Two trains $A$ and $B$ are moving towards each other with speeds $72 \ km/h$ and $36 \ km/h$ respectively. The train $A$ whistles at $640 \ Hz$ frequency. Before the trains meet,the frequency of sound heard by a passenger in Train $B$ is (Speed of sound in air $= 340 \ m/s$). (in $Hz$)

An observer is riding on a bicycle and moving towards a hill at $18\,km\,h^{-1}$. He hears a sound from a source at some distance behind him directly as well as after its reflection from the hill. If the original frequency of the sound as emitted by the source is $640\,Hz$ and the velocity of sound in air is $320\,m/s$,the beat frequency between the two sounds heard by the observer will be $...Hz$.

Two cars moving in opposite directions approach each other with speeds of $22 \, m s^{-1}$ and $16.5 \, m s^{-1}$ respectively. The driver of the first car blows a horn having a frequency of $400 \, Hz$. The frequency heard by the driver of the second car is ..... $Hz$ (velocity of sound is $340 \, m s^{-1}$)

If a source emitting waves of frequency $F$ moves towards an observer with a velocity $V/3$ and the observer moves away from the source with a velocity $V/4$,the apparent frequency as heard by the observer will be ($V=$ velocity of sound).

With what speed should a sound source move towards an observer so that the observer hears half the original frequency $(v)$?

$A$ $SONAR$ system fixed in a submarine operates at a frequency of $40.0 \; kHz$. An enemy submarine moves towards the $SONAR$ with a speed of $360 \; km/h$. What is the frequency (in $kHz$) of sound reflected by the submarine (in $; kHz$)? Take the speed of sound in water to be $1450 \; m/s$.