Calculate the minimum distance in air required between the source of sound and the obstacle to hear an echo. Take speed of sound in air = 350 m s−1.
The sensation of a sound persists in our ears for about 0.1 second after the existing stimulus ceases to act. Hence, to hear an echo distinctly, it must reach the ears at least 0.1 second after the original sound.
Now, we know that, d=vt2=350×0.12=17.5 m