$A$ train whistling at a constant frequency $n_0$ is moving towards a station at a constant speed $v$. The train passes 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 expected curve.

$A$ train is approaching a platform with a speed of $10 \,ms^{-1}$ while blowing a whistle of frequency $340 \,Hz$. What is the frequency of the whistle heard by a stationary observer on the platform (in $\,Hz$)? (Given speed of sound $= 340 \,ms^{-1}$)

$A$ railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n)$ of the sound heard by the observer is plotted against time $(t).$ Which of the following best represents the resulting curve?

$A$ train moving at a speed of $220\, m s^{-1}$ towards a stationary object,emits a sound of frequency $1000\, Hz$. Some of the sound reaching the object gets reflected back to the train as an echo. The frequency of the echo as detected by the driver of the train is ...... $Hz$. (Speed of sound in air is $330\, m s^{-1}$)

$A$ stationary source emits sound waves of frequency $500\, Hz$. Two observers moving along a line passing through the source detect sound to be of frequencies $480\, Hz$ and $530\, Hz$. Their respective speeds are,in $m\,s^{-1}$ (Given speed of sound $= 300\, m/s$)

With what velocity an observer should move relative to a stationary source so that a sound of double the frequency of the source is heard by the observer?