$A$ person observes two moving trains,$A$ reaching the station and $B$ leaving the station with equal speed of $30\,m/s$. If both trains emit sounds with frequency $300\,Hz$,(Speed of sound : $330\,m/s$) the approximate difference of frequencies heard by the person will be $..........Hz$.

Two sources $A$ and $B$ are sounding notes of frequency $660 \, Hz$. $A$ listener moves from $A$ to $B$ with a constant velocity $u$. If the speed of sound is $330 \, m/s$,what must be the value of $u$ so that he hears $8$ beats per second?

Statement $-1$: Due to the motion of the listener,the frequency of the sound waves (as received by the listener) emitted by a stationary source is affected.
Statement $-2$: Due to the motion of the source,the wavelength of the sound waves (emitted by the source) as received by a stationary listener is affected.
Statement $-3$: If the receiver and the source are both moving,the observed frequency must be different from the original frequency of the source.
Treat the motion of the source or listener as always along the line joining them for all the above cases.

$A$ source of sound emits a sound wave of frequency '$f$' and moves towards an observer with a velocity $\frac{V}{3}$,where $V$ is the velocity of sound. If the observer moves away from the source with a velocity $\frac{V}{5}$,the apparent frequency heard by him will be:

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

$A$ vehicle sounding a whistle of frequency $256 \,Hz$ is moving on a straight road towards a hill with a velocity of $10 \,ms^{-1}$. The number of beats per second observed by a person travelling in the vehicle is (velocity of sound $= 330 \,ms^{-1}$).