16 March 2000
The incredible noise of snow flakes!
by Kate Melville
It sounds absurd but then again I doubt too many humans can actually 'hear' snow falling (with the possible exception of avalanches).
However for aquatic animals snow can cause an enormous racket just below the surface. These 'noisy' snowflakes also pose problems for sensitive sonar readings.
The researchers who discovered this natural aquatic noise pollution did so by analysing underwater recordings made during winter storms. They attribute this sub-surface noise to oscillating bubbles, way too small and short-lived to be seen by the human eye. What they actually found was that as a snowflake falls on water, it deposits a tiny amount of air underneath the surface. Before reaching the surface and popping these bubbles send out a piercing sound. The way all this occurs, according to Andrea Prospertti of Johns Hopkins university is, "If you submerge a pocket of air trapped in a snowflake, that pocket of air cannot just sit there.
The bubble has to adjust its volume, and it will do so by oscillating. And when it oscillates, it emits noise."
This sound, ranges from 50 to 200 kilohertz and so is too high-pitched to be heard by humans (we tend to max out at nothing higher than 20 kilohertz). But to porpoises and other aquatic animals that can detect the elevated frequencies, snowflake noise may be very annoying as it can add up to 30 decibels to underwater noise levels
However rather than being just a piscatorial problem, the issue also impacts on matters both military and civilian. The deal here is that snowflake noise can create electronic "clutter" for people using sonar devices to do things like track migrating fish or nuclear attack submarines. So stay tuned folks but we suspect any further noise about this discovery might end up being muffled by its military applications.