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The doppler effect is the change in frequency of a sound wave as the source of the wave approaches an observer.
As an object emitting sound waves approaches, each successive wave occurs at a position closer than the last, creating an increased frequency.
While sitting in a car, when another car goes by while honking its horn, the sound waves generated by the passing car will increase in pitch as it approaches. As the passing car moves away from the observer its horn decreases in pitch.