Mach wave

In fluid dynamics, a Mach wave, also known as a weak discontinuity,[1][2] is a pressure wave traveling with the speed of sound caused by a slight change of pressure added to a compressible flow.

Thus, it is possible to have shockless compression or expansion in a supersonic flow by having the production of Mach waves sufficiently spaced (cf.

A Mach wave is the weak limit of an oblique shock wave where time averages of flow quantities don't change (a normal shock is the other limit).

If the size of the object moving at the speed of sound is near 0, then this domain of influence of the wave is called a Mach cone.

Mach angles may also occasionally be visualized out of their condensation in air, for example vapor cones around aircraft during transonic flight.

Schlieren photograph of an attached shock on a sharp-nosed supersonic body. The Mach angle is acute, showing that the body exceeds Mach 1. The angle of the Mach wave (~59 degrees) indicates a velocity of about Mach 1.17.
A sonic boom produced by an aircraft moving at M=2.92, calculated from the cone angle of 20 degrees. Observers hear nothing until the shock wave, on the edges of the cone, crosses their location.