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I understand why there are peaks in integer multiples of the fundamental in the Fourier transform. But why is there often a peak in half the fundamental?

I am testing for the guitar specifically. I am doing an STFT on an guitar audio monophonic signal and then get the 4 strongest frequencies in each note. I always get the fundamental in those 4 and of course some multiples of it but also half of it.

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  • $\begingroup$ At the risk of sounding like a total dofus: you have taken into account that guitar is a transposing instrument, sounding one octave lower than written? $\endgroup$
    – user28833
    May 31 '17 at 15:05
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When I play A3 (220Hz) in my guitar, the fifth string which is A2 (110Hz) also vibrates a bit: it is what is called Sympathetic resonance. Besides other non-linear effects, this could be the case.

This phenomenon also occurs sometimes when recording bat calls, you can observe resonance at half the fundamental frequency. Rhinolophus sedulus calls Here, the fundamental frequency is at 65 kHz (upper orange tag) and there is resonance, probably generated by the microphone, at half that frequency (lower orange tag).

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  • $\begingroup$ If I check not immediately after the onset but 10 or 15 frames later, then I actually stop getting that half frequency. Could that be the case? $\endgroup$
    – pavlos163
    May 30 '17 at 16:43
  • $\begingroup$ Do you see a continuous (progressive) attenuation of that half-harmonic as time goes by? $\endgroup$
    – Fusho
    May 30 '17 at 16:48
  • $\begingroup$ Yes. At a faster rate than other harmonics. $\endgroup$
    – pavlos163
    May 30 '17 at 17:02
  • $\begingroup$ Most probably something in the path from string being picked to microphone is in resonance. And that "something" (probably a lower string, but it could be the body of the guitar for example... or both) has a big impedance at that half-harmonic frequency, that is why it gets attenuated faster. $\endgroup$
    – Fusho
    May 30 '17 at 17:12

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