If the sound source (e.g. projectile) has a higher speed than the speed of the sound then the formula
– f’=fc/(c-c_k) – cannot be used.

The wave cone you can see in the picture is formed if you increase the speed of the wave source. The sound phenomenon when the sound source takes over the congested wave fronts is called supersonic bang. It can be heard when a supersonic aircraft speeds up and it breaks through the sound barrier.