The sensation of sound persists in our brain for about 0.1 s. To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 s.If d is the distance between the observer and the obstacle and V is the speed of sound, then the total distance travelled by the sound to reach the obstacle and then to come back is 2 d and the time taken is t=(Total distance travelled)/Speed of sound
If we take the speed of sound to be 340 m/s at the temperature, say at 22°C in air, then the total distance covered by the sound from the point of generation to the reflecting surface and back should be
d=(340 x 0.1) / 2
= 17 m
Thus , for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17 m.