An observer moves towards a stationary source of sound with a velocity one-fifth of the velocity of sound. The percentage change in the apparent frequency is ..... $\%$

What should be the velocity of a sound source moving towards a stationary observer so that the apparent frequency is double the actual frequency? (Velocity of sound is $v$)

$A$ car is moving at a velocity of $17 \ m/s$ towards an approaching bus that blows a horn at a frequency of $640 \ Hz$ on a straight track. The frequency of this horn appears to be $680 \ Hz$ to the car driver. If the velocity of sound in air is $340 \ m/s$,then the velocity of the approaching bus is: (in $m/s$)

$A$ sound source,emitting sound of constant frequency,moves with a constant speed and crosses a stationary observer. The frequency $(n)$ of sound heard by the observer is plotted against time $(t)$. Which of the following graphs represents the correct variation?

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

The frequency changes by $10\%$ as a sound source approaches a stationary observer with constant speed $v_s$. What would be the percentage change in frequency as the source recedes from the observer with the same speed? (Given that $v_s < v$,where $v$ is the speed of sound in air)