from the June 05, 2006 edition -
http://www.csmonitor.com/2006/0605/p16s01-stgn.html
Battle of the whispers
A Boston tourist attraction becomes a research lab to resolve a
19th-century sotto voce mystery.
By Peter N. Spotts | Staff writer of The Christian Science Monitor
It's an odd formula that math teachers shouldn't scrutinize too
closely, but for physicist William Hartmann, it worked: S + Ta = Rp,
where S is scientist, Ta is tourist attraction, and Rp is research project.
When the Michigan State University physicist and his family
arrived in New England to begin a sabbatical at Harvard in 1976,
"it was the Bicentennial, and we did all the Boston sites," he
recalls. Then he heard about a 30-foot, walk-through,
stained-glass globe that played odd tricks with sound. "We'd
done everything else," he says, "so we went to see it."
For Dr. Hartmann, who specializes in acoustics, the globe instantly
morphed from tourist attraction to research target. "I resolved that
if I ever came back to Boston, I'd study its acoustics," he says. In
2001, he returned to Boston for a sabbatical at Boston University, and
Monday, he and two BU colleagues are presenting their initial findings
on the globe's sleights-of-ear at the annual meeting of the Acoustical
Society of America in Providence, R.I.
The globe, known formally as the Mapparium, is part of the Mary Baker
Eddy Library for the Betterment of Humanity, located in the Christian
Science Publishing Society building at the First Church of Christ,
Scientist's Boston headquarters (home of this newspaper).
The globe lets visitors look at the world inside-out from a glass
bridge. Built in 1934, the Mapparium reflects the political boundaries
of the day. Now it may have also become the vehicle for settling a
135-year-old dispute between two eminent British scientists about
whispering galleries -- locations inside domes or specially designed
rooms where a whisper can readily be heard a far distance.
"I'm very familiar with whispering galleries," Hartmann says, noting
the fine gallery at Chicago's Museum of Science and Industry, for
example. "But this one has some unique properties."
He and BU researchers Steven Colburn and Gerald Kidd took measurements
with microphones, noise generators, and an acoustically correct
manikin "head." The globe was officially closed at the time, but
church officials opened it for the team.
"It's kind of a favorite place for me when we have visitors from out
of town," says Dr. Kidd. "To have a chance to be in there for hours
was just fabulous."
Many of the results being reported Monday describe measurements of how
the globe's aural illusions and their strength vary with location and
orientation between listener and sound source. Ultimately, the team
hopes to tie those observations to other factors, such as sound
frequency, then use acoustic theory to explain the experience the
Mapparium delivers.
The team spent several hours trying to identify the various effects by
walking the glass bridge with tiny microphones in their ears,
recording what they heard at various locations. Then they returned
with more sophisticated gear to test more rigorously.
The whispering-gallery effect -- a person at one end of the bridge can
hear clearly the whispers of someone at the other end -- is the most
obvious aural illusion. But there are others: The team identified
points of "acoustic symmetry" or hot spots along the bridge where
sound is naturally amplified. For two people conversing at a constant
volume, the volume will seem twice as loud if they stand two meters on
either side of the bridge's center, compared with the volume if one
person stands at the center of the bridge.
Another is the "opposite ear" effect. With a miked manikin near one
end of the bridge, the team moved a sound source toward the opposite
end. "For a range of locations, the sound appears to come from its
true direction," Kidd says, "then it flips over and sounds as if it
comes from the opposite side."
As for the 135-year-old debate: In 1871 Britain's Astronomer Royal,
George Airy, unveiled a theory explaining the whispering effect in the
dome of St. Paul's Cathedral in London. He held that, in a spherical
dome, the sound at one location reflects to an identical spot opposite
the source. Fellow scientist Lord Rayleigh replied: Bosh! Noting that
whispers can be heard throughout the whispering gallery, not just
opposite the whisperer, he said the sound "crept" around the inner
perimeter of the dome.
Hartmann searched for Rayleigh's skipping phenomenon, holding a mike
on a boom in various locations as close to the Mapparium's stained
glass as he dared.
At the end of the team's stay, and within the limits of Hartmann's
setup, the final score: Airy 1, Rayleigh 0.
www.csmonitor.com | Copyright 2006 The Christian Science Monitor.
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[TELECOM Digest Editor's Note: I've not seen the one in Boston,
although I hope I may sometime have the funds and the health to
go visit it. But I have seen the one at the Museum of Science and
Industry in Chicago and it is really fascinating. One person stands
in a small chamber at one end of a long hall. The other person walks
to the other end, about a hundred feet away to a similar chamber. The
'chamber' is actually just a piece of glass about six feet tall. You
stand there facing it and talk in a whisper or a very soft voice; the
person at the other end -- a hundred or more feet away down a busy
hallway full of people can hear you whispering or talking very softly
quite well -- you can have a conversation with each other. I do not
know what makes it work. From the description shown above, apparently
the one in Boston operates the same way. PAT]
Peter N. Spotts (csm@telecom-digest.org)