Question

The minimum distance between the source and the reflector, so that an echo is heard, is approximately equal to

• A. $10.2m$
• B. $17.2m$
• C. $34.2m$
• D. $1.72m$

Answer 1

The correct option is B

$17.2m$

Step 1: Given data

The minimum time condition for the echo to persist in our brain, $t=0.1s$

The speed of sound in the air, $s=344m{s}^{-1}$

Step 2: To find the minimum distance between the source and the reflector

The total distance traveled by the sound to reach the reflector and then to come back is, $dis\tan ce\left(2d\right)=speed\times time$

$dis\tan ce=344m{s}^{-1}\times 0.1s$

$Totaldis\tan ce\left(2d\right)=34.4m$

Therefore, the minimum distance between the origin point and the reflecting surface($d$) should be half of the total distance i.e , $17.2m$.

Hence, Option B is correct.