In the given situation , an observer is moving towards a stationary source of sound , therefore by Doppler's effect the apparent frequency f′ of whistle heard by person (observer) will be ,
f′=f(v+vov) ,
where , f= original frequency of sound ,
vo= velocity of observer ,
v= velocity of sound in air ,
It is clear from above formula that , f′>f ,the frequency of whistle heard by the person is greater than the original frequency of whistle .