a person standing in a bus queue hears the siren coming towards and then passing him. An ambulance travelling at 12m/s in a built up area emits sounds of frequency 1000 Hz from its siren. Calculate the frequency heard by the person when the ambulance moves towards him
In this question we use the concepts of doppler effects. By applying the formula for frequency of direct sound from source is f' = f0((v+v0)/(v+vs)).
After the sound moves away from the source the formula becomes f1' = f0((v+v0)/(v-vs)).
You can further substitute the values and get the answer.