These are the Spectra of those same three notes from the trumpet, one from the lowest notes on the trumpet(A3#) From both the record of the note itself, and from the spectrum, we A strong series of harmonics. There are wiggles of much shorter than the period of the note.
one from the mid (F5#) Again the plot would suggest a lot of higher harmonics and the spectrum bears this out. Note that the spectrum of peaks extends all the way out to the limits of hearing.
and one from the highest(A5#). This we would expect to be much "purer"-- ie fewer higher harmonics ( althought he second harmonic should be strong)
The trumpet is an instrument with very rch harmonics. This is both because of the flare of the trumpet which reduces the knee frequency for the higher harmonics, but also because of the very righ harmonic structure of the lips opening and closing, letting in the air in bursts. Again, the junk below about 100 Hz is all noise-- either from the room, or from the electronics in the computer or noise from the computer itself.
The absolute amplitude in dB is simply how far below the loudest sound that the microphone could record without clipping. It is the relative amplitudes of the notes that are important.
As explained in the notes on the Fourier transform in the course notes, the widths of the peaks arise out of the finite time that the note is recorded and the fact that the recording time is not an exact multiple of the period of the note. Both of these effect broaden out the peak.
The number of samples 16384. Since there are 44100 samples per second, this is about .4 seconds. The resolution -- the minimum frequency that can be sample is one whose period is this sample time, so the minimum frequency is about 2.5Hz. The smooth curve at low frequencies is an interpolation by the program and cannot be trusted (eg below about 50Hz)
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The image files were created under Mandriva Linux using audacity and xv.