[Phys-L] Re: sound frequency in helium

["Fayngold, Moses" <fayngold@ADM.NJIT.EDU>]

 The statement that it is frequency, which is primary fixed characteristics of vocal cords, rather than wavelength, does not make much sense to me - for two reasons. First, I do not see a physical mechanism maintaining the constant frequency. In particular, it is not clear to me how we can adjust muscular strain to do it. Second, and more important, if I understand correctly, the fixed frequency, by mere definition of the word fixed, is being maintained fixed expressly for the point of remaining constant under different conditions. Then how come that it changes with changing chemical composition  of gas in resonaiting cavity? What do the words "fixed frequency" mean in this context?

Moses Fayngold




-----Original Message-----
From:   Forum for Physics Educators on behalf of Brian Whatcott
Sent:   Tue 4/19/2005 7:56 PM
To:     PHYS-L@LISTS.NAU.EDU
Cc:
Subject:        Re: sound frequency in helium


At 12:25 PM 4/19/2005, John M., you wrote:
> > I do not know much about the anatomy of the vocal chords, but if we
> > can assume that their sizes are fixed and in first approximation do
> > not depend on muscular effort, then each cord (more accurately,
> > corresponding resonating cavity?) must produce a system of standing
> > waves of fixed lengths regardless of surroundings.
>
> That doesn't follow from the assumption that muscular effort is
> irrelevant.  Moreover muscular effort IS relevant.  As John Denker
> and others have pointed out, in vocal sound the frequency is primary.
> The vocal cords vibrate at a specific frequency that can be altered
> via muscular effort and produce a source waveform that is rich in
> harmonics.  The wavelength of each harmonic is a secondary quantity
> determined by the frequency and the speed of sound for the gas in the
> vocal tract.  Harmonics with wavelengths that resonate well within
> the vocal tract given its shape and size are reinforced and those
> that do not are suppressed.  Usually there are two or three bands of
> harmonics that are emphasized via resonance.  These are called
> "formants."  A little googling on "formants" may be useful.
>
> BTW, pipes are different.  In the case of pipes it is the wavelength
> that is the primary quantity.  It is determined by the construction
> of the pipe.  The frequency is a secondary quantity determined by the
> wavelength and the speed of sound for the gas inside the pipe.
>
> John Mallinckrodt
> Cal Poly Pomona


Taking a wild analogical leap, I see that cadmium selenide which
usually emits in the infra-red when illuminated with white light,
is persuaded to re-emit in the visible - the red or yellow ,in fact
when it is divided to nano-particles in the few tens of nanometer
length scales. The powder is used as fluorescent tagging for cells.
The effect is ascribed to increased quantum gaps in particles
  this small.    [as reported by Phillip Ball, NS April 16, 05, p33]



Brian Whatcott    Altus OK    Eureka!
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