In 2018, glaciologist Peter Neff recorded a video of himself dropping a piece of ice down a bore hole in an Antarctic glacier. The clip went viral. More than 10 million viewers have since watched––and listened––in bewilderment at the sound the produced by the ricocheting ice.
Last week paleoclimatologist John Higgins replicated Neff’s ice drop, reigniting the internet with the simple joy of the naturally-produced sci-fi sound.
The ice cores were drilled to extract ice cores to glean information about the atmospheric composition of ancient Earth. “Once you have all of these bore holes that you’re done with, you’ve done all the science, the logical human thing to do is throw some ice down a deep hole to see what it sounds like,” Neff said with a chuckle. “And that’s what we did. It’s an unexpected sound.”
Fascination with the sound inspired acoustics researchers to explain the phenomenon. “I had never heard anything like this recording before, especially the ‘ricochet’ sound, and I have to admit that we were stymied for a few days,” said Mark Bocko, an electrical and computer engineering professor at the University of Rochester. “After digging into some of the darker recesses of my old acoustics textbooks, I was able to work out the details and this turned out to be a straightforward but really striking illustration of sound dispersion in acoustic waveguides.”
A post on the Rochester Newscenter explained Bocko’s findings:
According to Bocko’s analysis:
As the piece of ice falls down the hole, it scrapes and bounces off the edge of the borehole. You can hear the frequency of this sound decrease as the ice chunk picks up speed the further down the hole it gets. The decrease in frequency is the Doppler effect, the same effect that causes a car horn to drop in pitch as it drives past you.
After the ice chunk hits the bottom of the borehole, you can hear a “ricochet” noise, which is caused by the slightly different ways the sound from the impact propagates back up the borehole. The acoustic wave for the “heartbeat” impulses travels straight up the borehole, while the other sound waves bounce back and forth off the side-walls of the borehole on their way up. This causes different frequencies to travel at different speeds. The high frequencies travel fastest and get to the top first while the low frequencies lag behind and arrive later.
The spacing of the “heartbeat” noises after the ice impacts is determined by the depth of the hole and the speed of sound in air. In this case, the speed of sound in air at -20 degrees Celsius is 318.9 meters/second; it takes sound about half a second to make one round in the 80-meter-deep borehole.
Read a full explanation of Bocko’s findings here.