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Frozen Lake Mendota

UW-Madison employee Ray Spiess walks to his office with his lunch across frozen Lake Mendota last January. When ice is forced to vibrate, move, or crack, it generates a sound wave.

M.P. KING, STATE JOURNAL ARCHIVES

Q: Why do frozen lakes make sounds?

A: Lake ice is dynamic; it moves. When it is forced to vibrate, move or crack, it generates a sound wave. That sound propagates throughout the ice as well as into the water below and into the atmosphere.

How does the ice move?

Lake ice expands and contracts with temperature changes. This movement can result in the ice cracking. This cracking will generate a noise, sometimes sounding much like a loud, booming thunderclap.

If there is no snow on the lake, throwing a rock or other objects onto the ice can generate interesting sounds. There can be a pinging sound when the rock hits the ice.

The ice will be floating on water, if it’s not too near a shallow shoreline. The rock striking the ice will cause it to vibrate, and the water below will move as well allowing the ice to vibrate like a drum head.

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As the vibrations, or sound waves, move through the ice, the high frequencies will move faster than the low frequencies so the sound will change with time. If you are far enough away the sound waves will separate and the higher frequencies will reach your ears before the lower frequencies. The pitch of the sound will change, resulting in a pinging or chirping sound. The sound could also propagate across the ice to the far shore and then reflect back to you, much like an echo. This works best on clear ice.

Snow will dampen any of the sounds the lake ice will make. Cold snow can make a squeaky sound when you walk on it as the snow crystals get crushed below your boots.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.