You will have noticed that as a car travels quickly towards you, the sound of its engine gets higher, and then becomes deeper after the car has passed by. This is called the Doppler Effect. What happens is that as the car approaches, the frequency of the sound of its engine increases as the wavelength of the sound decreases. Each successive sound wave is a little shorter as the car comes towards you. Then as the car moves away, the process is reversed; the frequency decreases while the wavelength of the sound becomes longer.
You will not notice the Doppler Effect if a supersonic aircraft flies past, because you cannot hear the sound of the plane until it is actually flying away from you.
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