When we imagine a frozen lake, we almost all think of a still, lifeless place. But the truth is quite different. Lakes can sing!
This phenomenon is known as the Doppler effect. That’s right, like in the famous Bing Bang Theory episode. Here is a brief explanation:
But what do ambulances, airplanes and stars have to do with a frozen lake? First of all: ice doesn’t move at all! Well, that’s not entirely wrong. However, I’m sure you’ve all heard the ice cubes in your cola, iced tea or gin and tonic (or whatever you like to drink) crack.
The sound is caused by the difference in temperature, which stretches and expands the ice until it cracks. In a lake, this simply happens on a larger scale. The air above the ice is several degrees below zero, and the water below is usually 4 above zero, so cracks appear all over the surface.
That still says nothing about the ambulance! Because the decisive factor is still missing, namely this: sound travels much faster in ice than in the air. A frozen lake is therefore a gigantic membrane over which short sound waves travel at very high speed from all directions, including past us. The higher the speed, the shorter the sound waves and the higher the sound we hear. Almost like an ambulance.
With just a few stones (and a frozen lake nearby), you can even become a sound engineer yourself and re-enact the Battle of Hoth.