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The reason that a jet sounds different when it is flying towards you versus when it is flying away from you is called the Doppler Effect. Since sound moves in waves, as the jet is flying towards you, the space between each sound wave, called the wavelength, is compressed due to the plane moving towards you. After the plane passes you, the wavelength is expanded since the plane is moving away from you. Since sound waves with different wavelengths sound different, the jet will sound different before and after it passes you.