Question

The minimum distance between the sound and the reflecting surface, in order to hear an echo, must be

• A. $0.65m$
• B. $1.65m$
• C. $16.5m$
• D. $165m$

Answer 1

Answer: (C)

$16.5m$

Let the distance between the source of sound and the obstacle be $x$ metres .

Thus the distance travelled by sound to reach back to the source $d=2x$

As the minimum time required to hear an echo is equal to $0.1sec$

As velocity of sound in air $v=330m/s$

Thus $d=vt$

$2x=330\times 0.1⟹x=16.5m$