Battle of the whispers
A Boston tourist attraction becomes a research lab to resolve a 19th-century sotto voce mystery.
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.