Muffled shots tell a lot about snow

A hush falls over a snowy landscape. That’s because snow mutes sounds passing over it. A U.S. Army researcher has devised a way to use that phenomenon to measure important characteristics of the snow itself.

The way gunshot sounds change over snow reveals snow’s traits. Albert

Possible uses for the new technique range from curbing noise pollution to tracking the winter activities of hostile military forces, says its inventor Donald G. Albert of the Army’s Cold Regions Research and Engineering Laboratory in Hanover, N.H.

The method may also help scientists assess snow-air interactions in atmospheric chemistry and studies of Earth’s climate (SN: 1/6/01, p. 15).

To develop the technique, Albert and his colleagues during two winters recorded gunshots fired over snowfields. The loud crack of a pistol shot becomes a “muffled whoomp,” Albert says. “What we’re doing is dissecting the whoomp to find out what the snow is like,” he explains.

Sound waves from the blast rattle the air inside small pores in the snow. Those vibrations sap [the pulse’s] high frequencies, leaving mainly the lower-pitched ones to arrive at microphones 60 meters from the shooter.

Using equations developed by other scientists, Albert fit his recorded waveforms to a pair of snow traits–depth and flow resistivity, or how readily air moves through the snow.

Albert’s acoustic measurement of flow resistivity may aid specialists in snow-air interactions, he says. Knowing resistivity will enable them to compute snow’s permeability–its most important trait in interactions with air but a hard one to measure.

At Army listening posts, the new method could also make acoustic sensors smarter. Using familiar ambient sounds, the computerized devices would be able to recognize changed snow conditions. And that, Albert says, would better enable them to accurately discern whatever activity, friendly or not, might be going on. He reports his findings in the January Journal of the Acoustical Society of America.

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