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Singing pipes



Dwight asked for an explanation of "singing pipes". Let me improvise.
ALSO SEE THE POST SCRIPTUM NOTE AT THE END.

Perhaps an analogy will lead to a correct explanation. Consider a pendulum
on a windy day; it gets energy from air (collisions with flowing molecules)
and it swings continously. Now suppose the air is replaced by a more viscous
medium, such as water in a river or honey. Then the pendulum does not swing.
It is suspended at an angle for which the restoring force of gravity and the
pushing force (from flowing molecules) are equal.

Now consider a pipe with a region in which the cross section is reduced.
In that region speed is higher and pressure is lower. This creates two
conflicting "motivations". The blowing air has an "extra tendency" to enter
the low pressure region but by doing this it gains speed and the pressure
is lowered again. The region of pressure oscillations is a source of waves.
The narrow cross section region is located in a proper place to feed standing
waves of several frequency (fundamental+harmonics). The role of convection
is to supply energy; a blower of any kind should do the same, I suspect.

If I had time and equipment I would investigate a relation between the sound
intentsity and other variables, such as the nature of the fluid, the blowing
rate, etc. It would also be challenging to explain the data with some kind of
a theory.
Ludwik Kowalski
Subject: Re: Singing pipes

Greetings everyone! I saw an advertisement a while ago about a
singing pipe. You place a pipe that has a wire mesh inside of it over a
bunsen burner. When you remove the pipe from its heat source, it lets
off a tone/moan.

POST SCRIPTUM:

The alternative explanation was offered in a message from William Beaty.
It came when I was ready to mail the above suggestion. So we have an ideal
situation for a student research project. Which explanation is better?
Both theoretical and experimental efforts are needed.