Acoustic Dispersion isn’t a term I was familiar with until earlier this week. There was a video that I believe went viral a few years back (video below). It was of a man throwing a rock at a frozen lake and the lake making this amazing noise (something similar to the pew pew noise of Star Wars blasters). Well, this lake happens to be in Alaska and is only a short distance from where we live!

I have been wanting to try this out since I learned that the lake was so near to us. The only issues is that this phenomenon only seems to happen if the lake is frozen, but not covered in snow. Also, I have been told the lake can’t be “too” frozen either. (You know, whatever that means.) Over the weekend we did get almost a foot of snow. However, all last week was around freezing, with no snow! So we made our way to Edmunds Lake!
Ok, so I am horrible at rock throwing (lets blame it on the baby I am wearing). The “singing” my rocks make is no where near as loud as the noise in the viral video. I had hoped to get my husband out to the lake before it snowed. He is quite good a skipping rocks. However, (to my delight) we got a TON of snow last Saturday. Thus we will likely have to wait until next fall to attempt this experiment again.

As you see in my video, Jasper simply throwing the rock at the ice next to use does not make the noise we are looking for. Instead the ice simply cracks. You must through the rock across the lake. This will cause the ice to make the “pew pew” sound we are talking about.

This is where the science comes in. Short sound waves travel faster than long sound waves. The higher frequencies reach the ear first. Because you are standing far from where the rock skips across the ice, you are able to hear the pew pew sound because the higher and lower frequencies split and the lower ones take longer to reach your ear, creating the pew pew. However, if you just chuck the rock at the ice by your feet, you are not far enough away from the noise for your ears to pick up the differences in speed in higher and lower frequencies.

Why do we not hear similar noises when throwing rocks at the ground or other objects? This is because the water under the ice is not solid. This causes the ice to vibrate similar to a cymbal when it is struck.
IMG_7096.JPGThis noise is not unique to Edmunds Lake. We tried it at both Mirror Lake and Reflections Lake. While both made the noise, the Glen Highway being so near to the lakes kept us from hearing the “rock singing” as well. In addition, I have been told the Edmunds Lake has more hot springs than Mirror and Reflections. This likely keeps the lake from freezing as fast, thus creating a better cymbal-like structure, and therefore making the noise louder. A friend of mine tried this experiment at Man Made Lake. It did not work at all. Perhaps this is because the lake is, well, man made and has no hot springs at all. There for it is likely very solid.
While this concept is a bit beyond my 4-year-olds understanding, I think this will be on our lesson plans for the future! In the mean time, I had to drag him away from the lake. He thought the noise was pretty awesome!

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