplitude. This is mathemetically true and easily verified by spectrum analysis.
This widely verifiable fact seems to be strong evidence for “ultrasonic” hearing being actually quite common.
Therefore, human hearing that spans only “twenty to twenty-kay” seems likely another audio myth.