$A$ motorcyclist is moving around a circular track of radius $50 \ m$ with a constant speed of $25 \ m/s$. $A$ static siren at point $Y$ emits sound of frequency $n$. How many times (approximately) in an hour will the motorcyclist hear the sound of the actual frequency $n$?

$A$ stationary source is emitting sound at a fixed frequency $f_0$,which is reflected by two cars approaching the source. The difference between the frequencies of sound reflected from the cars is $1.2\%$ of $f_0$. What is the difference in the speeds of the cars (in $km/h$) to the nearest integer? The cars are moving at constant speeds much smaller than the speed of sound,which is $330 \ m/s$.

When both source and listener are approaching each other,the observed frequency of sound is given by (where $V$ is the speed of sound,$V_L$ and $V_S$ are the velocities of the listener and source respectively,and $n_0$ is the radiated frequency):

$A$ source of sound is travelling at $\frac{100}{3} \, m/s$ along a road,towards a point $A$. When the source is $3 \, m$ away from $A$,a person is standing at a point $O$ on a road perpendicular to the path of the source. The distance of $O$ from $A$ at that time is $4 \, m$. If the original frequency is $640 \, Hz$,then the apparent frequency heard by the person is ...... $Hz$ (speed of sound is $340 \, m/s$).

$A$ train whistling at a constant frequency $n$ is moving towards a station at a constant speed $v_s$. The train goes past a stationary observer on the station. The frequency $n'$ of the sound as heard by the observer is plotted as a function of time $t$. Identify the correct curve.

Two trucks heading in opposite directions, each with a speed of $0.1 u$, approach each other. The speed of sound is $u$. The driver of the first truck sounds his horn with a frequency of $495 \,Hz$. Let $v_1$ and $v_2$ be the frequencies heard by the driver of the second truck when the trucks are approaching each other and when the trucks have passed each other, respectively. The magnitude of $v_1 - v_2$ is (in $\,Hz$)