From: Ronald Wong (ronwong@inreach.com)
Date: Thu Jan 22 2004 - 13:11:24 PST
Message-Id: <l03102800bc35ce660f94@[209.209.18.39]> Date: Thu, 22 Jan 2004 13:11:24 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re Helium
Irene asked:
> I was wondering if anyone can give me any information on what causes your
>voice to sound squeeky when you "talk" using the helium gas.
>
>Irene Hirota
>
Irene:
A footnote to Paul's fine explanation as to why helium "raises" the pitch
of someone's voice.
If, while someone was talking, it was possible to remove their voice box
and hold it up into the air while they continued to talk, you'd have made
an interesting discovery. It suddenly became difficult to hear what they
had to say.
After recovering from this startling transformation, you would quickly see
why. Like a vibrating string, the amount of air displaced by the vibrating
vocal chords is quite small.
So the question arises: How do we hear what someone has to say without
sticking an ear into their face?
The answer lies in the universal phenomena of resonance.
Our head and lungs are full of cavities which are normally filled with air.
Of these,the largest is the oral cavity extending from our vocal chords to
our lips.
Any enclosed body of air has a set of frequencies at which it will tend to
vibrate. The dimensions of the enclosed air will determine what these
resonant wavelengths are. Since the dimensions of the cavities of our head
are fixed, the wavelengths at which they will resonate are fixed.
When our vocal chords vibrate, they produce a whole range of frequencies
from the very low to the very high. Those frequencies whose wavelengths
correspond to the resonant wavelengths of our head's cavities will set all
the air in these cavities vibrating at those frequencies. The consequence
of all this air moving at one and the same time is the intensification of
the sound produced by the vibrating vocal chords. It's this family of
frequencies that we hear and associate with the speaker.
Since the dimensions of the cavities differ from
one individual to another, the "family of frequencies"
(technically, the quality of sound) is different for
different speakers. That's what enables us to distinquish
one person from another just by the sound of their voice.
Have one of your students who plays a wind instrument bring it to class one
day and have him/her "play" a particular note without connecting the mouth
piece to the instrument. The vibrating lips (in the case of a
trumpet/trombone/french horn/etc.) or reed (in the case of a wood
instrument) doesn't produce a very loud sound. Then have the student
connect it to the instrument and play it again. Like I said - resonance.
The student can then change the "dimensions of the enclosed air" and
demonstrate how the resonant frequency is changed as the dimensions get
larger or smaller.
A sound wave's speed is equal to the product of it's WAVELENGTH and frequency.
Since the resonant WAVELENGTH of the cavities ARE FIXED, any increase in
the speed of sound (like heating the air in the lungs to a higher
temperature or, simpler yet, filling it with helium) will lead to an
increase in the frequency at which the cavities will resonate - i.e. Paul's
explanation.
This has nothing to do with the vibrations of the vocal chords.
If you've actually done this demonstration in your class, then you will
have noticed that your vocal chords continued to vibrate in helium just as
they had in air. You continued to produce the same family of vibrations in
your vocal chords as before. The only difference was that the cavities of
your head were now only going to resonate at the higher frequencies
produced by your vibrating vocal chords. It was these frequencies that were
now being intensified. These were the ones your students were now hearing
and that's the reason for the temporary increase in the pitch of your voice.
The next time you do this demonstration, finish it by filling the balloon
with a little carbon dioxide and taking it in. Don't tell the kids how it
differs from helium. Just do the demonstration with "another" gas (the
carbon dioxide).
The first time you do this you might want to do it
sitting down. The carbon dioxide's mass is heavier
than the average air molecule and, if you inhale too
much, will sit in your lungs until it is all expelled.
You'll feel a little dizzy until then (unless you
pass out before hand - physics can be so much fun!).
The kids will be amazed at the sudden transformation of your voice in the
opposite direction.
Leave it to them to explain this one to you.
A. You'll find out who was paying attention to the first
part of your demonstration - and whether they understood it.
B. It'll also give them an opportunity to speculate on
the nature of this "other" gas (just like "real" scientists).
Gung Hay Fat Choy
ron
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