Question

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

• A. $17.2m$
• B. $10m$
• C. $6m$
• D. $4m$

Answer 1

The correct option is A

$17.2m$

We can hear the original sound and the reflected sound(echo) separately only if there is a time-interval ($t$) of atleast $\frac{1}{10}$th of a second or 0.1 second between them. Taking the speed of sound in air, ($v$) to be $344m{s}^{-1}$, the distance travelled by sound, ($d$)to produce echo = $\frac{vt}{2}=\frac{344\times 0.1}{2}=17.2m$.
Therefore, the distance between the source and obstacle should be atleast $17.2m$.