Inaudible sound might be the next frontier in wildfire defense

A decade ago, two college students built a fire extinguisher that snuffed out a fire using nothing more than a booming 10-inch subwoofer. The internet lapped it up, and Jimmy Fallon even booked a demonstration for The Tonight Show.

But since that brief viral moment, there hasn’t been much more than a whisper about the technology.

It’s not for lack of trying. The college kids weren’t the first to prove the concept. DARPA was on the case in 2012; and a search of the scientific literature reveals dozens of researchers investigating the idea.

One startup now claims to have cracked the problem. Sonic Fire Tech has built an acoustic fire suppression system that doesn’t just extinguish flames; it that might also protect homes and other buildings from wildfires. The startup has raised a $3.5 million seed round from investors, including Khosla Ventures and Third Sphere, TechCrunch has exclusively learned.

Wildfires cost the U.S. as much as $424 billion annually. The problem has become so acute in places like California that insurers are refusing to renew policies after repeated blazes have reduced large swathes of the state to ashes.

Sonic Fire Tech has been developing its technology over the last several years. Michael Thomas, who is chairman of the startup’s board, had been tinkering with the idea of using sound to fight fires, and when he hit a wall, he reached out to Geoff Bruder over LinkedIn. Bruder had worked for NASA, where he focused on heat and acoustics. 

“This is kind of a new age founding story,” Bruder, the startup’s CEO and CTO, told TechCrunch.

Bruder was intrigued with Thomas’s idea and set about building a prototype. “I got a subwoofer and some parts from Home Depot and AutoZone and said, ‘Hey, let’s see if we can do any better than other people had,’” Bruder recalled. “We knocked a fire out from seven feet in my driveway.”

The startup soon ditched the subwoofer and moved to lower frequencies. The problem with audible frequencies is that any system powerful enough to suppress a fire would be damaging to people’s hearing, Bruder said. “You’ve basically got to throw a speaker design in the trash and start from scratch,” he said.

There are competing theories as to how exactly acoustic energy can disrupt combustion, but the soundless demonstrations certainly suggest that Sonic Fire Tech is onto something.

The new system uses a reciprocating piston much like those inside a car’s engine, but it is significantly larger. An electric motor turns a crankshaft, which pulses the two-foot piston to produce infrasound, the technical term for sound that’s below people’s hearing range, or about 20 Hz. 

“Since we designed everything ourselves, we dropped the frequency to where we’re below audible range, which helps us transmit further, and it makes it safe,” Bruder said.

Sonic Fire Tech’s current record is 25 feet. A bigger system could work as far away as 330 feet, Bruder said. The company plans to sell and install its system for about 2% of a home’s value, and it’s talking with insurance companies to qualify the technology. 

To protect a house, Sonic Fire Tech routes infrasound from a single generator through rigid ducts that sit on the roof’s ridge and under the eaves. On the ridge, they fire down the pitch to catch any fires that might start in debris in the gutters. Under the eaves, they are aimed toward the ground to suppress any flames that pop up near the walls. The system turns on when sensors detect a flame.

A home-based system draws around 500 watts of electricity, and in case of a power outage, Sonic Fire Tech is drawing up plans to use lead-acid batteries for backup. Unlike sprinkler systems, it doesn’t require a source of water, which can be in short supply in wildfire country.

The startup is working with PG&E and Southern California Edison to demonstrate the technology on homes, and it has signed a letter of intent with a chemical storage facility. 

“The natural progression is, if we get certified as a sprinkler replacement, then you can just run a run a pipe into your house and protect your kitchen and everywhere you would need to protect,” Bruder said.