[Phys-L] Re: Singing Rod Demo....

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

I too find the argument from observables (concerning evanescent amplitude)
more convincing than an argument from "what ought to be".
(i.e intuition)

Specifically: if, I say IF, a metal rod struck sharply on its side
shows a loud, fast decaying tone, followed by a quiet slowly decaying tone;

then I can reasonably propose that the initial fast decay is caused by
coupling of the transverse vibration into the environment
or
a strongly dissipative mode in a metal rod.
and
  I can propose that the quiet slowly decaying mode is due to
  a longitudinal mode that couples loosely to the air
or
  a weakly  dissipative mode in the longitudinal axis.

These are proposals or assertions.
What could be more natural (even "scientific") than
  to design methods of distinguishing between these various possibilities?

The very first experimental variable is of course air pressure.
A reasonable vacuum chamber, along with a vibration sensor immediately
distinguishes the relative contributions  of air damping and
internal dissipation.

But what of the other propositions: transverse vs longitudinal vibrations?
  The engineer immediately moves to applying strain gage rosettes:
perhaps two on two sides of a central diameter normal to the
  waistline of the rod's long axis?
     This instrumentation distinguishes axial from transverse vibration
by the sign of the outputs, and is easily demonstrated by watching the
  results of applying a bending force by hand, then applying a longitudinal
stretch.

Brian W



At 01:54 PM 11/30/2005, Bernard Cleyet wrote:
> And what about the relative amplitudes?
>
> bc, who prays he is not exhibiting a reading failure
>
> John Denker wrote:
> > [Ralph] von Philp wrote in part:
> >
> > >   Another teacher strongly believed
> > > that the waves must be transverse because the impressive volume
> of the sound
> > > is too great to be achieved longitudinally.
> >
> >
> > I've been thinking some more about that part of the argument.  We
> > know that argument leads to the wrong conclusion, but beyond that
> > there are good scientific reasons -- as well as pedagogical reasons --
> > for asking _how_ that argument goes wrong.
> >
> > The argument as quoted above doesn't provide much detail, but one
> > might assume that the physical basis of the argument goes something
> cut
>
> > In contrast, the longitudinal mode couples just fine.  Yeah,
> > only the end of the rod couples to the sound field, but it
> > couples efficiently.  To a first approximation, locally near
> > the end of the rod the air thinks there is just suddenly
> > more metal.  The air is displaced.  It's not quite the ideal
> > symmetric monopole radiator, but it's close.
> >
> >    The volume of the rod is not changing nearly as much as you
> >    might naively think, because while the end is getting longer
> >    the middle of the rod is getting thinner.  But these two
> >    regions are separated by more than one wavelength (in air)
> >    of the sound, so the cancellation argument we used in the
> >    transverse case does not apply.
> >
> > Similar physics considerations apply to the design of loudspeakers.
> > If you move some object (like a speaker cone) to push on the air,
> > you have to ask where the object moved _from_, and what effect
> > that had on the air.  That's why (at the most basic level) speakers
> > consist of a lot more than just the voice-coil and speaker cone.
> > Without the _enclosure_, the performance would be terrible.
> >
> > =


Brian Whatcott    Altus OK    Eureka!
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