Archive for March 15th, 2012

15
Mar
12

sonic illusions from ancient civilizations

Post from Alan Brown:

When Lincoln Center concert hall opened in 1962, not everyone loved the sound. It took nearly 40 years and numerous minor and two major renovations to create the right acoustic environment for the New York Philharmonic Orchestra.

This story helped me put the accomplishments of ancient civilizations into perspective.

While attending the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver, I stepped into a session on archeoacoustics—the study of acoustics in ancient structures—that left me spellbound. It turned out that past civilizations could teach us a lot about sound.

Humanity’s connection with sound traces back to our deepest roots. Paleolithic cave artwork is most often found in caves with the greatest echoes, where small groups of people might have bonded while playing music, singing, and dancing. Similarly, artwork along canyons, such as Utah’s Horseshoe Canyon, occurs in places with greatest echoes.

Archeology’s focus on the physical world blinded past generations of researchers to the sonic quality of the ruins they were investigating.

Not any more.

Take, for example, the sonic illusions at the 1,000-year-old Mayan ceremonial complex at Chichen Itza, a few hours south of Cancun. The main pyramid features 91 steps. If you stand in front and clap your hands, the stairs scatter the sound and reflect it back, with each step having a slightly different time delay. Instead of the sharp clapping sound we expect, it comes back as a chirping sound.

Researcher David Lubman claims the chirps are similar to those of the quetzal, a sacred bird of the Maya. (Click here to listen to two quetzal chirps followed by two handclaps.)

The nearby ball court, about the size of two football fields, is bound on opposite sides by two small temples. They form a whispering court that lets a person in one listen to a conversation in the other. They also let speakers project words onto the playing field, or sound like a whooping bird flying left to right.

In the Peruvian Andes, 1,500 years earlier, the Chavin people built a ceremonial building that also played sonic tricks. One example, according to Stanford University graduate student Miriam Kolar, is the waterway under the stairs leading to the entrance. Pouring water into the channel creates a sound like thunder.

Building itself is a complicated maze of hallways, vents, and chambers. The surprise is that they all work together to amplify sound and direct it towards three openings in the front. Think of it as the ancient equivalent of a bass reflex speaker.

She tested her theory by having musicians play two conch sell trumpets. She said the sound felt like it was coming out of the floor, and vibrated her whole body.

How did they learn to do this? No one knows. Perhaps it was all trial and error. But like Lincoln Center, they eventually got it right.




The Editor

John G. Falcioni is Editor-in-Chief of Mechanical Engineering magazine, the flagship publication of the American Society of Mechanical Engineers.

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