I was experimenting with sound analysis lately and from what I see when I plot spectral data of an audio file is that apart from notes that were actually picked there are some other notes with quite high local amplitude.
For example I have a sample where D major chord is played with some nasty distortion. After looking onto the spectral plot I can observe a note D, F# and A as expected. But as I go higher some other notes have higher amplitude than threshold, one of those is E.
Tried same algorithm for recorded guitar chord without distortion, but it sounds like there are some other effects like chorus or flanger and again additional note was found.
So, the question is: why I can see additional notes when analysing spectrum even though they were not picked during recording ?
Note: The note E has some harmonics further in spectrum, so it cannot be treated like noise.