Consider two sound sources $S_1$ and $S_2$ having the same frequency $100\,Hz$ and the observer $O$ located between them as shown in the figure. All three are moving with the same velocity in the same direction. The beat frequency heard by the observer is .... $Hz$.

$A$ 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:

$A$ bus is moving with a velocity of $5 \, ms^{-1}$ towards a huge wall. The driver sounds a horn of frequency $165 \, Hz$. If the speed of sound in air is $335 \, ms^{-1}$, the number of beats heard per second by the passengers in the bus will be:

$A$ source of sound $S$ emitting waves of frequency $100 \, Hz$ and an observer $O$ are located at some distance from each other. The source is moving with a speed of $19.4 \, m s^{-1}$ at an angle of $60^{\circ}$ with the source-observer line as shown in the figure. The observer is at rest. The apparent frequency observed by the observer is .... $Hz$ (velocity of sound in air $330 \, m s^{-1}$).

$A$ train $S_1$,moving with a uniform velocity of $108 \ km/h$,approaches another train $S_2$ standing on a platform. An observer $O$ moves with a uniform velocity of $36 \ km/h$ towards $S_2$,as shown in the figure. Both the trains are blowing whistles of the same frequency $120 \ Hz$. When $O$ is $600 \ m$ away from $S_2$ and the distance between $S_1$ and $S_2$ is $800 \ m$,the number of beats heard by $O$ is: [Speed of sound $= 330 \ m/s$]

$A$ source of sound is moving with a velocity of $50 \ m/s$ towards a stationary observer. The observer measures the frequency of sound as $500 \ Hz$. The apparent frequency of sound as heard by the observer when the source is moving away from him with the same speed is (Speed of sound at room temperature $350 \ m/s$). (in $Hz$)