In the animation below, the source is at rest. Every period T the source emits a wave crest, shown spreading out to the left and to the right. The spatial separation of the wave crests is one wavelength in either direction.
A listener moving toward the source would run into more waves per second than a listener at rest, thus hearing a higher frequency than a listener at rest.
Why, then, would a listener moving away from the source hear a lower frequency than a listener at rest?
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