Question

Calculate the minimum distance in air required between the source and the obstacle to hear an echo. Take speed of sound in air = 350 m s^-1......≧∇≦

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bhai kasay ho....?​

Answer 1

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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 is 350m/s in the question, the distance traveled by sound in 0.1 second is 35m, which is calculated from the formula 

Distancetraveled=velocityofsound×timetaken. 

This is twice the minimum distance between a source of sound and the reflector. That is 35m. 

So, if the obstacle is at a distance of 17.5 m at least, the reflected sound or the echo is heard after 0.1 second, distinctly