Dune tunes

Science World, Feb 7, 2005 by Britt Norlander

Lon Beale climbs to the top of a towering sand dune. Then, he straps a board to his feet and leaps off the edge. His sandboard cuts into the dune, sending sand rushing down the slope. As the sand slips, a thunderous boom cracks from the dune. "It roars like a dinosaur," says Beale, director of Sand Master Park--a sandboarding center in Oregon.

This roar has been heard by desert dwellers for centuries. But scientists have never known how it forms. Now, Melany Hunt, a mechanical engineer and colleagues at California Institute of Technology, may have solved the mystery.

When sand on the dunes moves--pushed by strong winds or even sandboarders like Beale--the grains rub together and bounce off each other. These interactions create sound waves (vibrating energy waves) that travel through the sand.

In booming dunes, the sound waves move downward and hit a layer of wet sand about 2 meters (6 feet) beneath the surface. There, some of the waves get reflected, or bounce back, toward the sandy surface. When the sound waves reach the top of the sand, they get reflected again and travel back downward.

As the waves bounce back and forth through the upper sand layer, waves of a certain frequency (number of vibrations per second) become amplified, or louder, explains Hunt. These particular waves combine to create a loud boom with a single pitch (how high or low a note sounds). "It's like playing one key on the piano," says Hunt.

This sound can't be created on smaller dunes, Hunt says, possibly because those dunes don't have a wet layer below. But large dunes can form strong waves. "You can even feel the vibrations [travel] up your legs," says Beale. "It's an eerie feeling."

Did You Know?

* According to the 1923 book Tales of Travel, Marco Polo had heard the booming sounds made by sand dunes. In some ancient cultures, the people believed that spirits made these sounds.

* The sounds made by sand dunes during the summer differ from the tunes made in the winter. Scientist Melany Hunt believes that the difference may be due to the amount of moisture in the sand, which varies with the seasons.

Resources

* Download movies at Melany Hunt's Web site to see scientists sliding down the dunes to make "booming music": www.me.caltech.edu/hunt/

* Visit this site to learn about how sand dunes form, and the animals and plants that call the dunes home: www.nps.gov/whsa/factsheets.htm