$A$ bus moving with a uniform speed of $72 \,km/h$ towards a building blows a horn of frequency $1.7 \,kHz$. If the speed of sound in air is $340 \,m/s$, what will be the frequency of the echo heard by the bus driver (in $\,kHz$)?

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 .... $Hz$.

This question has Statement $1$ and Statement $2$. Of the four choices given after the Statements,choose the one that best describes the two Statements.
Statement $1$: Bats emitting ultrasonic waves can detect the location of a prey by hearing the waves reflected from it.
Statement $2$: When the source and the detector are moving,the frequency of reflected waves is changed.

$A$ rocket is moving at a speed of $200\; m s^{-1}$ towards a stationary target. While moving, it emits a wave of frequency $1000\; Hz$. Some of the sound reaching the target gets reflected back to the rocket as an echo. Calculate:
$(1)$ the frequency of the sound as detected by the target and
$(2)$ the frequency of the echo as detected by the rocket.

An observer starts moving with uniform acceleration $a$ toward a stationary sound source emitting a whistle of frequency $n$. As the observer approaches the source,the apparent frequency $n'$ heard by the observer varies with time $t$ as:

Two cars $X$ and $Y$ are approaching each other with velocities $36 \; km/h$ and $72 \; km/h$ respectively. The frequency of a whistle sound emitted by a passenger in car $X$,heard by the passenger in car $Y$,is $1320 \; Hz$. If the velocity of sound in air is $340 \; m/s$,the actual frequency of the whistle sound produced is ........ $Hz$.