Question

give reason - we cannot hear an echo if the reflected sound reaches our ears before a gap of 0.1 s.

Answer 1

0.1 seconds is known as the persistence of hearing.it is during this gap that the sensation of sound prevails in our ears so the echo gets superimposed on the sound hence we cannot distinctly hear the echo

Answer 2

Because 0.1 sec is the persistence of hearing and if any sound vibrates in our ear less than 0.1 second it can't be detected .