Vibration Time Wave form : Multiple Frequencies - Linear system

High Frequency Riding a Low Frequency :
When two independent frequencies are present in a linear system, they cannot add together in amplitude or frequency. When this occurs, the two frequencies mix, and the high frequency will ride the low frequency, At first glance, amplitude modulation appears to be present.

High frequency riding on low frequency

Since harmonics are not present and the high frequency is riding the low frequency, there is not
a cause and effect relationship. The two signals are generated independently. It is important to note that a high frequency which is an exact multiple of a low frequency will cause the amplitude of the high frequency peaks to be the same in each period of the low frequency. A high frequency that is not a multiple will cause the amplitude of the high frequency peaks to vary during each period of the low frequency.