English: Now the source is moving at the speed of sound in the medium (vs = c, or Mach 1). assuming the speed of sound in air at sea level is about 330 m/s . The wave fronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives where ${\displaystyle f=\left({\frac {c+v_{r}}{c-v_{s}}}\right)f_{0}=infinityHz\,}$. An observer behind the source will detect ${\displaystyle f=\left({\frac {c-v_{r}}{c+v_{s}}}\right)f_{0}=0.5f_{0}\,}$.