Page History

...

Unlike ordinary sound waves, the speed of a shock wave varies with its amplitude. The speed of a shock wave is always greater than the speed of sound in the fluid and decreases as the amplitude of the wave decreases. When the shock wave speed equals the normal speed, the shock wave dies and is reduced to an ordinary sound wave.

Reference 2: http://www.grc.nasa.gov/WWW/k-12/airplane/machang.html

In case that:
C = speed of sound
V = speed of the moving object
M = Mach number, would be M
So if an object move twice time faster that sound speed the Mach number would 2. |

When an object travels slower than sound, the ratio in this equation is greater than one, and the equation does not have a real solution. Traveling at the speed of sound makes the ratio equal one and results in a Mach angle of ninet¥y degrees. Above the speed of sound, the ratio is less than one and the Mach angle is less than ninety degrees. The faster the object moves, the narrower the cone of high pressure behind it becomes. Measuring the vertex¥ angle is a way to determine the speed of a supersonic object.
Reference 3: 3 screenshot from the video bellow.