An engine sounding a whistle of frequency $2000 \,Hz$ is receding from a stationary observer at $72 \,km/h$. What is the apparent frequency heard by the observer (in $\,Hz$)? The velocity of sound in air is $340 \,m/s$.

When the source of sound and the observer both are moving towards each other,the observer will hear:

$A$ stationary tuning fork is in resonance with an air column in a pipe. If the tuning fork is moved with a speed of $2 \ m/s$ in front of the open end of the pipe and parallel to it,the length of the pipe should be changed for the resonance to occur with the moving tuning fork. If the speed of sound in air is $320 \ m/s$,the smallest value of the percentage change required in the length of the pipe is. . . . . .

$A$ person with a vibrating tuning fork of frequency $338 \,Hz$ is moving towards a vertical wall with a speed of $2 \,ms^{-1}$. The velocity of sound in air is $340 \,ms^{-1}$. The number of beats heard by that person per second is

Two sources of sound of same frequency are placed at a distance of $100\,m$ from each other. An observer moving in between them hears $4$ beats per second. If the distance between the sound sources is increased to $400\,m$,then the number of beats heard by him will be:

When an observer moves towards a stationary source with velocity $V_1$,the apparent frequency of the emitted note is $F_1$. When the observer moves away from the stationary source with velocity $V_1$,the apparent frequency is $F_2$. If $v$ is the velocity of sound in air and $\frac{F_1}{F_2} = 2$,then $\frac{v}{V_1}$ is equal to: