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Re: Waves



At 02:22 PM 7/5/00 -0700, Leigh Palmer wrote:
...
>
>>but I don't think I can give a good answer to your "why"
>>question. The transmission of transverse waves in a stiff string is
>>also dispersive, by the way.
>
>I find it hard to say "why" dispersion occurs even when the mathematics
>seems clear. This is strange because it's usually the other way round
for me.

I'm not basing my statement on theory, I'm stating an empirical
observation. Remember my ice example? This result may be simply
expressed in a theory of sound transmission in solids, but I am
unaware of it. I have never subscribed to the dictum "Never trust
an experimental result until it has been confirmed by theory".


I missed the fact that you attribute the ice example to dispersion. You
told us about this a couple of years back - no, I don't have that kind of
memory, it's just that I recently looked over an old phys-l thread on
culvert whistlers because a student wanted to do an investigation into the
phenomenon - and I took your comment then to be another example of the same
waveguide phenomenon that was under discussion.

As I understand it, the culvert whistle is not due to dispersion, but
rather to the wavefront being multiplied by reflections from the sides of
the tube. The sound propagating in a large slab of ice, where reflections
occur from the top and bottom surfaces would presumably create the same
effect, as would the spherical shell cavity between the earth and the
ionosphere for an electromagnetic pulse.

Mark