A source is moving towards an observer with a speed of 20 m / s and having frequency of 240 Hz . The observer is now moving towards the source with a speed of 20 m / s . Apparent frequency heard by observer, if velocity of sound is 340 m / s , is

Options

A. 270 Hz
B. 240 Hz
C. 268 Hz
D. 360 Hz

Answer

A

Explanation

The perceived frequency heard by the observer depends upon the relative motion between observer and source. If the source and observer are approaching each other, then velocity of the source `v_(s)` is positive and velocity of the observer `v_(0)` is negative, the perceived frequency will be higher than the original.
Perceived frequency is given by
`n' = n ((v + v_(0))/(v - v_(s))) " " [ :' n' = n (v - (v_(0))/(v -v_(s))]`
Here, `v = 340 ms^(-1), v_(0) = 20 ms^(-1), v_(s) = 20 ms^(-1), n = 240 Hz`
Hence, `n' = 240 ((340 + 20)/(340 - 20))`
`= 240 xx (360)/(320) = 270 Hz`