When the source of sound and the observer both are moving towards each other,the observer will hear:

$A$ bat flies at a steady speed of $4 \,ms^{-1}$ emitting a sound of $f = 90 \times 10^{3} \,Hz$. It is flying horizontally towards a vertical wall. The frequency of the reflected sound as detected by the bat will be (take velocity of sound in air is $330 \,ms^{-1}$)

The frequency of sound heard by an observer moving towards a stationary source with certain speed is $n_1$ and if the observer moves away from the same source with same speed,the frequency of sound heard by the observer is $n_2$. If the speed of sound in air is $340 \ m/s$ and $n_1: n_2 = 71: 65$,then the speed of the observer is: (in $km/h$)

Two trains are moving towards each other at speeds of $20 \, m/s$ and $15 \, m/s$ relative to the ground. The first train sounds a whistle of frequency $600 \, Hz$. The frequency of the whistle heard by a passenger in the second train before the trains meet is ...... $Hz$ (the speed of sound in air is $340 \, m/s$).

Two sources $A$ and $B$ are sending notes of frequency $680 \ Hz$. $A$ listener moves from $A$ towards $B$ with a constant velocity $u$. If the speed of sound in air is $340 \ ms^{-1}$,what must be the value of $u$ so that he hears $10$ beats per second (in $ms^{-1}$)?

When both source and listener are approaching each other,the observed frequency of sound is given by (where $V$ is the speed of sound,$V_L$ and $V_S$ are the velocities of the listener and source respectively,and $n_0$ is the radiated frequency):