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Most of the websites i have looked at, say that the frequency of middle C note is approximately 261.626 Hz. and that middle C on guitar is at the 5th string 3rd fret. but my tuner says the frequency at the 5th string 3rd fret is 130.81 Hz.
The frequency of beating is calculated as the difference between the frequencies of the two notes. For the example above, |200 Hz - 300 Hz| = 100 Hz. As another example, a combination of 3425 Hz and 3426 Hz would beat once per second (|3425 Hz - 3426 Hz| = 1 Hz). This follows from modulation theory.