Catherine Mwangi

A high school girl in Mexico just won a gold medal for putting out fires with a speaker. It plays one deep, low hum and the flames die in about 5 seconds. DARPA tried the same idea back in 2012 with a rig so big and clunky they never took it any further, and hers runs on a 12-volt battery and a speaker she can carry around. Sound is just air being pushed back and forth. A low note at 30 hertz means the air around the flame is flipping direction 30 times every second. Flames only exist in a thin layer where fuel vapor mixes with oxygen. Shake that layer hard enough and it keeps getting stretched thinner, the heat gets pulled away faster than the flame can make more, and eventually the flame rips right off the fuel. Whatever tears off is too cool to keep burning, so it just goes out. DARPA's report put a number on how little it takes. Once the air at the flame is moving about half a meter per second, which is barely a breeze, the flame goes out, and pretty much any pitch can get there if you crank the power high enough. Low notes just get you there way cheaper, because a speaker moves a lot more air on each push when the pitch is low. Two engineering students at George Mason figured this out by trial and error in 2015. They started with super high pitches, 20,000 to 30,000 hertz, and all the flames did was shiver. It only worked once they dropped down to 30 to 60 hertz, which is basically the bass on a hip-hop track. Ángela Karime's build is a 12-volt battery, a frequency generator, an amplifier and a speaker, tuned to 30 hertz. She named it Vortex Tech and talks about it shooting out little rings of air, and that's real physics. When loud, low sound gets pushed through a narrow opening, the air curls up into rings like a smoke ring, and those rings carry a puff of air way farther than the sound would get on its own. In her tests it put out small flames from about 2 meters away. She went through five prototypes and over 100 tests in about a year, and she says it's put out burning wood, flammable liquids, electronics and cooking oil. The cooking oil one is a big deal. Throw water on a grease fire and it flashes into steam and flings burning oil all over your kitchen, which is exactly the kind of fire where you'd want something that doesn't put anything into the pan. Now, sound can't cool anything down, so a big fire sitting on red-hot fuel can come right back the second the speaker shuts off. The George Mason guys worried about the same thing in 2015, and it's why this is still a prototype for small flames and the red extinguisher under your sink isn't going anywhere yet. Space might end up being where this matters first. Up there water floats off in blobs, while sound works the same with or without gravity, and acoustics researchers have already published experiments on fighting fires with sound on spacecraft. She's still in high school, and she's already working on a version that spots a fire and puts it out by itself. The gold in Indonesia also came with a special award that gets her into another international competition.