What should be the minimum distance (in m) between source and reflector in water so that echo is heard distinctly ? (The speed of sound in water $= 1400 m s^{- 1}$ )

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Solution

The correct option is D 70
The sensation of any sound persists in our ear for about 0.1 seconds. This is known as the persistence of hearing. If the echo is heard within this time interval, the original sound and its echo cannot be distinguished. So the most important condition for hearing an echo is that the reflected sound should reach the ear only after a lapse of at least 0.1 second after the original sound dies off.
As the speed of sound in water is 1400 m/s in the question, the distance traveled by sound in 0.1 second is 35 m, which is calculated from the formula
$D i s t a n c e t r a v e l e d = v e l o c i t y o f s o u n d \times t i m e t a k e n$ . That is, 140 m. This is twice the minimum distance between a source of sound and the reflector. So, if the obstacle is at a distance of 70 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly.

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What should be the minimum distance (in m) between source and reflector in water so that echo is heard distinctly ? (The speed of sound in water =1400ms−1)

What should be the minimum distance (in m) between source and reflector in water so that echo is heard distinctly ? (The speed of sound in water $= 1400 m s^{- 1}$ )