The repetition of sound caused by the reflection of sound waves. For example, if we shout or clap near a suitable reflecting
object such as a tall building or a mountain,
we will hear the same sound again a little
later. This sound which we hear is called an
echo.
The sensation of sound persists in our
brain for about 0.1 s. To hear a distinct echo
the time interval between the original sound
and the reflected one must be at least 0.1 s.
Given,
Sound is propagating with speed, v m/s
Hence, the total distance covered by the
sound from the point of generation to the
reflecting surface and back should be at least,
D=speed×time
D=v×0.1
D=v10
Thus, for hearing
distinct echoes, the minimum distance of the
obstacle from the source of sound must be
half of this distance, that is,
D2=v20