Mon Prof Augmenté
Car Passing With Sound Waves
Age 18 · Science
A car moves to the right along a road, sending out repeating wavefronts. Because the car moves, the wavefronts bunch closer ahead and spread farther apart behind, so a person at the roadside hears a higher pitch as it approaches and a lower pitch after it passes.
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In the picture
- Badge 1 sits on the car.
- Badge 2 sits on the compressed wavefronts ahead of the car.
- Badge 3 sits on the stretched wavefronts behind the car.
- Badge 4 sits on the pedestrian at the roadside.
Examples
- An ambulance siren sounds higher as it approaches and lower after it passes.
- A race car engine note drops as it speeds past.
Key takeaways
- Motion between a source and an observer changes the observed frequency.
- Approaching motion raises pitch, while receding motion lowers pitch.
Quiz: Car Passing With Sound Waves (Age 18)
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As a sound source moves toward a stationary observer, how does the pitch the observer hears compare with the pitch when the source is not moving?
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Why does a passing car’s sound change from a higher pitch to a lower pitch?
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If a sound source is moving away from an observer, how does the observed frequency compare with the frequency emitted by the source?
The answers, each with its explanation, are in the app. This lesson has left the free week, so its quiz is part of the subscription.
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