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Question
State two conditions necessary for hearing an echo.
Solution
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The persistence of hearing for human ears is 0.1 second. This means that we can hear two sound waves clearly if the time interval between the two sounds is at least 0.1 s. Thus, the minimum time gap between the original sound and an echo must be 0.1 s.
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The above criterion can be satisfied only when the distance between the source of the sound and the reflecting surface would satisfy the following equation:
Velocity = `"Distance travelled by sound"/"Time taken"`
V = `(2"d")/"t"`
d = `("vt")/2`
Since, t = 0.1 second, then d= `("v"xx0.1)/2` = `"v"/20`
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