Vibration Time Wave form : Multiple Frequencies-Non Linear System (Pulse)

A pulse is caused by a hit or an impact. Pulses fall into one of three categories: empty pulses, frequency modulation, and amplitude modulation.This occurs when excited or generated frequencies are not present. It is called empty because it contains no generated or excited frequencies. A pulse is identified by a series of spectral lines. The repetition rate of the pulse is equal to the difference frequency between the spectral lines. The empty pulse has a low level spectral line at shaft speed, and the amplitude increases with each succeeding harmonic.
Empty Pulse

Frequency modulation is a time-varying frequency. This frequency modulation can appear as a series of bursts or beats. Generated or excited pulses are usually caused by a once-per-revolution impact or excitation.
Generated or Excited Pulse