From: Ronald Wong (ronwong@inreach.com)
Date: Sat Dec 02 2000 - 18:07:09 PST
Message-Id: <l03102804b64f408860c8@[209.209.19.185]> Date: Sat, 2 Dec 2000 18:07:09 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Donald Duck & He
In response to the question:
>...haven't your students asked why a voice gets "Donald Ducky"
>when helium is inhaled?
>
Rob Lee said:
>The natural frequency of vibration for you vocal cords
>is not changed by the He, rather the medium in your resonant chambers,
>(sinuses, throat, mouth, et. al.) is now helium with a much faster speed
>of sound. Since the frequencies are the same with a faster speed of
>sound, the wavelengths change causing different resonant frequencies to
>be emitted.
to which Paul D. added:
>...
>The speed of sound in helium is higher than the speed of sound in air by a
>factor of 2.7. (The square root of the ratio of the molecular masses)
>
>This changes the resonant frequency of the mouth and vocal cavity, which
>remains the same size but now contains a medium with a higher speed of
>sound.
>
>The vocal cords vibrate with a range of frequencies (see the Exploratorium
>exhibit "Frequency Analyzer.") The frequencies that match the resonant
>frequencies of the vocal tract are amplified. With helium higher
>frequencies are amplified than with air. And so the voice sounds higher
>pitched.
Well, the cat's out of the bag.
Of course, you'll have to explain why the speed of sound should be faster
due to the lower molecular mass - a good review, if you've earlier
explained what determines the speed of sound and why.
After you've explained the "Donald Duck effect" to your students, you're
ready for some more fun (assuming you've demonstrated the effect in the
first place - you really should).
>From their standpoint all you've done is demonstrated an effect and then
offered them a hypothesis.
You should ask your students to predict what would happen if someone
inhaled a gas heavier than air - Carbon dioxide for instance - and, equally
important, why.
Now comes the test. Since it's heavier than air and will sit in your lungs
if you let it, take the usual precautions and then give it a go.
The effect is quite memorable when it's done by a woman (for obvious reasons).
---------------------
For those of you who do not care for such entertainments, stand a tube
upright on a table (thus creating a "closed pipe"). Hook up a small speaker
to a signal generator and point it down the tube. Adjusting the frequency
output of the generator, determine the fundamental, resonant frequency of
the air in the tube. Record it. Squirt carbon dioxide into the tube and
wait a little while so that it can warm up to room temperature (got to keep
those variables under control). Find the fundamental frequency for this new
condition by twiddling around with the SG again.
If you use this approach, you might want to have your students predict the
resonant frequency of the tube with the carbon dioxide in it based on the
suggestion that the speed of sound depends on the square root of the ration
of the molecular masses times the speed of sound in air (it's a
"suggestion" unless you've explained to them earlier why the speed would be
dependent on molecular mass. In that case, from the students' standpoint,
it would be another part of the original hypothesis. Verifying their
prediction validates the hypothesis).
--------------------
Enjoy - ron
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