Question

For hearing an echo, the distance from the obstacle should be:

• A. less than 10 m
• B. between 10 m and 15 m
• C. 17 m or more
• D. none of these.

Answer 1

Answer: (C)

17 m or more

The sensation of sound persists in our brain for about 0.1 s. Thus, to hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1s. If we take the speed of sound to be 344 m/s at a given temperature (say at 22 ºC) in air, sound must go to the obstacle and reach back the ear of the listener after reflection in 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., 34.4/2 = 17.2 m