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Diagram of the recorded sound is composed of many points
which are called samples. The microphone level was read 44100
times per second during the recording. There are 4410 points on
the present diagram showing a tenth of a second long section of
the recording.
Discreet Fourier transformation was computed on a one
second long recording section. The diagram of the transformation
shows the frequency spectrum of the sound which pipe emits. Clearly
can be seen that the largest share of energy corresponds to the
frequency which is a little higher than 200 Hz. This is also the
pitch we hear when the pipe sings. There is a minor peak at the
frequency around 50 Hz and a very small one at around 620 Hz
(a third harmonic?). The peak at 50 Hz could be due to the
noise of the electric equipment (computer, printer, monitor) in
the room where the recordings took place. Mains electricity is
AC, 50 Hz.
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Diagram of the recorded sizzle of the boiling liquid
nitrogen surprises us. We would expect a diagram of a noise, i.e.
an unperiodic one. But we can clearly count circa five periods
on the diagram which is one tenth of a second long. The period
of the oscillation is therefore around 0.02 s and its frequency
around 50 Hz.
Frequency spectrum of a noise should be more or less
constant at all frequencies. The spectrum of the recorded sizzle
shows a large peak at 50 Hz. Obviously, the 50 Hz sound oscillations
are not caused by the pipe since they exist when the pipe is silent.
Obviously the electrical equipment in use contributes the 50 Hz
peak in the frequency spectra.
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