Researchers at the National Institute of Standards and Technology (NIST) were taking pictures of a doghouse covered in soot — the black carbon component of smoke that is left behind after a fire — when they noticed something weird.
“Every time we shined the light pretty close to the surface, you're hearing this popping noise,” said Ryan Falkenstein-Smith, a mechanical engineer at the institute. “We said to ourselves, ‘Well, that's kind of interesting.’”
As they continued to take pictures, they found areas with more soot also had more intense popping noises. That’s because soot absorbs all of the light to which it is exposed. It heats up very rapidly, expands and releases energy very rapidly, before finally contracting and making the popping sound.
Up until recently, researchers didn’t have many tools to assess the size and spread of soot. Sending soot samples to a lab could be costly and time-consuming. If they analyzed soot photos with software, they could only guess how much was there.
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The surface color in images could also pose problems.
“If the surface is originally black and then it has soot deposited on it, how do they know if the blackness that they register on their image is from the original surface or from the soot?” Amy Mensch, another mechanical engineer at NIST, said.
That brought NIST researchers back to the question at hand: Is there a correlation between the amount of soot and the volume intensity of sound?
Turns out, there is. Falkenstein-Smith and Mensch took soot samples to the lab, measured their weights, and then shone a flash of light on each of them — like the flash from a professional camera. They found that the more soot there is, the louder the sound. Even when the surface the soot was on or the intensity of the light shone at it changed, the correlation stayed the same.
Even though these measurements were tested in a lab, Somesh Roy, who studies soot at Marquette University in Wisconsin, said the accessibility of the flash of light inherent in this new method could be a game-changer for fire researchers.
"It's reproducible, it is portable...there is a scope for making a lot of measurements at low cost, even if the accuracy may not be super great," he said. "We can have a really fine data at one location, or we can have good data at multiple locations. We have to use both of these things, but these types of devices can give us good data at many, many locations."
Roy said by knowing how much soot is present on a surface, fire investigators may be able to better understand how a fire spread, where it spread to and where the the fire started — all without directly observing the fire.
Knowing soot levels also allows air quality researchers to determine the safety of the air after a fire has occurred. Soot, which is referred to by atmospheric scientists as black carbon, is one of the leading causes of climate change after carbon dioxide emissions. The Environmental Protection Agency measures air quality by the amount of particulate matter in the air or on a surface, which includes soot particles. But unlike other airborne chemicals that may evaporate over time, soot does not.
“Soot is mostly carbon, so it's a solid,” Mensch said. “It sticks around. Once it lands on a surface, it stays there, and it can have heavy metals or other hazardous components.”
Mensch said this new research on the sound of soot will help fire investigators figure out if homes are safe to live in after soot exposure.
“Even if your home isn't burned down by the fire, if it's near the fire, if it's in your community, it could be inundated by the smoke and it could have ruined almost every surface inside,” she said.
Roy said this research could allow fire researchers to assess the air quality and its impacts before it's too late.
"Depending on what type of things are burning, the toxicity, the properties of these particulate matters can change, (and) these particulate matters have a lot of climatic and global health repercussions," he said. "Having good measurements is essential for us to make our understanding of this whole process and their interactions more strong."
The soot sound research method, which has been published by NIST, has a provisional patent that's been filed and will eventually be put together for a patent application. In the meantime, researchers plan to look at the photos of the soot samples they tested and analyze how the visual correlates to the audio response. They also plan to deploy this sound technique in future experiments.
