Re: Singing pipes

[William Beaty <billb@eskimo.com>]

On Tue, 18 Mar 1997, Leigh Palmer wrote:


> What can be said confidently is that the tubes are powered by convection.
> When mine are laid over sideways they become utterly silent.

I don't know, maybe lack of convection just spoils the mechanism.  If so,
it wouldn't be *powered* by convection.

After seeing one of these in action at Brett Caroll's demo at AAAS in
Seattle, I came up with the following.  I have no idea if it's correct.

Think about the physics of hot-wire anemometers.  Also think about the
structure of oscillators: an amplifier with feedback and a loop delay.

A hot wire instantly heats the boundary layer of air around it by
conduction.  Then the energy transfer slows, because the boundary layer is
approximately the same temperature as the wire.  If a relative motion of
air is then started, the boundary layer will be partially removed, which
supplies low-temperature air to the immediate surroundings of the wire,
and energy is transferred from wire to air once more. 

In the howling tube demo, sound acts like microscopic wind.  With no wind,
the wire is surrounded by a hot boundary layer.  With wind, the wire heats
the incoming air, there is a net expansion of air, and so the hot wire has
done mechanical work.  Whenever the velocity component of the sound wave
is high at the wire's location, the hot wire will create a pulse of
expanding air.  If a sound wave travels past the wire, the wave *might* be
amplified, if the pulse pressure peak is within 90deg of the peak pressure
of the sound wave.  A heated wire mesh *might* act like a sound amplifier.

Why would the howler-tube stop working with no convection?  One
possibility: after a couple of cycles of sound has passed, the hot
boundary layer around the wire might be oblong.  So, whether there is
sound there or not, the hot wire is still surrounded by hot air, since the
hot layer has grown to encompass all relative positions of the air.  If
there was gentle wind, the hot layer would be translated in one direction. 
Vibration of the air would then expose the wire to "new" air during one
excursion of the air vibratory motion.

Another possibility.  Suppose the sound wave *does* always expose the wire
to cool air, even without average wind.  But this would occur both on
positive and negative excursions of the sound.  The hot wire would
generate two positive pulses when the sound wave underwent a single
complete cycle.  The wire would not amplify the sound, instead it would
generate a second harmonic.  But if there was net wind superposed upon the
sound, the air motion would not be symmetrical.  There would still be two
pulses generated for each pulse of sound, but one of the pulses would be
much larger.  The spectrum of this wave would contain one-F energy as well
as two-F.  If the hot wire was placed in the right spot in a resonant
cavity, the system would oscillate and the wire would be cooled.  There
might also be a 2-F frequency component created.

Sensible?  After trying Brett's demo using a shattered 12v tail light
bulb, I discovered that the bulb filament is too small, and the lifetime
in oxygen too short.  To build an electric version, I suppose nichrome
wire is necessary.  I have visions of a large solar collector, wind-driven
aiming mirrors, and a set of pipes which slowly and *loudly* plays a tune.
"Thermal chimes" rather than wind chimes.

......................uuuu / oo \ uuuu........,.............................
William Beaty  voice:206-781-3320   bbs:206-789-0775    cserv:71241,3623
EE/Programmer/Science exhibit designer        http://www.eskimo.com/~billb/
Seattle, WA 98117  billb@eskimo.com           SCIENCE HOBBYIST web page