Philadelphia has not been much of a winter wonderland for the past few years; this year, however, Mother Nature has decided to make up for lost time. After a winter storm, there is always something calming about taking that first walk in the fresh snow. If you stand still and listen, you might notice an unusual tranquility. Why does the world seem so quiet right after a new snowfall? Read on to learn more about the acoustic snowflake.
Wilson Bentley famously pioneered photo-microscopy of snowflakes in the late 19th century; several snowflake photos of the thousands he took are included below. You can also discover more beautiful snowflakes here.
One of Bentley’s many observations was that snowflakes are six-side crystalline forms; when packed together, this structure helps to form open spaces or pores. An apt descriptor of snow cover is by the percentage of total open space in the material, or porosity. Below are two examples of snow cubes with different porosities. The snowpack on the left has a denser structure and therefore has less porosity than the example on the right, which is less densely packed within the same volume.
Sound is an acoustic wave that propagates by transferring energy through a medium. When an incident wave traveling in one medium (air) encounters the boundary of a second medium (snow), it splits into reflected and transmitted waves. As this transmitted wave finds its way through snowflake pores, it bounces through the snowflake structures, and sound energy is converted to heat energy through friction. Through this process, sound energy is absorbed, and reflected sound levels decrease in turn. Acoustically absorptive panels, that you may see in auditoria or even restaurants (if you’re lucky), work in a similar fashion.
An absorption coefficient is a way to describe how well a material absorbs sound: a higher number indicates more absorption. It ranges between 0-1 and is frequency dependent. Snow absorbs more sound at mid-frequencies (500 to 2000 Hz) than low frequencies (below 500 Hz). The acoustical absorption properties of snow is similar to a 3/4″ acoustic wall panel. The beautiful white landscape is actually acting like sound-absorbing panels made by Mother Nature! The figure below shows the absorption coefficients of the two snow samples compared to a typical acoustic wall panel as well as plaster on a lath wall.
Rumor has it that Punxsutawney Phil did see his shadow on February 2nd – so we are in for 6 more weeks of winter. Snow what?! There are many more beautiful benefits to snow despite the never ending shoveling!