$A$ 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 $340 \, m/s$).

An Indian submarine $(I)$ and an enemy submarine $(E)$ move towards each other in motionless water. The Indian submarine moves at $50 \ km/h$,and the enemy submarine at $70 \ km/h$. The Indian submarine sends out a sonar signal (sound wave in water) at $1000 \ Hz$. The sonar wave reflects off the enemy submarine and returns to the Indian submarine. If the speed of sound in water is $5500 \ km/h$,what is the frequency of the reflected signal detected by the Indian submarine in $kHz$ (in $kHz$)?

$A$ source of sound of frequency $n$ is moving towards a stationary observer with a speed $S$. If the speed of sound in air is $V$ and the frequency heard by the observer is $n_1$,the value of $n_1/n$ is

$A$ sound source $S$ is moving along a straight track with speed $v,$ and is emitting sound of frequency $v_{0}.$ An observer is standing at a finite distance,at the point $O,$ from the track. The time variation of frequency heard by the observer is best represented by (where $t_{0}$ represents the instant when the distance between the source and observer is minimum).

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?

$A$ train standing at the outer signal of a railway station blows a whistle of frequency $400 \ Hz$ in still air. The train begins to move with a speed of $10 \ ms^{-1}$ towards the platform. What is the frequency of the sound for an observer standing on the platform (in $Hz$)? (Sound velocity in air $= 330 \ ms^{-1}$).