(of a rotating mechanical system) Is that speed at which torsional vibration excitation from rotating components is equal to a natural frequency of the system. The torsional vibration frequency will be equal to the speed multiplied by the order number. (See definition) As an example, a 4-cycle, 4-cylinder in-line engine with an order number of two operating at 1,000 RPM would generate a frequency of excitation of 33 Hz (2 x 1,000/60 sec/min). If the natural frequency of the system is also 33 Hz, then the critical speed, for that system, would be 1,000 RPM.