Minimum distance for echo
Class x > Physics > Sound
If d is the distance between the observer and the obstacle and V is the speed of sound, then the time taken for echo
t = \frac { total\quad distance\quad travelled }{ Speed\quad of\quad sound } = \frac { 2d }{ V }
or, d =\frac{Vt}{2}
Now putting t = 0.1 s and V = 340 m/s, we get
d = \frac { 340\times 0.1 }{ 2 } = 17 m.
Thus, to hear an echo, a minimum distance should be 17 m.
