Question

The echo will be heard if the original sound reflected by an obstacle reaches our ears after a minimum time of:
A) $10s$
B) $5s$
C) $1s$
D) $0.1s$

Answer 1

Echo is the reflected sound which is produced when the sound waves travel in a medium and get struck with the obstacle or any surface, then human ears hear the same sound produced after some time. It is the reflection of sound waves.

Step-by-Step Explanation:
Step I:
When sound waves travel in a medium, they bounce just like a ball bounces off from the ground. Human ears have the ability to hear the sound produced by the reflection of sound waves after at least$0.1\sec$ .
Step II:
But in case if the sound wave that is reflected, reaches the human ears before $0.1\sec$ , then echo can not be heard or distinguished. Another condition for echo to be heard is that the distance between the reflecting surface and the sound source should be minimum $17m$ and not less than that.
Step III:
Another point to be noted is that the direction of the reflected sound waves can be different or it can be changed, but the echo heard will be of the same nature as that of original sound.
Step IV:

Therefore, option D is the right answer.

Note:
Sometimes echo and reverberation can be considered similar. But they are different. Where echo is the result of the reflection of sound waves, reverberation is the reflection of sound waves that are produced when the echoes are superimposed. Another difference is that the echoes can be easily distinguished or easy to understand, but reverberation can not be distinguished.