You will not find this in the literature. It is from my research. I teach this in my class lectures.
There are two independent sources of false beats:
1) Acoustical (interference and resonances from other parts and strings of the piano)
2) Mechanical (in the string, differences in the vertical minus horizontal frequencies inherent in the string movement).
The sensor does not 'hear' the acoustic false beat so it enables you to zero in on the mechanical false beat in the string under test.
Here is a picture from an actual parallel recording, one with sensor and one with mic.
Below that is a view of the spectra going on. Notice the false beat close to the actual desired frequency.
Regards,
Prof. Steve
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Steven Norsworthy
Cardiff By The Sea CA
(619) 964-0101
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