Which of the following conditions is necessary to hear an echo clearly?
Time interval must be at least 0.1 s
The sensation of a sound lasts in our ear for about 0.1 s.
An echo is heard as a separate sound only if the reflected sound reaches the ear at least 0.1 s after the original sound; in air this needs the reflecting surface to be at least about 17 m away.
A soft surface absorbs sound and weakens the echo.
Hence, the answer is that the time interval must be at least 0.1 s.
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