The diagram below represents waves emitted by a source of sound, S, which is stationary relative to the air.
The velocity of the sound waves relative to the air is v.

The next diagram represents waves emitted by a source which is moving (with velocity v_s) relative to the air.

Motion of the source produces a change in the wavelength, shorter wavelength in front of source, longer wavelength behind.

Motion of the observer produces a change in the apparent frequency of the waves.

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