Tiny white snowflakes floating gently onto the glistening surface of a body of water.
For humans, it's hard to imagine a more quiet and peaceful scene. But to water animals with a keener sense of hearing, these falling flakes can create an enormous racket just below the surface, new research indicates. Noisy snowflakes also can pose problems for electronic "ears" by blurring sensitive sonar readings.
A team of researchers from four universities, including Johns Hopkins, produced these findings about snowflake sounds by analyzing recordings made underwater during winter storms. The researchers attributed the subsurface noise to some unlikely culprits: oscillating bubbles, too small and short-lived to be seen by the naked eye. Their report was published in a recent issue of the Journal of the Acoustical Society of America.
The researchers concluded that as a snowflake falls onto a body of water, it deposits a tiny amount of air just beneath the surface. Before the bubble reaches the surface and pops, it sends out a piercing sound. "If you submerge a pocket of air trapped in a snowflake, that pocket of air cannot just sit there," explained Andrea Prosperetti, the Charles A. Miller Jr. Distinguished Professor of Mechanical Engineering at Johns Hopkins and a co-author of the article. "The bubble has to adjust its volume, and it will do so by oscillating. And when it oscillates, it emits noise."
This screeching sound, ranging from 50 to 200 kilohertz, is too high-pitched to be heard by human ears, which generally pick up nothing higher than 20 kilohertz. But the snowflake noise could be quite annoying to porpoises and other aquatic animals that can detect the higher frequencies, said Lawrence A. Crum, chair of the Acoustics and Electromagnetics Department at the University of Washington's Applied Physics Laboratory in Seattle. Falling snow can add 30 decibels to underwater noise levels, Crum said.
Crum, the lead author of the study, launched the research project several years ago when he and a colleague used sensitive hydrophones--underwater microphones--to capture the sound of snowflakes falling on a motel swimming pool in Roanoke, Va. When the recordings were analyzed, the acoustical "fingerprints" of the snowflake sounds were identical to those of bubbles.
Prosperetti, an internationally respected expert in bubble physics, was asked to join in the research because, with the same group, he had earlier conducted studies of underwater noise generated by raindrops. He developed a theoretical foundation to explain how tiny bubbles produced by falling snow could generate the type of sounds recorded by Crum and his colleagues. To do so, Prosperetti had to brush up on his snowflake science.
"Snow is incredibly complex," Prosperetti said. "The shape and size of a snowflake depends very critically upon the temperatures to which it has been exposed--not just the temperature on the surface of the Earth but also in the cloud where it formed. We know that snowflakes are made of many ice crystals arranged together in such a way as to leave a lot of space for air because the density of a snowflake is about one-tenth the density of water. That means nine-tenths of the volume of a snowflake is air. When a snowflake strikes the water, it melts, and the air inside is freed up as a bubble. When that bubble oscillates, it makes noise."
Beyond its impact on water animals, this snowflake noise can create electronic "clutter" for people who use sonar devices to track migrating fish or to distinguish between natural and man-made underwater sounds. The new research could help engineers develop equipment that can filter out sounds made by snow falling on water.
The other co-authors of the journal article on snowflake noise were Hugh C. Pumphrey of the Department of Meteorology at the University of Edinburgh in Scotland and Ronald A. Roy of the Department of Aerospace and Mechanical Engineering at Boston University. The research was funded in part by the Office of Naval Research.