Answer:
The Doppler Effect
Step-by-step explanation:
The Doppler effect is a change in the frequency of sound waves that occurs when the source of the sound waves is moving relative to a stationary listener. Because the car is moving towards you, the sound waves get bunched up in front of the car and spread out behind it. Sound waves that are closer together have a higher frequency, and sound waves that are farther apart have a lower frequency. The frequency of sound waves, in turn, determines the pitch of the sound. Sound waves with a higher frequency produce sound with a higher pitch, and sound waves with a lower frequency produce sound with a lower pitch. As the car approaches, the sound waves get closer together, increasing their frequency. This listener hears the pitch of the engine get higher. As the car speeds away, the sound waves get farther apart, decreasing their frequency. This listener hears the pitch of the engine getting lower.
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