An observer is moving towards a stationary source of sound. In this case:

$A$ train approaching a railway platform with a speed of $20 \ ms^{-1}$ starts blowing the whistle. The speed of sound in air is $340 \ ms^{-1}$. If the frequency of the emitted sound from the whistle is $640 \ Hz$, the frequency of sound to a person standing on the platform will appear to be: (in $Hz$)

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

$A$ whistle producing sound waves of frequencies $9500 \ Hz$ and above is approaching a stationary person with speed $v \ ms^{-1}$. The velocity of sound in air is $300 \ ms^{-1}$. If the person can hear frequencies up to a maximum of $10,000 \ Hz$,the maximum value of $v$ up to which he can hear the whistle is ... $ms^{-1}$.

$A$ source of sound of frequency $500 Hz$ is moving towards an observer with velocity $30 m/s$. The speed of sound is $330 m/s$. The frequency heard by the observer will be .... $Hz$

$A$ whistle of frequency $540 Hz$ rotates in a horizontal circle of radius $2 m$ at an angular speed of $15 rad/s$. What is the highest frequency heard by a listener at rest with respect to the center of the circle (in $Hz$)? (Velocity of sound in air $= 330 m/s$)