$A$ stationary source is emitting sound at a fixed frequency $f_0$,which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is $1.2 \%$ of $f_0$. What is the difference in the speeds of the cars (in $km/h$) to the nearest integer? The cars are moving at constant speeds much smaller than the speed of sound,which is $330 \ m/s$.

$A$ car moving towards a wall at a velocity of $72 \,km/hr$ sounds a horn of frequency $124 \,vib/sec$. What frequency will the driver hear? (Speed of sound in air = $330 \,ms^{-1}$)

$A$ stationary source of sound $A$ is producing sound of frequency $170 \, Hz$. Another source of sound $B$ producing sound of frequency $240 \, Hz$ is moving towards the source $A$ on a straight path with a uniform speed of $20 \, ms^{-1}$. An observer between $A$ and $B$ is moving towards the source $A$ along the straight path $BA$. If the number of beats heard by the observer is zero, the speed of the observer is . . . . . . $ms^{-1}$. (Speed of sound in air $= 340 \, ms^{-1}$).

An engine is moving on a circular track with a constant speed. It is blowing a whistle of frequency $500 \ Hz$. The frequency received by an observer standing stationary at the centre of the track is

When both the source of sound and the observer approach each other with a speed equal to $10 \%$ of the speed of sound,then the percentage change in frequency heard by the observer is nearly (in $\%$)

$A$ man standing on a platform observes that the frequency of the sound of a whistle emitted by a train drops by $140 \, Hz$. If the velocity of sound in air is $330 \, m/s$ and the speed of the train is $70 \, m/s$,the frequency of the whistle is .......... $Hz$.