$A$ train whistling at a constant frequency $n$ is moving towards a station at a constant speed $v_s$. The train goes past a stationary observer on the station. The frequency $n'$ of the sound as heard by the observer is plotted as a function of time $t$. Identify the correct curve.

$A$ source emits a sound of frequency $400 \ Hz$,but the listener hears it to be $390 \ Hz$. Then:

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

The pitch of the whistle of an engine appears to drop by $20 \%$ of its original value when it passes a stationary observer. If the speed of sound in air is $350 \ m/s$,then the speed of the engine in $m/s$ is:

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400 \ Hz$ in still air. The train begins to move with a speed of $10 \ ms^{-1}$ towards the platform. What is the frequency of the sound for an observer standing on the platform (in $Hz$)? (Sound velocity in air $= 330 \ ms^{-1}$).

The driver of a car travelling with speed $30 \, m/s$ towards a hill sounds a horn of frequency $600 \, Hz$. If the velocity of sound in air is $330 \, m/s$,the frequency of reflected sound as heard by the driver is .... $Hz$.