雪害の記録 2006 Apocalypse

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(USA)Sound theory from snow study

2005年03月06日 | 雪害の記録04-05
The white stuff actually does bring quiet ・mostly because it absorbs sound waves

Snow falls on Bard College freshman Julie Shore as she walks along a campus path in Red Hook, N.Y., last week.
PHILADELPHIA - Freshly fallen snow is really nature's white noise machine.

That's right. It really is quiet after a snowfall, and not just because everyone is hiding indoors.

Scientists have discovered that in the first hours after a snowfall, the flakes have a rare acoustic quality to absorb sound waves.

Sometimes it lasts for just a few hours ・as in a slushy snowfall. Other times, when it's covered by freezing rain, it heads to the opposite end of the acoustical spectrum and echoes like concrete.

Researchers worldwide, particularly where they get lots of winter weather, say they study the sound of snow in hopes of figuring out everything from avalanche triggers to noise insulation.

"After a fresh snowfall, it's very quiet and then once it's blown by wind and sunshine, it hardens and loses its effectiveness," said Gilles Daigle, who researches sound acoustics for Canada's National Research Council, a governmental research agency in Ottawa.

"It's a quality of life issue here," he said, adding that he started studying snow acoustics in the 1970s, as part of an effort to reduce noise pollution.

"We take into account where we should construct the roads and what kind of zoning to allow around it ・it's all about land use planning."

The reason snow works so well as a sound insulator is because all the crystals are shaped differently. So when they fall on each other, they stack like differently shaped blocks, leaving lots of air between the flakes. The air works like the holes in a sponge, trapping sound waves and dampening vibrations.

"The grains themselves aren't very spherical, so when it first falls, it's very fluffy," said Douglas Durian, who studies the physics of granular materials at the University of Pennsylvania in Philadelphia. "It's the fragility of the packing that gives rise to the tremendous ability to dampen vibration."

All of which is more easily done with fallen snow. Falling snow, however, is much harder to study. Snowflakes themselves are tiny hard crystals and may actually scatter sound more than absorb it. But they don't stay still and are unpredictable, say scientists.

It may seem like the study of snow acoustics may be silly, when it just lasts for a day or so. But to Jerome Johnson, it was a matter of saving lives.

Johnson works for the U.S. Army Corps of Engineers Cold Region Research and Engineering Lab in Fairbanks, Alaska. A few years ago, he studied snow acoustics for Washington state.

"The goal was to develop ways to trigger avalanches artificially as a safety mechanism without using traditional means, such as artillery rounds," he said.

Now he studies snow structure and strength as a way to help figure out how to get vehicles to drive more easily through soft snow.

HoustonChronicle
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