“Skipping rocks on a frozen lake like Lago Bianco in Switzerland will often produce strange, sci-fi-like sounds. Roberto Moiola/Sysaworld/Getty Images
Ice is kind of weird. For one thing, despite being solid, it’s less dense than liquid water. And for another, it’s been known to make some rather jarring noises. Skip a stone across a frozen lake and you might hear a high-pitched sound that’s both familiar and otherworldly. It’s like the chirp of an exotic bird. Or a laser blast from a galaxy far, far away.
Lifestyle YouTuber Cory Williams knows all about the aural quirks of frozen water. In 2014, Williams struck internet gold and went viral when he filmed himself tossing rocks onto an ice-covered Alaskan lake. This simple action produced high-pitched, futuristic noises.
Williams’ video documents a classic example of acoustic dispersion. Sound waves are made up of multiple frequencies, including high ones and low ones. When a sound travels through air, its component frequencies usually travel together at the same rate, so they all reach the human ear more or less simultaneously.
But sometimes, when a sound wave passes through a solid medium (like ice), those high and low frequencies get separated. Being faster, the high-frequency wavelengths zip ahead of their low-frequency counterparts. As a result, you may hear a gap between the high notes and the low notes contained within the same sound. That’s acoustic dispersion in a nutshell.
The phenomenon is easy enough to reproduce. If you bang a hammer against a metal rod, wire or slinky that’s long and thin, the high-frequency vibrations will pulse through the object at a much quicker pace than the lower ones, creating a shrill twang. Those classic laser blast sound effects from the original Star Wars trilogy were made using this very method. ("Pew! Pew!")
Extra-large ice sheets can also lend themselves to acoustic dispersion, which explains the crazy noises Cory Williams set off by chucking rocks at a frozen lake. If you’d like to imitate his experiment, though, you’ll need to stand a good distance away from the iced-over body of water.
Space exaggerates the gap between high- and low-frequency wavelengths. Across a short distance, the former won’t have much of a chance to pull ahead. But on big, wide sheets of ice or lengthy strips of metal, the degree of separation increases between high-frequency waves and their slowpoke counterparts.
Another way to induce acoustic dispersion is by skating across frozen lakes. By gliding over the ice, a skater can produce a series of whistling twangs that’ll evoke the sounds of space battles in "Return of the Jedi." Yet sportsmen who’ve done this say you can’t personally hear the noises while you’re out there cutting figure eights because — in this situation — you’ll be too close to the source of the sound: yourself.
Ice doesn’t need human help to make freaky chirps, by the way. The surfaces of frozen bodies of water naturally buckle and crack, which tends to set off loud dispersive noises. But if you hear these in the wild, feel free to pretend you’re standing on the ice planet Hoth. We won’t judge.
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Nobody wants to wind up walking on thin ice. To that end, a team of researchers looked at acoustic dispersion as a means of assessing the thickness of ice sheets over lakes, ponds and canals. Their verdict? If you throw a rock onto the surface and the resultant "Pew! Pew!" noise sounds any higher than the piano key E4, the ice isn’t safe to walk on.