Question

A person standing at a point 12m from a wall that produce an echo will not hear it.

Answer 1

The person may not hear it due to maybe if tearticles of air are tightly packed or the person maybe has lost his ears or the frequency of the sound produced are low

Answer 2

To hear an echo,the original sound and the reflected sound should be separated by a time interval of at least one tenth of a second. If two sounds occur within one-tenth of a second, you will not hear them as separate sounds. As a human ear cannot distinguish sounds if they are not separated by one-tenth of a second.