What are "sidebands" in a frequency spectrum?

Sidebands in a frequency spectrum refer to frequencies that appear around a fundamental frequency as a result of modulation effects. When a periodic signal is modulated, either through amplitude or frequency modulation, additional frequencies are generated that are spaced symmetrically around the fundamental frequency. This phenomenon occurs because the modulating signal influences the bandwidth of the main frequency, creating these sidebands.

For instance, if you have a primary frequency and you modulate it with another frequency, the resulting spectrum will display not only the original frequency but also additional frequencies that are offset from it. These sidebands can provide important diagnostic information in vibration analysis, revealing underlying conditions of machinery that might not be apparent from the fundamental frequency alone. Understanding these sidebands is essential for diagnosing issues such as misalignment, imbalance, or bearing faults in mechanical systems.