Vibration Time Wave form : Multiple Frequencies-Non Linear System (Amplitude Modulation ) .

Two or more independent frequencies can be mixed together if a machine has some form of non linearity or other problem. There are many forms and degrees of frequency mixing. Examples of amplitude modulation, sum and difference frequencies, pulses, and frequency modulation are discussed in the following sections.

Amplitude Modulation :
Amplitude modulation occurs when two frequencies are added together algebraically. Frequencies will not add in a machine that behaves in a linear manner. Therefore, a problem must exist before amplitude modulation can occur. There are several forms of amplitude modulation; one form is a beat. A beat occurs when the amplitudes of two frequencies are added together.

When the amplitudes of the two frequencies go into phase, they add together. Then, as the two frequencies go out of phase, the amplitudes subtract until they are 180 degrees out of phase. The two frequencies continue to go into and out of phase, forming a time varying amplitude signal called a beat.

Beat-Amplitude Modulation of Two Frequencies.

Amplitude modulation also occurs when two frequencies are not exact multiples

Two Similar Frequencies with Second Harmonic.

Beat of Two Similar Frequencies.