With what velocity should an observer approach a stationary sound source,so that the apparent frequency of sound appears double the actual frequency? (Let $v$ be the velocity of sound).

With what velocity should an observer move relative to a stationary source so that they hear a sound of double the frequency of the source?

$A$ source of sound of frequency $450 \text{ cycles/sec}$ is moving towards a stationary observer with $34 \text{ m/sec}$ speed. If the speed of sound is $340 \text{ m/sec}$,then the apparent frequency will be ..... $\text{cycles/sec}$.

The frequency of a car horn changes from $400\, Hz$ to $500\, Hz$ as the car approaches a vertical wall. If the speed of sound is $330\, m/s$,then the speed of the car is $.....\, km/h$.

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

When a train is approaching a stationary observer,the apparent frequency of the whistle of the train is $n_1$,and when the train is moving away from the observer,the apparent frequency is $n_2$. The frequency of the whistle noticed by the observer when he moves with the train is . . . . . . .