Hi folks, Haven't posted for ages! I was interested in an acoustic phenomenon yesterday. I took a small group of my photography students out in my car do do some landscape work. We went to a scenic area beside the reservoir. A small side part of the reservoir was frozen, about a quarter inch thick. The day was quiet and still, and very cold, and the part we were at, in a little valley. One of the students rolled a coin across the ice, throwing it from a distance about six feet from the water's edge and about six feet above. The coin rolled a considerable distance. All of us were amazed at the eerie sound which was produced as the coin rolled over the ice. Its volume and duration were quite startling. It was a cross between a metallic sound and a weird bird call, and continued at the same volume as long as the coin rolled on the ice. I guess that the ice must have functioned as a kind of soundboard. But what bemused me was, where did all the energy come from to make that much noise? The coin - a UK penny, was only thrown (or rolled) from a height of about six feet. How could it set the ice so effectively in motion? I conclude that conditions must have been just right for an absolutely optimal transfer of kinetic energy into acoustic energy. The students were so taken with this that they all started throwing coins -pennies and two pence pieces(and some round mints)- on to the ice, to listen. I felt tempted to return later and make a recording. Ice would certainly form a very rigid and brittle soundboard, and I guess if it was not too thick, or too big an area, the whole sheet could be set in motion by a rolling penny. Would there be air between the ice and the water? I'm not sure. It was a fascinating effect which we all enjoyed. Best regards, David Boyce.
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