Question

In order to hear distinct echoes in the air, 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. equal to 20 m

Answer 1

The correct option is A

at least 17.2 m

The perception of hearing for a human ear is $\frac{1}{10}th$ of a second = $0.1s$.
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 sound travels twice the distance between the obstacle and source, the distance between source and obstacle = 17.2 m.