$A$ 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 = $330 \,ms^{-1}$)

$A$ source of sound is travelling with a velocity of $40 \, km/h$ towards an observer and emits sound of frequency $2000 \, Hz$. If the velocity of sound is $1220 \, km/h$,what is the apparent frequency heard by the observer in $Hz$?

When the source of sound moves towards a stationary observer,the wavelength of sound received by him

$A$ source of sound and a listener are approaching each other with a speed of $40 \, m/s$. The apparent frequency of the note produced by the source is $400 \, cps$. Then,its true frequency (in $cps$) is (velocity of sound in air $= 360 \, m/s$).

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

$A$ train is moving with a constant speed along a large circular track. The engine of the train emits a sound of frequency $f$. What is the frequency heard by the guard at the rear end of the train?