An echo is the phenomenon of repetition of sound of a source by reflection from an obstacle. For hearing an echo, the obstacle must be situated at a suitable distance from the source. Due to persistence of hearing the sound must take at least 1/10* of a second in reaching the listener. If d is the minimum distance of the reflecting object from the source, then time t taken to cover the distance 2d must be 1/10 th second. '
Therefore, t = 2d/V or d= Vxt/2 = 340 x 1/20
= 17 m.
Echoes are not heard in a small room because the distance is less than the minimum distance of 17 m required for hearing the echo distinctly.