Ancient peoples around the world seem to have designed their sacred spaces not only for ceremonial sights, but for ceremonial sounds as well, archaeologists say.
In Peru, for example, a 3,000-year-old Andean ceremonial center's design was optimized for the blare of a priest's conch-shell trumpet. In Mexico, the Chichen Itza temple site features a staircase that can make hand claps sound like the chirp of a quetzal bird. And one of the best-known ancient monuments of all, England's Stonehenge, has a layout that's acoustically pleasing as well as astronomically significant.
The big question is, did ancient societies really have acoustics in mind when they built their monuments?
"That is a challenge," said David Lubman, a California-based acoustical scientist and consultant. Much of the evidence is circumstantial, or based on interpretations of ancient myths. But when the acoustical resonances fit so well with the purpose of a ceremonial space, it's hard to resist making a connection.
"Whether or not you have historical evidence, you have another form of evidence," said Miriam Kolar, a researcher at Stanford University's Center for Computer Research in Music and Acoustics.
Theater for the ears
Researchers discussed their efforts to unravel the mysteries of ancient acoustics today at the annual meeting of the American Association for the Advancement of Science, held in Vancouver, British Columbia.
For the past few years, Kolar and her colleagues have been focusing on Chavin de Huantar, a pre-Inca site in Peru that served as a regional religious center. People apparently came to a circular plaza to worship, and to hear an oracle's pronouncements issuing from a stone gallery.
The Stanford team conducted a detailed acoustical study of the gallery's cross-shaped passageways. They found that the central duct between the gallery and the plaza would serve as an acoustic filter system, accentuating the tones produced by the priests' ceremonial conch trumpets, known as "pututus."
"There was theater going on," Kolar said. The thrilling effect of the trumpet calls and the oracle's words may well have been heightened by the psychoactive effects of the San Pedro cactus that the Chavin people consumed during their rituals.
The chirping staircase
There are theatrical touches as well at Chichen Itza, a Maya temple complex going back more than 1,000 years, Lubman said. One of the most prominent monuments is the Temple of Kukulkan, also known as El Castillo: Some researchers have argued that the temple's staircase was constructed so as to create a "feathered serpent" shadow during the spring and autumn equinoxes. Lubman says the staircase can produce an aural as well as a visual effect: When you clap your hands at just the right spot, the echo comes back sounding much like the chirp of the quetzal bird, which was sacred to the Maya.
The acoustician played an audio clip demonstrating that the bird's chirp and the clap's echo sounded remarkably similar. He speculated that a priest might have clapped his hands loudly to seek counsel from a quetzal. Worshipers would have been impressed to hear the chirp of a spectral bird, apparently coming from inside the temple. "Only priests were trained to interpret what the quetzal said," Lubman said, half-jokingly.
Lubman has been studying Chichen Itza's acoustics for more than a decade. That's such a long time that the quetzal research "should be old news," he said. "But the darn bird keeps chirping." He noted that Chichen Itza has another interesting acoustic feature: Its ball court is designed like a "whispering gallery," so that a low utterance in one corner of the court could be heard clearly in another corner.
The bottom line? Maybe the ancient Maya were more in tune with sacred sounds than we are today. "Now, many things go through our eyes before they get to our minds, but that wasn't true in the ancient world," he said.
The Stone Age and Stonehenge
Steven Waller, a researcher at California-based Rock Art Acoustics, theorized that acoustics may even have had something to do with the placement of the stones at Stonehenge, a monument that's at least 5,000 years old. "What struck me was that the layout of Stonehenge reminded me of an interference pattern," he told his AAAS audience.
Waller said he was even more intrigued when he considered the legends of ancient Britain. One legend suggests that Stonehenge was created when two pipers lured maidens into a circle with their magic tunes, and then turned them into standing stones. He noted that some of Stonehenge's monoliths are sometimes called "piper stones."
Could ancient acoustics have been behind some of these legends? To find out, Waller conducted an experiment in which he put blindfolds on experimental subjects and had them walk around an open field in a circle while two flutes played an identical tone (1100 Hz, or C-sharp). The sound waves from the two flutes interfered with each other in such a way that the sound alternated between loud and soft in different locations. When the walkers were asked to map out the area, they came up with a pattern of obstacles and archways much like an ancient stone circle.
"It's as if there was something blocking the sound ... a ring of invisible objects, massive objects, blocking the sound," he said.
Waller also analyzed the placements of stones at Stonehenge and other neolithic stone circles, and found the acoustic parallel he was looking for. "The pillars actually cast acoustic patterns that mirror an interference pattern," he said.
The leading hypothesis about Stonehenge is that it served as a religious center that was laid out to mark the astronomical alignments for Earth's seasons, and Waller doesn't take issue with that. "My theory doesn't necessarily conflict with the solar alignment theory," he said. But is there any evidence to show that Stonehenge's designers really did have acoustics in mind? Waller can only point to the circumstantial connections — for example, the fact that cave paintings were often put in the locations that had the best acoustics for ceremonies, or the fact that some ancient peoples thought echoes emanated from spirits inside stones.
"They didn't know about sound waves reflecting," he said.
Waller said the important thing is to be mindful of the contributions that acoustics can make to the study of sacred spaces. Some of those spaces are already in danger of disappearing. For example, Waller worried that some of the modern-day renovations aimed at making cave paintings in France more accessible to tourists may actually destroy the acoustic qualities that led the painters to those spots in the first place.
"Nobody has been paying attention to the sounds," he said. "We've been destroying the sounds."
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Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.