Most flame spotters scout for wildfires with their eyes. But researchers have shown they can now map distant flames with electronic ears, too.

Even a low-intensity grassland fire “hums” loudly enough to be monitored just by sound. That’s the finding of a new study in the July 16 Geophysical Research Letters.

Researchers deployed acoustic sensors near a prescribed burn in Idaho. That was a fire set intentionally to remove dried vegetation that might otherwise fuel a later wildfire.

As the fire burned, those sensors picked up a low-frequency rumble.

By tracking this infrasound — frequencies too low for the human ear to hear — the sensors mapped the flames’ spread. These data show the technique can detect even relatively weak fires, such as those in the early stages of taking hold.

It’s not the first time acoustic monitoring has detected a blaze. Last year, another group reported using it to map a prescribed burn in a Florida forest. But that fire had burned far hotter. Because the Idaho fire was in a damp landscape, it burned less intensely.

The new Idaho data are “encouraging for the broader use” of the technique, says Jake Anderson. He’s a geophysicist at Boise State University in Idaho who led the new study.

Picking up pressure waves

In their early stages, wildfires can be very hard to spot. They often are far from towns and low to the ground. That’s why many forests have lookout stations. Fire spotters used to perch in these structures, high above the ground, scanning the horizon for telltale smoke or flames. Today, satellites and airborne platforms track most fires.

“A lot of those things require direct line of sight,” says Cody Evers. He’s an environmental scientist at Portland State University in Oregon who did not take part in the new work. Such line-of-sight tech has its limits: Clouds and trees can shroud smoke and flames.

One alternative is to scan for unseen flames with sound waves, especially those at frequencies below the range of human hearing.

Fires emit pressure waves due to a process called puffing. They steadily emit plumes of hot gas as they also draw in cold air. The resulting waves oscillate with frequencies ranging from 1 to 20 hertz. (Human ears are sensitive to frequencies between roughly 20 and 20,000 hertz.)

Infrasound allows you to get “very fast alerts about how a fire is changing,” Anderson says. “And it can do so in poor-visibility conditions.”

Arrays help the ‘ears’ home in on fires

To field test the idea, Anderson and his co-workers placed more than 90 infrasound sensors across a swath of southwestern Idaho. This area was due to undergo a prescribed burn. The team designed and built its instruments. Each sensor weighed less than a kilogram (2.2 pounds) and was roughly the size of a paperback book. The team set the units up in eight arrays, each having from three to 44 sensors.

By having sensors in spread-out arrays, a computer can use their data to estimate where a sound is coming from, explains Madeline Hunt. She’s a geophysicist at Boise State and a coauthor of the new study. This concept should be familiar, she says: “With two ears, you can get a better idea of the direction to something, more than if you were just listening with one ear.”

The largest array, located nearly 2 kilometers (1.2 miles) from the fire, accurately tracked the flames’ spread over several hours. A low rumble tended to arrive from the same direction as the helicopter that had ignited the fire. The rumble persisted, however, even after the helicopter left to refuel. That shows the fire emitted that infrasound, the team concludes.

The next step will be to study a true wildfire. That’s far more challenging than an intentional burn, Anderson notes. After all, researchers may not be able to safely access all the affected terrain. The ideal solution would be to have tech “that’s installable by firefighters themselves” as they deploy into the field.

But, warns Evers at Portland State, winds might still limit this system’s success. Why? Large fires often develop in the presence of high winds. And wind produces lots of infrasound, too. So lots of wind around these sensors, Evers says, “could create all sorts of problems.”

Sound fire-fighting?

This new tech might find use beyond just tracking a fire. For instance, in some cases it might also help fight one.

Sonic Fire Tech, a company based in Cleveland, Ohio, is marketing fire-suppression systems based on infrasound. Earlier this year, the company showed how a small indoor cooking fire could be extinguished using its tech. There’s no mess from overhead sprinklers. And the sound waves don’t pose a danger to people or pets, the company says.

It’s highly unlikely, however, that such technology would work well on prescribed fires or blazes in nature, Anderson says. Sound waves spread spherically, he explains. So they will dramatically lose power if they’re not confined.

“I would be surprised,” he says, “if anytime soon we see a long-range, outdoor infrasound tool for extinguishing fires.”