Something is missing in question. But I will try to explain you about Doppler effect to reach your answer.
Answer:
It is defined as "an increase in the frequency of sound, light or other waves as the source or observer moves towards each other".
This effect causes the sudden change in pitch noticeable in passing siren.
You hear a siren from a few blocks away. The source is a police car that is racing in your direction. As the car approaches, zooms past you, and then speeds off into the distance, the sound of its siren keeps changing in pitch. First the siren gets higher in pitch, and then it suddenly gets lower.
