A stationary normal shock wave is classified as going in the normal direction of the flow direction. For example, when a piston moves at a constant rate inside a tube, sound waves that travel down the tube are produced. As the piston continues to move, the wave begins to come together and compresses the gas inside the tube. The various calculations that come alongside of normal shock waves can vary due to the size of the tubes in which they are contained. Abnormalities such as converging-diverging nozzles and tubes with changing areas can affect such calculations as volume, pressure, and Mach number.