I am using the plot spectrum tool on the Flute E5 sound but it is showing that it is an octave higher (E6) on the plot spectrum thingie. It’s also the same for the other instruments: Cello and Violin. The plot spectrum is showing a different than the note it is playing.Why is that? I got the sound off of http://theremin.music.uiowa.edu/MISpiano.html
Without knowing what the exact audio samples are, my guess is that you are assuming that the note name will correspond with the highest peak in the spectrum. That is often the case, but not always. For a sound that is rich in harmonics, it is quite common for the first harmonic of the fundamental frequency to have higher amplitude than the fundamental.
Another possibility is that they may be basing the note names on a different octave numbering systems. Middle C (MIDI note 60, 261.63 Hz with the A440 standard) is usually referred to as C4, though some refer to it as C3, while others refer to it as C5.
Oh I see. So the first harmonic of the fundamental frequency have a higher amplitude than the fundamental. All right. Thanks!