$A$ car blowing a horn of frequency $350\, Hz$ is moving towards a wall with a speed of $5\, m/s$. The beat frequency heard by a person standing between the car and the wall is ..... $Hz$ (speed of sound in air $= 350\, m/s$).

$A$ motor car blowing a horn of frequency $124\,vib/sec$ moves with a velocity $72\,km/hr$ towards a tall wall. The frequency of the reflected sound heard by the driver will be .... $vib/sec$ (velocity of sound in air is $330\,m/s$).

$A$ train is moving with a constant speed along a large circular track. The engine of the train emits a sound of frequency $f$. What is the frequency heard by the guard at the rear end of the train?

$A$ train,standing at the outer signal of a railway station,blows a whistle of frequency $400\; Hz$ in still air.
$(i)$ What is the frequency of the whistle for a platform observer when the train $(a)$ approaches the platform with a speed of $10\; m s^{-1}$,$(b)$ recedes from the platform with a speed of $10\; m s^{-1}$?
$(ii)$ What is the speed of sound in each case? The speed of sound in still air can be taken as $340\; m s^{-1}$.

$A$ vehicle with a horn of frequency $n$ is moving with a velocity of $30\, m/s$ in a direction perpendicular to the straight line joining the observer and the vehicle. The observer perceives the sound to have a frequency $n + n_1$. Then (if the sound velocity in air is $300\, m/s$):

Three musicians experiment with the Doppler effect. Musician $A$ rides in a car at a speed $u$ directly away from musician $B$ who is stationary. Musician $C$ rides in a car directly toward $B$ and travels at the same speed as $A$. Musician $A$ plays a note at frequency $f_A$ on his trumpet. $B$ hears the note,adjusts his trumpet,and plays the same note he heard. Choose the incorrect statement.