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Re: dispersion at Chichenitza



At 10:47 AM 1/10/00 -0500, kyle forinash wrote:
there is a distinctive chirp in the echo; lower sounds
return faster creating an echo that rises in pitch.
...
Does anyone know exactly how this works?

If you had reported a sound that *falls* in pitch there would be a
beautiful explanation. It would be a great demonstration of
diffraction-grating physics.

You don't need to travel to the Yucatan to observe acoustical
diffraction. Any building with exterior lattice-work on a sufficiently
coarse scale will do nicely. Actually most such buildings are nicer than
pyramids, because they give you more options as to the angle of incidence.


Check out the article "Singing Stairs" in the January 16, 1999, issue
of Science News. It summarizes a paper given by David Lubman at the
October, 1998, Acoustical Society of America meeting. (I was at that
meeting, but unfortunately missed this talk).

Lubman notes that the *decreasing frequency* of the chirp is reminiscent
of the call of the quetzal bird; thus his theory that the Mayan's
constructed the stairs deliberately to mimic the bird's call.

This theory is somewhat in dispute.

Andy


A short while back on a tour of the pyramid at Cholula (just outside
Puebla, MX) our guide (an elderly man missing some teeth making his Spanish
somewhat difficult) told us a similar story. The actual sound depends
somewhat on how far away you are standing from the foot of the stairs. One
position which he demonstrated was supposedly the quetzal and another
position gave the call of the eagle (of some significance in central
Mexico). He told the story as if this was "engineered" by the builders.
It is a decreasing frequency sound. It is an interference effect. Reminds
me of the echo tube.

Dewey




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Dewey I. Dykstra, Jr. Phone: (208)426-3105
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