These animations show how pulses with higher frequencies can
produce harmonics in an open pipe. Introductions are given in
Open
vs closed pipes (Flutes vs clarinets) or Flute
acoustics. A pulse of high pressure reflects at the open
end of a pipe and returns as a pulse of low pressure. A pulse
of hlow pressure reflects as a pulse of high pressure. In a
pipe open to the air at both ends, a complete cycle of vibration
is the time taken for the pulse to travel twice the length L
of the pipe (once in each direction). The pulse travels at the
speed of sound v, so the cycle would repeat at a frequency of
v/2L.
Note that this situation is not symmetric from right to
left. A high density pulse moves from left to right, which
is air flow to the right. Then a low density pulse moves from
right to left, which is also flow to the right.
What happens if we send pulses with twice the frequency?
In the next animation, we send a negative pulse half a period
after the positive pulse. Because negative and positive pulses
are different, this doesn't change the period. Unlike the
animation above, it is symmetrical. A Fourier spectrum of
the first animation would have a DC term, whereas this one
does not.
