English: The sound source has now broken through the sound speed barrier, and is traveling at 1.4 times the speed of sound, c (Mach 1.4). Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer actually hears the sound it creates. As a result, an observer in front of the source will detect ${\displaystyle f=\left({\frac {c+v_{r}}{c-v_{s}}}\right)f_{0}=-2.5f_{0}\,}$ and an observer behind the source ${\displaystyle f=\left({\frac {c-v_{r}}{c+v_{s}}}\right)f_{0}=0.42f_{0}\,}$.