Question

Why is an echo not heard when the distance between the source of sound and reflection body is less than 10 m?

Answer 1

The impression of sound lasts on the eardrum for `1/10` of a second. In `1/10` of a second, the sound travels a distance `332"m"/"s"xx1/10"s"` = 33.2. Thus, the minimum distance between the source of the sound and the reflecting body should be `33.2/2` = 16.6 m.
As the distance 10 m is far less than 16.6 m, therefore the ear cannot make out when the original sound had died and echo has been received. Thus, no echo is heard.