Burp or a bark?
Some desert sand dunes boom, burp and bark when people slide down them and others never make a sound. Figuring out which dunes make noise, especially in the fall, and why, is a tradition bordering on obsession at Caltech in Pasadena.
“It’s not a quick boom; it’s more like a rumble that goes on for at least 60 seconds,” said Kathy Brantley, who wrote her thesis on tuneful dunes after participating in Caltech’s Summer Undergraduate Research Fellowship, SURF, program. She brought to her studies a few important assets: an interest in the physics of ice floes and avalanches, and skiing experience which taught her the essential hand-butt coordination required to slide down the dunes and get their rumbles flowing. The sounds last, Brantley said, as long as a rider slides -- and sometimes for up to a minute more.
The Southern California researchers are amassing data on a long-recorded phenomenon. The ancient Chinese called rumbling sand hills ming- sha-shan. Marco Polo and other medieval desert rats, whose tales were often dismissed as desert delirium, were not hallucinating when they reported sounds resembling kettledrums north of Kabul, Afghanistan, and in the Gobi Desert.
In his 1941 book “The Physics of Blown Sand and Desert Dunes,” British investigator R.A. Bagnold described the vibes as “the song of sirens who lure travelers to a waterless doom, the tolling of underground bells in sand engulfed monasteries.” Studies by Bagnold and others have shown that sand of an appropriate grain size and smoothness emanates sound when put into motion. In some instances, its vibrations can be felt through the body. Beach sands, in contrast, meekly squeak when walked upon.
Caltech mechanical engineering professor Melany Hunt is picking up on the work of engineer Bob Sharp, who began the first of 50 trips to Kelso Dunes in the Mojave National Preserve in the 1950s. Hunt said the knowledge gleaned from sand flows will yield insights into the physics of other flows, including Los Angeles’ destructive hill slides.
She leads students to the sweltering Kelso area, as well as to Dumont Dunes near Baker, Eureka Dunes in Death Valley National Park and Big Dune in Beatty, Nev. They bury microphones called geophones in the sand, then, using a computer sound card back in the air-conditioned lab, play back their vibes.
But there is more to coaxing out dune tunes than good hand-butt coordination. Like surfers eyeing waves, dune-tune enthusiasts must ride the right dune at the right time.
They have learned that dry sand is more musical than moist, and that dune tunes are louder in the fall than in the spring. Small, young, shiftless dunes usually keep quiet while older dunes more than 200 feet high produce a wide range of sound.
The Caltech crew uses the burp test to audition dune sand for musical potential. Some technique is involved, but they can detect a noisemaker by gently shaking a jarful of dry sand collected from near the top. No burp? A silent dune.
