When the source of a sound is moving relative to the listener, the frequency heard by the listener is different than the frequency at the source. When the sound is approaching the listener, the frequency is higher--when the sound is moving away, the frequency is lower. An example would be the approach and passing by of a train whistle while sitting at the gate. As the train approaches, the whistle is heard as a high pitch, yet once the train passes, the pitch decreases.
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