$A$ train moves towards a stationary observer with a speed of $34 \ m/s$. The train sounds a whistle and its frequency registered by the observer is $f_1$. If the train's speed is reduced to $17 \ m/s$,the frequency registered is $f_2$. If the speed of sound is $340 \ m/s$,then the ratio $f_1/f_2$ is:

An engine sounding a whistle of frequency $1152 \ Hz$ is receding from a stationary observer at $72 \ km/h$. If the velocity of sound in air is $340 \ m/s$,then the frequency of the note heard by the observer is: (in $Hz$)

$A$ car travelling at a speed of $54 \ km/h$ towards a wall sounds a horn of frequency $400 \ Hz$. The difference in the frequencies of two sounds, one received directly from the car and the other reflected from the wall, noticed by a person standing between the car and the wall is (speed of sound in air is $335 \ m/s$):

$A$ drone fitted with a siren is flying directly away from the drone operator and towards a distant building at a speed of $15 \,m/s$. The siren produces a sound of frequency $780 \,Hz$. What is the frequency that the operator hears in the echo reflected from the building (in $\,Hz$)? [Speed of sound is $340 \,m/s$.]

An observer is standing $500 \,m$ away from a vertical hill. Starting between the observer and the hill, a police van sounding a siren of frequency $1000 \,Hz$ moves towards the hill with a uniform speed. If the frequency of the sound heard directly from the siren is $970 \,Hz$, the frequency of the sound heard after reflection from the hill (in $Hz$) is about, (velocity of sound $= 330 \,m/s$):

An observer moves towards a stationary source of sound with a speed $\frac{1}{5}^{\text{th}}$ of the speed of sound. The wavelength and frequency of the waves emitted by the source are $\lambda$ and $f$ respectively. The apparent frequency and wavelength heard by the observer are respectively,