Ans. ECHO
When sound is incident on the surface of a medium, its bounces back into the first medium. This phenomenon is called echo.
When we clap or shout near a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is an echo and is a result of reflection of sound.
The sensation of sound persists in our brain for about 0.1s. To hear a clear echo, the time interval between our sound and the reflected sound must be at least
0.1s. If we consider speed of sound to be 340 ms~1 at a normal temperature in air, we will hear the echo after 0.1s. The total distance covered by the sound from the point of generation to the reflecting surface and back should be at least
340 ms'1 x 0.1s = 34.0 m. Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17m. Echoes may be heard more than once due to successive or multiple reflections.