Amplitude modulation is defined as the multiplication of one time-domain signal by another time-domain signal. The signals may or may not be complex in nature, i.e., either or both signals may contain harmonics components. It is impossible to have amplitude modulation unless at least two different signals are involved. The signals may be electrical in nature, or they can be vibration signals. Modulation is inherently a non-linear process, and always gives rise to frequency components that did not exist in either of the two original signals.
Tip excerpted from Introduction to Machinery Vibration by Glenn D. White