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What is echo? Calculate the minimum distance for echo to be produced.
Echo
If we stand in front of a hill and shout at the top of his lungs, we will hear sound twice. First we will hear sound directly coming from the shout, and second after a small time interval, when the sound reflects from the hill, and comes back to us. This reflection of sound that arrives the listener with a delay after the direct sound is known as echo.
The least distance to hear 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 0.1s.
Since sound travels at about 340 m/s in air, for the reflected sound to be audible separately, sound must travel at least
$ 340 \times 0.1 s = 34m $.
Since, the sound travels forwards and backwards, the distance of the wall from the person is half the distance the sound travels.
Therefore, we can hear an echo of our own voice when the nearest reflecting surface is at least $\frac{34}{2} m$
= 17m away.
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