Question

Why human beings can't hear a sound if the frequency is lesser than 20Hz or greater than 20kHz?

Answer 1

This is because the top end of our hearing range has evolved to help us accurately detect the direction of a sound. The ear facing away from the sound hears less of the higher frequencies because they are absorbed by your skull. The human ear can hear frequencies up to 20kHz but we can't scream higher than 3kHz or so.

Answer 2

because our ear drum is able to sense only sound waves having 20Hz to 20kHz frequency.