English: The same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving to the right with a speed vs = 0.7 c (Mach 0.7). The wave-fronts are produced with the same frequency as before. However, since the source is moving, the center of each new wavefront is now slightly displaced to the right. As a result, the wave-fronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) of the source. An observer in front of the source will hear a higher frequency ${\displaystyle f=\left({\frac {c+v_{r}}{c-v_{s}}}\right)f_{0}=3.33f_{0}\,}$, and an observer behind the source will hear a lower frequency ${\displaystyle f=\left({\frac {c-v_{r}}{c+v_{s}}}\right)f_{0}=0.59f_{0}\,}$