 The shock waves that develop during the starting or unstart process may be visualized with schlieren or shadowgraph optical techniques.The design of some air intakes for supersonic aircraft can be compared to that of supersonic wind tunnels, and requires careful analysis in order to avoid unstarts.At high supersonic speeds (usually between Mach 2 to 3), intakes with internal compression are designed to have supersonic flow downstream of the air intake's capture plane.