Question

In order to produce an echo, the minimum distance between the source of the sound and the reflecting body should be :

• A. 14 m
• B. 16 m
• C. 17 m
• D. 20 m

Answer 1

The correct option is C 17 m

As 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 we take the speed of sound to be 344 m/s at a given temperature, say at 22 ${}^{0}C$ in air, sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s.
Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back 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., $\frac{34.4}{2}\approx 17m$