Question

In order to hear distinct echoes, the distance of the obstacle from the source of sound should be ___.
(Take the speed of sound in air to be $344m{s}^{-1}$).

• A. at least 17.2 m
• B. at most 17.2 m
• C. equal to 17.2 m
• D. equal to 8.6 m

Answer 1

The correct option is A at least 17.2 m

Humans can detect two sounds distinctly if the time interval between them is more than 0.1 s.
$t\ge 0.1s$

Speed of sound in air is, $v=344m{s}^{-1}$

The distance traveled by sound is:
$s=vt$
$s\ge 344\times 0.1$
$s\ge 34.4m$

Echo travels twice the distance between obstacle and source. Hence, the minimum distance between the source and obstacle for the echo to be heard is $17.2m$.