Can you help me with this question?

On the way home from school, you stop for a stoplight. Suddenly you hear the siren of an ambulance in the distance. As you turn, you see the approaching vehicle so you wait patiently as the ambulance eventually speeds through the intersection. Describe the sound you hear throughout this process. Explain your answer in terms of the Doppler effect.

1 Answer
Mar 6, 2018

The sound you hear as the siren gets closer will increase in pitch and that will decrease as it moves away from you.


Sound is a longitudinal pressure wave. As the ambulance moves closer to you, the air molecules get compressed together. The wavelength of the sound (these pressure waves) decreases, and the frequency increases. That results in a higher sound pitch.

After the ambulance passes you, that process reverses. The air molecules hitting your eardrum get farther apart, the wavelength increases and the frequency decreases. Therefore, the sound pitch is lower.

This is the Doppler Effect.

Hope this helps!