$A$ 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 $330\,m/s$).

Two cars are moving on two perpendicular roads towards a crossing with uniform speeds of $72\; km/hr$ and $36\; km/hr$. If the first car blows a horn of frequency $280\; Hz$,then the frequency of the horn heard by the driver of the second car when the line joining the cars makes a $45^o$ angle with the roads will be .... $Hz$.

The driver of a bus approaching a big wall notices that the frequency of his bus's horn changes from $420\, Hz$ to $490\, Hz$,when he hears it after it gets reflected from the wall. Find the speed of the bus (in $kmh^{-1}$) if the speed of sound is $330\, ms^{-1}$.

An obstacle is moving towards the source with velocity $v$. The sound is reflected from the obstacle. If $c$ is the speed of sound and $\lambda$ is the wavelength,then the wavelength of the reflected wave $(\lambda_{r})$ is

$Assertion :$ The Doppler formula for sound waves is symmetric with respect to the speed of the source and the speed of the observer.
$Reason :$ The motion of a source with respect to a stationary observer is not equivalent to the motion of an observer with respect to a stationary source.

$A$ source $(S)$ of sound has a frequency of $240 \ Hz$. When the observer $(O)$ and the source move towards each other at a speed $v$ with respect to the ground (as shown in Case $1$ in the figure),the observer measures the frequency of the sound to be $288 \ Hz$. However,when the observer and the source move away from each other at the same speed $v$ with respect to the ground (as shown in Case $2$ in the figure),the observer measures the frequency of the sound to be $n \ Hz$. The value of $n$ is: