Question

An echo will be heard if the minimum distance between the source of sound and obstacle is?

• A. $10$m
• B. $1$m
• C. $15$m
• D. $17$m

Answer 1

The correct option is D $17$m

A sound, when heard, remains in our brain distinctively till 0.1 s. So to hear the echo, minimum time difference between sound wave at source and echoed sound wave should be 0.1s.

Let the distance between source and obstacle be $x$, and speed of sound be $340m/s$

So for minimum time difference of 0.1s,

$2x=340\times 0.1⟹x=17m$

So option D is correct.