Explain how an echo is heard?

If we want to hear a distinct echo, the reflected sound or echo must reach the ear 0.1 s after the direct original sound. If we take the sound speed to be 344 m/s, the sound must go to the obstacle and reach the ear of the listener on reflection after 0.1s. This implies that the total distance covered should be at least 344 (m/s) × 0.1 (s) = 34.4 m. Thus for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance i.e., 17.2 m from the source.