When a source of sound is stationary (v = 0) the wavefronts form concentric circles around the source. The wavelength is the same in all directions.
When a source of sound is in motion at a speed less than the speed of sound (v < c) the wavefronts are squashed together on one side. The waves behind the source have a longer wavelength, while those in front have a shorter wavelength. An observer behind the source would hear the sound with a lower frequency, while an observer in front would hear a higher frequency than is being produced by the source. This shift in frequency is called the doppler effect.