In the animation below, the source is moving to the right. All the wave crests have the same speed.
Look to the right of the source. There the separation of the wave crests is less than one rest wavelength because the source moves right between the emission of one wave crest and the next. This shortening means a stationary observer in front of a moving source hears a frequency higher than the source frequency.
Look to the left of the source. There the motion to the right means that the separation of wave crests to the left is greater than one rest wavelength. This lengthening means a stationary observer behind a moving source hears a frequency lower than the source frequency.
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