Question

An ambulance siren sounds different as it approaches you than when it moves away from you. what scientific term would you use to explain how this happens?

Answer 1

I believe the answer is: The Doppler Effect
According to the doppler effect, as the we move further away from the source of sound, The sound wave need a more time to arrive at our hearing senses.
This would create the illusion that source of sounds that located in further distance gave away sound with lower frequency.