A police car has passed by you with its siren blazing (529-Hz) while traveling at 28.1-m/s. What frequency would you hear after it has passed by?

Fr = ((V-Vr)/(V+Vs)) * Fs.

Fr = ((343-0)/(343+28.1)) * 529=488.9 Hz
= Freq. heard by receiver.

V = Speed of sound in air.
Vr = Velocity of the receiver.
Vs = Velocity of the source(siren).