The Doppler effect - A change in the frequency with which sound waves from a given source reach an observer when the source is in rapid motion with respect to the observer.
Approaching sounds seem higher in pitch (frequency) than sounds that are moving away. This is due to the fact that the wavelengths of sounds from an approaching source are progressively foreshortened, whereas those from a receding source are stretched.
The Doppler effect is used in various medical ultrasound scanning techniques (see Doppler ultrasonography).
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