BOOMING DUNES RECORDED
 
  "Park officials say that on windless days -- when sands at Eureka dunes are dry and unseen forces send columns of granules streaming down a steep bank -- the friction of grain against grain can create a sound resembling bass notes played on a pipe organ," write Bert and Jane Gildart in their guidebook to Death Valley National Guide.

"As skeptics, we've made many climbs to the top of the highest dunes in that area to listen. On several occasions we've met with less than the hoped for effects... But on another occasion we heard the 'singing sands.'

"We had departed just before sunrise on a late January day, and there wasn't a breath of air. Perhaps it was our walking that triggered the cascade, for as we watched the rapid descent of cascading sands, we heard the music and now concur that it resembles air resonating deeply in a column of pipes."

There have are only been about 30 "booming" sand dunes identified around the world, but they are capable of making quite a variety of loud sounds that resemble -- acording to witnesses --  foghorns, thunder, aircraft, pipe organs, etc.

While the phenomenon has reported for centuries, the causes of the sounds remain a mystery. Most believe the answer is friction - -tiny grains of sand rubbing together. But that's only part of the story, because the sound continues even after the movement has stopped. And further, the sound a sand dune makes in winter differs from the sound it makes in summer.

These are intriguing puzzles for California Institute of Technology mechanical engineering professor Melany Hunt, who has been researching the flow of particulates and granular materials, including the natural environment of both sand and debris flows.

Several times each summer, Hunt visits the Eureka Dunes in Death Valley with her  colleagues and students, carting a radar unit and geophones to test her theory that the loud sound that's generated is a resonance effect, much like a string being plucked on a musical instrument. 

Over a long period of time, whatever rain that falls in the desert environment percolates into the sand dune, eventually forming a band of moisture some two meters (6.6 feet) down. In time this sand hardens, forming a hard, cement-like crust. When the sand on the surface is disturbed, Hunt theorizes, the friction between sand grains creates a noise that reverberates, back and forth, between the dry sand on the surface and the wet sand below.

"That may be why smaller dunes don't make sound," says Hunt, "because they haven't been around long enough to form that hard layer of sand."

The loudest booming dunes are the tallest and the steepest. And the sound may vary by the season, she believes, because of seasonal changes in the amount of moisture in the sand.

~~~~
Source: Caltech

For a QuickTime movie of Hunt's students sliding down a booming dune, visit the website of Kathy Brantley
 


A FalconGuide to 
Death Valley National Park
A Guide to Exploring the Great Outdoors
by Bert and Jane Gildart
Falcon, 2005.

The lowest point in the Western Hemisphere and one of the most inhospitable places on earth, Death Valley is a land of mystery and stark beauty that attracts the curious and the faithful.

Travel writers and guidebook authors Bert and Jane Gildart confess to being among the latter group, returning to Death Valley National Park most winters to camp, hike and explore.

In this outdoor recreation guide, the Gildarts detail 30 of their favorite drives and hikes, from
the volcanic Ubehebe Crater in the north end of the park to the desert bighorn sheep range near Willow Spring to the south.

Descriptions of each outing include the distance, type of trip (paved drive, day hike, backcountry road, etc.), estimated length, type of vehicle needed and special features. Maps and photos are included, but the Park Service's maps and advice should be closely followed. Death Valley did not earn its name by being visitor-friendly.



Acts of God
The Old Farmer's Almanac Unpredictable Guide to Weather and Natural Disasters
by Bert and Jan