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

Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed of $1.0 \,m \,s^{-1}$ and the man behind walks at a speed of $2.0 \,m \,s^{-1}$. $A$ third man is standing at a height of $12 \,m$ above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency $1430 \,Hz$. The speed of sound in air is $330 \,m \,s^{-1}$. At the instant when the moving men are $10 \,m$ apart,the stationary man is equidistant from them. The frequency of beats in $Hz$ heard by the stationary man at this instant is:

$A$ submarine $(A)$ travelling at $18\, km/hr$ is being chased along the line of its velocity by another submarine $(B)$ travelling at $27\, km/hr$. $B$ sends a sonar signal of $500\, Hz$ to detect $A$ and receives a reflected sound of frequency $v$. The value of $v$ is close to ... $Hz$ (Speed of sound in water $= 1500\, ms^{-1}$)

$A$ source of sound is moving towards a stationary observer with $\frac{1}{10}$ of the speed of sound. The ratio of apparent to real frequency is

Obtain the equation of frequency observed by a moving observer and a stationary source.

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 $\%$)