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



And what about the relative amplitudes?

bc, who prays he is not exhibiting a reading failure

John Denker wrote:
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.

==========

Several people on this list have mentioned the advantages of
having a _theme_ in a physics course. One could imagine building
an introductory physics course around the physics of musical
instruments. Scaling laws, energy, force, leverage, mass,
resonance, waves ... there's a lot of good physics there. Also
I think it's good to remind phyiscs majors (as well as the other
99% of the population) that there's more to physics than quarks
and other esoteric stuff.

A starting point for information on the subject is:
http://www.catgutacoustical.org/

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