$A$ sound source,emitting sound of constant frequency,moves with a constant speed and crosses a stationary observer. The frequency $(n)$ of sound heard by the observer is plotted against time $(t)$. Which of the following graphs represents the correct variation?

$A$ train approaching a railway platform with a speed of $20 \, m \, s^{-1}$ starts blowing the whistle. The speed of sound in air is $340 \, m \, s^{-1}$. If the frequency of the emitted sound from the whistle is $640 \, Hz$,the frequency of sound as heard by a person standing on the platform is .... $Hz$.

$A$ car moving towards a cliff emits sound of frequency '$n$'. If the difference in frequencies of the horn and its echo heard by the driver of the car is $10 \%$ of '$n$',then the speed of the car is nearly (Speed of sound in air is $336 \ m/s$) (in $m/s$)

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

An observer moves towards a stationary source of sound,with a velocity one-fifth of the velocity of sound. What is the percentage increase in the apparent frequency?

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 ........ $Hz$.