Why fountains thunder and dunes boom.

Inanimate structures sometimes utter surprising sounds. Consider, for example, waterfall fountains. The smooth, falling walls of water can suddenly begin to flutter, emitting a loud roar that sounds like a helicopter taking off. Occasional thunder-like noise from dam spillways has rattled windows a half mile away. Then, too, some sand dunes quiver and hum, while others shake and boom.

These noises have caught the ear of several researchers, who discussed their findings this month at the annual meeting of the Acoustical Society of America, held in Houston.

Lee W. Casperson, an electrical engineer at Portland (Ore.) State University, studied two fountains in Dunedin, New Zealand, and developed a mathematical model describing smooth waterfalls. The model shows that the farther the water falls and the thinner the curtain of water, the more likely it is that the fountain or spillway will generate sound. Also, an enclosed chamber behind the waterfall increases the likelihood that the air behind will build up enough pressure to make the water oscillate, Casperson says. He believes his results can help designers make either loud or quiet fountains.

To quench his curiosity about why sand dunes rumble and boom, David R. Criswell climbed Sand Mountain, a dune just east of Fallon, Nev. A physicist from the University of Houston, Criswell studied the mechanics of how moving sand creates seismic and sound energy. He discovered he could make the dune sound like a bass violin by walking along the crest or sticking a shovel into the sand. "You can also feel it in your finger," he told SCIENCE NEWS.

Then he and four colleagues sat shoulder-to-shoulder on the crest and started inching down on their butts. "A very large sound started welling up from the center of the dune and lasted almost a minute," he recalls. The texture and shape of the sand grains may determine whether a dune booms, Criswell says, adding that high humidity seems to silence a dune.