Question

The minimum distance between the source and the reflector in air, to hear an echo, is approximately :

(a) 10 m (b) 17 m

(c) 34 m (d) 50 m.

Answer 1

17 m

Explanation: An echo is heard distinctly if it reaches the ear at least 0.1 s after the original sound.

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 2d and the time taken is,

$t=\frac{\text{Total distance travelled}}{\text{Speed of sound}}=\frac{2d}{V}$

or, $d=\frac{Vt}{2}$

Putting t = 0.1 s and V = 340 m/s in air at ordinary temperature, we get:

$d=\frac{340\times 0.1}{2}=17m$

Thus, to hear an echo distinctly, the minimum distance between the source and the reflector in the air is 17 m.