FIRE SILENCE: Sound can extinguish flame as this video proves, but scientists still are not sure how.
"I throw more power into my voice, and now the flame is extinguished," wrote Irish scientist John Tyndall about his experiments with sound and fire in 1857. Countless public demonstrations and a handful of lab tests later, researchers are still struggling to determine exactly how sound snuffs flames.
Sound travels in waves, which are simply variations of pressure in a medium—whether solid, liquid or gas. The energy from vibrating objects, such as speaker membranes, moves from particle to particle in the air in a repeating pattern of high- and low-pressure zones that we perceive as sound. According to the ideal gas law, temperature, pressure and volume are related; therefore, a decrease in pressure can lead to a corresponding decrease in temperature, which may explain how sound can extinguish a flame.
In 2004 Dmitriy Plaks and several of his fellow students at the University of West Georgia tested whether sound waves can douse fires in hopes of using sound to extinguish flames in a spacecraft. VIDEO They placed a candle in a large topless chamber with three bass speakers attached to the walls. The candle was lit and the Canadian rock band Nickelback's "How you remind me" was pumped through the subwoofers. Within roughly 10 seconds, once the song hit a low note, the flame was out, according to results published in 2005 in The Journal of the Acoustical Society of America.
"We don't know exactly what's going on," Plaks says, now a student at the Georgia Institute of Technology.
Physicist James Espinosa at Rhodes College in Memphis, Tenn., a former advisor to the student team, notes that the candle wasn't running out of oxygen to fuel the flame because the chamber was large and open to the air. He also doesn't believe that wind—which would actually displace the warm air around the candle with cooler air—had put out the fire, although only high-resolution thermal images would have been able to verify that.
There is another indication that the fire hadn't been extinguished by wind: frequency (the time it takes for succeeding peaks of a sound wave to pass a fixed point). "There's some special frequency at which a candle flame extinguishes," Espinosa notes. The students tested a range of frequencies from five to several hundred hertz. They found that the effective range was between 40 and 50 hertz, within the range of human hearing.
Plaks speculates that the pressure drop created by the sound wave was what extinguished the flame. Gary Ruff, project manager for fire suppression technologies at NASA's Glenn Research Center in Cleveland, agrees: if the difference between the high-pressure peak and low-pressure trough in the sound wave was large enough, the flame would go out.
Such acoustic fire suppression might prove useful in space, Espinosa suggests. "Not having to use water or toxic gas is a huge benefit" for spaceships, he says. But Ruff and NASA disagree with him: Generating the sound waves to extinguish a fire would require electricity, and astronauts would also have to be able to see the flames in order to direct sound waves at them. "[We are] looking for a very reliable, stand-alone system," Ruff says, such as chemical extinguishers.
Nevertheless, next summer Espinosa will try to extinguish a larger flame with a smaller speaker system. Instead of using the vibrating membrane of a subwoofer, he plans to create an electric arc (current that travels through the air between two electrodes), like that used for welding. This spark creates a shock wave that can be focused with an acoustic horn so that an array of such waves can be aimed at the fire.
Such a system might prove useful here on Earth for putting out fires in locations whose contents could be water-damaged by sprinkler systems, Espinosa says, such as museums that house valuable artwork or centers with data servers or other electrical equipment. "Sound is being used to cut pieces of metal, to destroy kidney stones," he adds. "It can do more than people give it credit for," including, apparently, firefighting.
