Mach wave

Summary

A Mach wave is the front along which an infinitely weak pressure disturbance spreads in a supersonic flow. It makes the Mach angle $\mu = \arcsin(1/M)$ with the local flow direction, where $M$ is the Mach number. Mathematically, Mach waves are the characteristics of the steady supersonic flow equations. Many weak Mach waves can add up to a shock wave, and spreading Mach waves form expansion fans.