Question

Statement I: Avinash is standing at a distance of 33 m from a wall. He claps and hears the echo of his own clap. (Take the speed of sound in air as 330 m <!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> $s^{-1}$)
Statement II: To hear an echo, the sound after reflection should reach the ear at least 0.1 seconds after the original sound is heard.

• A.
  Statement I is correct and statement II is incorrect.
• B.
  Statement I is incorrect and statement II is correct.
• C.
  Both statement I and statement II are correct.
• D.
  Both statement I and statement II are incorrect.

Answer 1

The correct option is C

Both statement I and statement II are correct.
The sensation of sound remains in the human ear for 0.1 seconds and this is known as the persistence of hearing in humans.
The time gap between direct sound and reflected sound should be greater than the time of persistence of hearing to hear an echo.
Here,
Time taken by the sound of Avinash’s clap to come back after reflection from the wall is the time taken for the sound to travel a distance twice the distance between Avinash and the wall.
So,
T = $\frac{Distancecoveredbythesound}{Speedofsound}$
T = $\frac{2\times 33}{330}$= $\frac{1}{5}$ = 0.2 seconds

Since T is greater than the time of persistence of hearing (0.1 s), Avinash will hear the echo of his clap. So, both statement I and statement II are correct.