What Are "Beat" Frequencies ?
What is meant by the term: "beat frequencies" ? If you work in a place with lots of machines, you have probably felt or heard one. Have you ever walked by a machine and felt or heard the vibration increasing . . . and then going away . . . and then increasing . . . and then going away . . . etc., etc. That is a "Beat" vibration.
Beat vibrations are caused by vibrations that have very close to the same period and similar amplitudes. 
  • The animation shown here shows two separate sine waves - the red 1x rpm component of a 2-pole motor and the line frequency being supplied to that motor. Either can cause mechanical vibration.
  • Note that the amplitudes are the same. The amplitudes must be at least similar to produce a beat. Otherwise, the dominant amplitude will be only slightly affected by the much lower amplitude and the effect will not be felt.
What does this interaction of these signals produce ?  
  • You can see here that when the peaks coincide (they are 'in-phase'), they add together and create more vibration than either single signal produces by itself.
  • When they oppose one another (they are out-of-phase), they cancel each other out and the vibration disappears - for a moment, anyway.
  • The "Beat" frequency is simply how often these separate signals go in-phase with one another. It might be 10 times a minute or 20 times a minute.
  • The difference between the two frequencies IS the beat frequency. By measuring the frequency of the beat (when you can feel or hear it), you can actually determine what spectrum resolution you need to separate the 2 peaks on a spectrum by simply counting the number of beats that occur in one minute or so (or portion of a minute) and dividing by 3.
For example, a 2-pole US motor is running at, say, 3580 rpm. Line frequency is 3600 cpm. The difference is 20 cpm. That means if the 1x rpm component and the line frequency component have similar amplitudes, there will be a beat frequency occurring at 20 cpm. If you have ever walked past a motor and heard it "humming", it is a beat vibration being generated by the interaction of mechanical and electrical frequencies.