The frequency of sound heard by an observer moving towards a stationary source with certain speed is $n_1$ and if the observer moves away from the same source with same speed,the frequency of sound heard by the observer is $n_2$. If the speed of sound in air is $340 \ m/s$ and $n_1: n_2 = 71: 65$,then the speed of the observer is: (in $km/h$)

An engine is moving on a circular track with a constant speed. It is blowing a whistle of frequency $500 \ Hz$. The frequency received by an observer standing stationary at the centre of the track is

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 .... $Hz$.

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?

$S_1$ and $S_2$ are two identical sound sources of frequency $656 \ Hz$. The source $S_1$ is located at $O$ and $S_2$ moves anti-clockwise with a uniform speed $4 \sqrt{2} \ ms^{-1}$ on a circular path around $O$,as shown in the figure. There are three points $P, Q$ and $R$ on this path such that $P$ and $R$ are diametrically opposite while $Q$ is equidistant from them. $A$ sound detector is placed at point $P$. The source $S_1$ can move along direction $OP$.
[Given: The speed of sound in air is $324 \ ms^{-1}$]
$(1)$ When only $S_2$ is emitting sound and it is at $Q$,the frequency of sound measured by the detector in $Hz$ is. . . . . .
$(2)$ Consider both sources emitting sound. When $S_2$ is at $R$ and $S_1$ approaches the detector with a speed $4 \ ms^{-1}$,the beat frequency measured by the detector is $\qquad$ $Hz$.

$A$ source of sound of frequency $256 \ Hz$ is moving rapidly towards a wall with a velocity of $5 \ m/s$. The speed of sound is $330 \ m/s$. If the observer is between the wall and the source,then the number of beats per second heard will be .... $Hz$.