Distortion
Distortion is the presence of frequency components in the response of a system that are not present in the excitation of the system, and it is caused by non-linearity in the system. An example is an imbalance in a rotor in a machine that generates a sinusoidal excitation force at the turning speed. If the machine is linear, the resulting vibration will be only at the turning speed, but if there are non-linearities in the machine, such as looseness, then harmonics of the turning speed will also be generated. In other words, the looseness of the machine parts distorts the driving force signal. This property is used to diagnose machine non-linearities such as looseness in machines.
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