Suppose that the speed of sound in air at a given temperature is $400 \ m/s$. An engine blows a whistle at $1200 \ Hz$ frequency. It is approaching an observer at the speed of $100 \ m/s$. What is the apparent frequency as heard by the observer in $Hz$?

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400\, Hz$ in still air. What is the frequency of the whistle for a platform observer when the train recedes from the platform with a speed of $10\, m/s$? (Speed of sound $= 340\, m/s$)

Two cars $A$ and $B$ are moving in the same direction with speeds $36 \, km/hr$ and $54 \, km/hr$ respectively. Car $B$ is ahead of $A$. If $A$ sounds a horn of frequency $1000 \, Hz$ and the speed of sound in air is $340 \, m/s$,the frequency of sound received by the driver of car $B$ is ................. $Hz$.

When the observer moves towards the stationary source with velocity $V_1$,the apparent frequency of the emitted note is $F_1$. When the observer moves away from the source with velocity $V_1$,the apparent frequency is $F_2$. If $V$ is the velocity of sound in air and $\frac{F_1}{F_2}=2$,then $\frac{V}{V_1}=?$

Two passenger trains moving with a speed of $108\, km/h$ cross each other. One of them blows a whistle whose frequency is $750\, Hz$. If the speed of sound is $330\, m/s$,then the passengers sitting in the other train,after the trains cross each other,will hear a sound whose frequency is .... $Hz$.

An observer is approaching with a speed $v$ towards a stationary source emitting sound waves of wavelength $\lambda_0$. The wavelength shift detected by the observer is (Take $c$ as the speed of sound).