[time-nuts] Whispering Gallery-Mode....??

[Poul-Henning Kamp]

In message <002601c8a604$43d8f0b0$2181a8c0 at APOLLO>, "David C. Partridge" writes:
>>From the paper:
>>From the paper:
>
>"... can be understood by comparison with the optical concept of "total
>internal reflection," or by the acoustic "whispering gallery"phenomenon
>experienced in large circular halls."
>
>The classic whispering gallery is in St. Paul's Cathedral, London.   If you
>stand near the wall on one side, you can hear someone standing in a similar
>location on the other side even when they talk very quietly - hence the term
>whispering gallery.


It was my impression that the resonators which operatte in WG mode
uses the _other_ pecularity about whispering galleries, an effect
very few people even know about, reasons which will be obvious in
a moment:

Empty the entire gallery of as much absorbant material as possible,
including people and their clothes.

Using a tone generator in the deep range, placed as closely to the
outher wall as possible, and approximately halfway up the height
of the inner fence.

The traditional instrument for this is a double bass or fagotto but
a good stable Basso Profundo ("Russian basso") can also do it.

You start as low as go can and sweep slowly upwards until you
suddenly hit a resonance of quite startling intensity.

There is a series of such resonances and they follow an odd-ball
mathematical sequence I can't remember at present, but basically
you have a standing wave which "knots" an integer number of time
around the outher circumference, but which is constrained by the
fence at the inner circumference from propagating out into the
larger volume of the rotunda.

It's hard to do if the rotunda is too big and it doesn't work if
the inner fence and the outher wall are not solid and smooth.


I have seen Wispering Gallery resonators which where essentially
doughnut shaped (or more correctly: looked like a slice of a de-cored
pineapple) and others which are spherical, but in both cases the
principle is the same: total internal reflection and no way for the
energy to escape due to the shallow angle of incidence.

Because the oscillation is circular and the circumference large
relative to the wavelength, the modes are close enough to give
a wide selection of resonance frequencies and far enough apart
to filter nearby frequencies very efficiently.

Poul-Henning


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