Below the wave speed, waves emitted from a moving source pile up in front and spread out behind, leading to a Doppler shift. Above the wave speed, waves accumulate at the sides, forming a shock wave.
For sound, v/c is the Mach number. For light, it is the fraction of the local speed of light. Objects travelling faster than the local speed of light (e.g. in water) emit Cerenkov radiation. The angle of the shock wave sharpens as the speed of the source increases.
