Question

In order to hear distinct echoes, the distance of the obstacle from the source of sound should be:

• A. at least 17.2 m
• B. at most 17.2 m
• C. equal to 17.2 m
• D. depends on the medium

Answer 1

The correct option is A

at least 17.2 m

Taking the speed of sound in air to be 344 m/s and the minimum time gap between direct sound and reflected sound to be 0.1 seconds, the distance travelled by sound to produce echo = 344 $\times$ 0.1 = 34.4 m. Since echo travels twice the distance between obstacle and source, the distance between source and obstacle = 17.2 m.