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This would be for an exhibit in a science museum like
our
wonderful Exploratorium here in San Francisco.
It would be a pipe organ that would play various tunes
through a single pipe by blowing gasses of different
densities
through it. Helium for high notes, sulfur hexafluoride for
low,
air for the middle and whatever other gasses in
between.
MIxtures would probably be necessary for "tuning" and
you'd have to recycle the gasses to
have it run all day every time a
museum patron hit the play button, (or it would get kind
of pricy) but it
would be a interesting way to demonstrate different gas
properties.
Gas-Tuned Trumpet
Gas-Tuned_20Trumpet
Shameless (self-)promotion of a related idea. [Wrongfellow, Jan 11 2012]
Underwater horn
Underwater_20horn
Yes yes, let us self-promote! [bungston, Jan 11 2012]
Variable Wind Organ
http://www.youtube....watch?v=JxhgZObqlLw
Weights move along the windchest to alter pressure. See annotation. [TomP, Jan 11 2012]
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Annotation:
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You could cheapen this by using air of different temperatures. Maybe a chart could show how the density of air at one temperature = some other gas at room temperature. Another cheap gas is steam, also available in a range of temperatures. |
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Pondering further: air at one temperature but different pressures also has different density. I think that different pressures of air could be produced by different rates of flow. This would be fun because there could be pinwheels over the pipe exit which would spin at different rates corresponding with the rate of flow and note produced. |
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Maybe museum-goers could hold their hands over the outflow as well. You would not want that option for the steam pipe. |
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I had thought of the temperature thing but wasn't
sure if that would work. I've seen plenty of
examples of people talking with helium and sulfur
hexafluoride but noted that speaking in the arctic
and in the desert sounds the exact same so a
range of about 150 degrees doesn't make an
audible difference. |
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But maybe you just need bigger temperature
differences. That would certainly be more
economically feasible than having to replace tanks
of gas. |
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You could have a video of the thing I suppose but
that's no fun. |
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Doing this on the cheap. Sounds like a good poser
for the physics smarties out there. The heat diff is
a good one but I think you'd encounter air flow
differences that would offset any pitch effects of
the varied air density. |
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Looks like Wrongfellow hit the basic premise first.
Not sure if that constitutes baked but I'll leave that
up to the audience. |
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Both ideas are pretty neat. |
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//I think that different pressures of air could be produced by different rates of flow.// [bungston]'s anno. |
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See [link] - The 'Variable Wind Organ' built by a Belgian organbuilder alters the pressure in the windchest by moving weights along it (further from or nearer to a pivot at one side - it is normal to have lead weights on the windchest, but they don't move.) This causes different harmonics to be played, or for just an eerie whoosh sometimes. |
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He has made 3, a prototype, the one in the video and a large one now installed in a church somewhere. |
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But if you just blow different gasses through a standard pipe, you won't be able to play a song. Whenever you change gas mixtures, the majority of the pipe will remain filled with the old gas for some time. A large portion of the air flow does not go through the main part of the pipe. That is just a resonator. I suspect that it could take a second or more for the gas inside the pipe to get fully replaced and the pitch to stabalize. The actual time will depend on the pipe size, type, etc. This could still be a very interesting demonstration. If the pipe was made with a clear tube and the different gasses were colored differently (maybe add some "smoke"), it would be interesting to listen to the pitch change while wathcing the gasses get mixed and displaced in the pipe. |
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Re arctic vs desert - exhaled air is pretty near body temperature at both sites. |
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Thinking more about things that traverse the pipe - one could use water with different amounts of aeration to affect density. |
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A pipe with options for water, aerated water, steam and different temperatures of air could point downwards into a pool, which would provide visual evidence of the speed with which fluids/gasses were emerging, and also let you recycle. |
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True that about the exhaled air. Temperature
would probably be the first place to start. You
could compensate for any differences in pressure
as necessary. I wish I had time to experiment with
this. |
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A question just popped into my head. Would the
pitch change or would it just be the same with
different octaves? Is the length of the wave in the
pipe changing due to the different medium or just
the harmonics within that wave? |
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//Would the pitch change// Yes. The frequency of the sound produced by ordinary fipple organ pipes is directly proportional to the speed of sound of the fluid in the pipe; all harmonics (including the fundamental) are shifted equally, and thus the overall pitch changes. The pitch is therefore affected by the molecular mass and the temperature of the gas. |
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However, pressure alone has no affect on the speed of sound, and therefore no affect on pitch. |
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Blowing harder or softer into an organ pipe affects the loudness, and the timbre (harmonic distribution), even to the extent of the lower harmonics being almost completely missing (as per [TomP]'s annotation). The same effect happens when you bow very fast near the bridge on a stringed instrument, or over-blow on a woodwind. |
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Blowing harder also causes the pitch to rise, but this is caused by end effects rather than the speed of sound within the pipe, as confirmed by the fact that the pitch of short, wide pipes is more strongly affected by blowing force than that of long, thin pipes. (Conversely, the effect on timbre, and the tendency to over-blow, are greater in long, narrow pipes). |
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Incidentally, the monochord is an ancient device for exploring and learning about vibrating strings, without the complexities of an ordinary instrument. It consists of a single string, whose tension and vibrating length can be varied (e.g. by weights and movable bridges), and is somewhat the string equivalent of this idea. |
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For completeness, your organ pipe (or a different one) could be fitted with a movable piston, like a swannee whistle, so the length and the speed of sound could be adjusted independently. |
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