Enlarge By Giovanna Dell'Orto, AP The ancient Maya city of Chichen Itza, with the main pyramid, El Castillo, in the background, in this June 2006 photo in Mexico. Crumbled ruins of pyramids litter Central America's jungles, trees growing from their tumbled staircase blocks. Why the ancient Maya abandoned these towering temples remains one of the big riddles of archaeology. But there is one other question: Why build them in the first place? "I think the pyramids were essentially echo machines, built to inspire spiritual feelings," says acoustics expert David Lubman, who will chair a meeting on the "archaeoacoustics" of Maya temples and other archaeological sites this Tuesday in Cancun. The Pan-American/Iberian Meeting on Acoustics will cover lots about the physics of sounds, but the session looking at the acoustics of Maya centers such as Chichen Itza, Palenque and the 1,000 B.C. Chavin culture of Peru (predecessors to the Incas), looks to be a noisy one. "Archaeologists used to give me funny looks when I talked about this," Lubman says. "But now they are definitely coming around." In 1998, Lubman published a Journal of the Acoustical Society of America study suggesting that when someone claps in front of the " El Castillo" pyramid at the famous site of Chichen Itza in Mexico's Yucatan, a chirp echo results that sounds like the song of a quetzal bird. Subsequent studies supported the idea, and gave rise to hordes of tourists now clapping away in front of the famous monument, once a temple to the feathered snake god, Kukulkan. The secret to the echo, a "pee-yooh" noise, as Lubman describes it, lies in the famously tall and narrow steps adorning the front of Maya temples. Unlike the echo you hear from shouting at the straight walls of a canyon, the tall steps on the pyramids tune the noise returned through an effect called "Bragg scattering," each riser bouncing back small echoes that add together to create a distinctive chirp. A 2004 report in the same journal, led by Nico Declercq of Belgium's Ghent University, supported the idea, and also found an explanation for another acoustic effect, where listeners seated on the bottom steps of the pyramid heard raindrop sounds generated by people's footsteps farther up the 100-foot-high temple pyramid. Both the quetzal and rain were sacred to the Maya, Declercq and colleagues noted, "probably due to the fact that Mayans originally lived for many centuries in the forest before getting involved in the construction of cities and religious sites." The Maya built monumental structures at least as far back as 600 B.C., such as the eight-acre acropolis, about three stories high, at Xocnaceh in Mexico's Yucatan. But pyramid building reached its height in the classic Maya culture that flourished roughly from 100 to 900 A.D., stretching from modern-day Mexico to El Salvador to Honduras. Pyramids at centers such as Tikal included one standing 212 feet high, the tallest building in the Western Hemisphere prior to the arrival of Columbus. "They all produced chirping echoes, and I don't think it was a coincidence" Lubman says. He theorizes that the ability of pyramids to recreate a sacred sound, the chirp of the quetzal, led to them popping up all over, like franchises, in the Maya world. "You had to have one. People came to centers for a spiritual experience in part," he says, comparing pyramid acoustics to medieval cathedrals designed to resonate to chanted hymns, or send glints of sunlight to worshipers through stained glass. "Acoustics are the new rage in archaeology," says Maya scholar Lisa Lucero of the University of Illinois at Urbana-Champaign, by e-mail. "There is little doubt that Maya builders took advantage of the great surround (sound) system, temples surrounding open plazas, for ceremonies (and) speeches," she adds. But like many archaeologists, Lucero can offer plenty of other of reasons for building pyramids. Across many ancient cultures, "leaders build taller buildings than their subjects to literally show they are closer to the gods, closer to the heavens. Also, as a stage to overlook their subjects, tall buildings serve this purpose." On Maya buildings, the tall risers and narrow treads could simply have restricted use of the pyramids (one theory popular with tour guides at the site of Uxmal is that the steps required commoners to crawl on their hands and knees to ascend to rulers) or had another use. Still, at the acoustics meeting, Francisca Zalaquett of the Universidad Nacional Autónoma de Mexico, will present evidence that the temples at the Maya site of Palenque allowed music to be projected a great distance from their heights. And a group led by Stanford's Miriam Kolar will report that Peru's Chavín de Huántar, a 3,000-year-old flat-topped pyramid, likely contained a series of "resonance rooms" that amplified the voice of a speaker from within the building. The ultimate test of the Maya pyramid birdcalls idea would be to reapply a smooth stucco coat to the outside of one pyramid, let it dry, and see how loud the echoes come back from the structure, Lubman says. Reapplying stucco to monumental buildings was one of the dry season duties of the ancient Maya, and he suspects it further amplified sounds by eliminating imperfections and gaps between stones on the pyramids, increasing the loudness of the echoed chirps. Lucero still sounds a cautious note. "Until we find an inscription that states that they recognized that clapping equals chirping, we will never know, period," she says. "They were not built to re-create birds sounds. That idea is for the birds." 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