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Re: wind instrument +- filtering of broadband noise



Thanks, However, intuitively the tube with the fisher burner only is easier for
me to "picture." The behavior is similar to a fipple in a flute of lips on a
horn -- The pulse traveling down blows the flame away (or reduces the flame
height as the gauge p. is now lower) and the vacuum following brings the flame
back, which heats the air producing a pulse -- has any one looked at the flame
with a strobe? The delay in heating the air may be important.

bc

Prediction: if I have the correct mechanism it should work best with a small low
pressure flame -- or at least begin to speak more quickly. The fisher burner
has a large volume between the orifice and the flame. Is this important? Will
it not work with short tubes because the mechanism requires low freq's?

Quick search yields:

Soon after the discovery of hydrogen, it was found that a hydrogen flame burning
in an open tube could excite a strong sound. This was called the singing flame,
and was a popular demonstration in the 19th century. An oscillation would be
reinforced if more heat were added near the time of greatest condensation, and
less at greatest rarefaction. Of course, the air would stream through the tube
as through a lamp chimney while this took place. In fact, a lamp chimney makes a
good resonator. Since excess pressure would cause the hydrogen to be pushed
back, diminishing the flame, and the opposite would happen at a rarefaction, it
might seem that the effect would be impossible. However, this does not consider
resonance in the hydrogen supply tube. On the high-frequency side of resonance,
the phase would be correct for reinforcement, while on the other it would damp
any oscillations. It is easy to see why the demonstration was somewhat tricky to
get working properly. There is an interaction between the oscillation in the
tube, and forced oscillations in the supply pipe, that make it work. Other forms
of resonator may also be used, and other gases. Wheatstone showed that the flame
was indeed intermittent, by using a rotating mirror. When the flame is observed
with the eye alone, this cannot be seen, of course. A related phenomenon
(Rijke’s) occurs when a wire gauze closes the bottom of a pipe, or is pushed
some ways up. The gauze is heated by a flame, and when the flame is removed, a
strong sound is produced. The sound can be maintained if the gauze is heated
electrically. Here, the effect depends on the combination of the steady air
motion upwards and the vibratory motion in communicating heat in the proper
phase, bringing cool air to the gauze at the proper time, which occurs only in
the lower half of the tube.

I think both of us deserve pats on our backs?

http://www.ex.ac.uk/~jbcalver/pipes.htm

Leigh Palmer wrote:

The phenomena of the singing tube and burner and the Rijke tube are
both cases in which the heat source is oscillating at the frequency
of the tone. These are examples of oscillators in which positive
feedback reinforces the oscillation of the source. The feedback is
in the form of a periodic reflection from the upper end of the tube.
The intensity of the oscillation can be tuned by adjusting the phase
of the feedback. In the case of the Rijke tube the optimum placement
of the screen is 1/4 of the length from the bottom of the tube, This
makes the reflection path 1-1/2 times the length of the tube, or at
the speed of sound, 3/4 of a cycle, giving a phase shift of 90
degrees. Getting the sign right involves knowing what the correct
phase shift is on reflection, and I can't quite get my head around
that one this afternoon. Thirty years ago Brian Pippard and I tried
to figure out a good explanation of how a Rijke tube works, but we
were not very successful. I think I've got it now, however. The
screen acts as a thermal reservoir and the surface of the wires
oscillates in temperature as a function of the air velocity which
flows past it. The flow is a combination of a steady convection and
an oscillating term. corresponding to the sound.

I think it likely that the burner version has a very similar
explanation for the phase shift, and that the burner flame output
also varies periodically, though the mechanism corresponding to the
heat capacity and thermal conductance combination of the Rijke tube
wire screen doesn't immediately strike me. As far as mode locking
goes, yes, I suppose you could use that term, but it does not
enhance conceptual grasp in my opinion.

Leigh