- Sound generators in motion produce a perceived higher frequency as they approach a point of observation, and a corresponding lowering of the frequency as the object recedes.
- On a train or inside a car, the passengers hear a steady frequency. But an observer will experience this phenomenon.
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In short, if you are standing on a curb and a car speeds by you the sound that you hear as the car approaches will be at its highest frequency when it is closest to you. As the car moves away in either direction the sound\'s frequency will lower.
