Stand in a field when new snow has covered the hillsides and you feel it, a sort of hush that comes over the landscape. It isn’t just imagination or the appreciation of a beautiful sight. Newly fallen snow absorbs sounds due to the trapped air between individual snow crystals. Snow cover as thin as .79” can muffle sound and thus change the feel of a landscaped.
Once it is blown around by the wind and hardened, snow’s sound muffling capacity diminishes. The magic of the initial moment is gone. But then there are days of sledding, cross-country skiing, and just plain tromping through the snow with the family dog. Winter is a time of rest for the soil and the pleasure of reading by a warm fire.
When snow is covered by freezing rain, it will reflect and thereby magnify sounds, much the same way concrete magnifies sounds. Loud sounds such as a pistol report can be used to measure the depth and permeability of snow cover. Scientists take advantage of these sound characteristics of snow to use radar for measuring the dryness or wetness of a snow pack and its other properties.
The acoustical properties of snow vary with the density and structure of the grains. In 1949, Zwikker and Kosten developed a theory about the acoustical properties of porous materials. They based their theory on the work of Lord Rayleigh in 1896on sound propagation in tubes. Extrapolating from this work, they created formulas for acoustic impedence of porous materials, likening the material to twisted tubes through which sound must travel.
The next step is to devise mathematical formulas and test them on samples of naturally fallen snow. Swiss scientists Othmar Buser and Walter Good, at the Switzerland’s Federal Institute for Snow and Avalanche Research, have made a start on this. They work with specimens of naturally fallen snow carved out in a uniform size and tested in the laboratory to determine the sound absorption properties of different types of snowfall.
In addition to changing sound, snow also creates sound. When the temperature reaches 14 degrees Fahrenheit or below, snow will squeak underfoot because of the crushing of ice crystals. Those of us who live in northern climates are very familiar with this phenomenon.
Snow also makes a distinctive sound when it falls on water, different from that made by other precipitation. Since this is only discernible under water, and most of us do not make a habit of swimming in snow storms, you have probably not noticed this particular property of snow.
In addition to changing the way the landscape looks and feels, snow can also change the sounds present there. The next snowstorm, stop and take a listen.
References and further information:
http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADP000151
http://en.wikipedia.org/wiki/Snow
http://iahs.info/redbooks/a162/iahs_162_0061.pdf
