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Dousing flames with low-frequency sound waves

02 Apr 2015
Fire-busters: Robertson and Tran drown out a blaze

A new type of extinguisher that uses sound waves to put out fires has been built by two engineering students in the US. Both chemical- and water-free, the invention offers a relatively non-destructive method of fire control, which could find applications in fighting small fires in the home, and the researchers now hold a preliminary patent application for their device.

While the concept of using sound waves to extinguish flames is not new, previous attempts to realize the principle – including efforts by teams at West Georgia University and the US Defense Advanced Research Projects Agency (DARPA) – had not been successful. Undeterred by this, as well as initial scepticism from their peers and faculty, Seth Robertson and Viet Tran – both final-year undergraduates at George Mason University in Virginia, US – elected to explore the concept, developing a series of prototype sonic extinguishers for a research project.

All about that bass

The principle behind the extinguisher is simple: as they are mechanical pressure waves that cause vibrations in the medium in which they travel, sound waves have the potential to manipulate both burning material and the oxygen that surrounds it. If the sound could be used to separate the two, the fire would be starved of oxygen and, accordingly, would be snuffed out.

Tran and Robertson explored the impact of different frequencies of sound on small fires. While ultra-high frequencies had little effect, the duo found that lower, bass frequencies – between 30 and 60 Hz – produced the desired extinguishing effect. Consisting of an amplifier and cardboard collimator to focus the sound, the duo’s final extinguisher prototype – which cost them only about $600 to develop – is a hand-held, 9 kg, mains-powered device with the capacity to quickly put out small, alcohol-fuelled fires.

“In my opinion, [Robertson and Tran’s] success has been down to their determination and willingness to try many different approaches to harnessing sound waves,” comments Brian Mark, who is also based at George Mason University and is the duo’s research supervisor, adding that the current prototype has been the result of many trials and experiments.

Catching on

Having acquired a preliminary patent application for the design, the researchers are now hoping to move onto further testing and refinements of their extinguisher, with the aim of taking steps towards a potential commercial application. Originally, Robertson and Tran envisaged their device as ideal for use on small fires in the home – for example mounted over a stove top – but are now investigating the possibility of applying the principle to broader applications. One possible use could be in space, where traditional extinguishing agents are hard to focus at a target fire. “Fire is a huge issue in space,” says Tran. “In space, extinguisher contents spread all over the place. But you can direct sound waves without gravity,” adds Robertson.

A possible complication may lie in the heat inherent in larger blazes. As the sonic extinguisher contains no coolant, it may be unable to prevent larger fires from reigniting after the sound is turned off. However, their duo’s work could potentially be applied to “swarm robotics where the device would be attached to a drone”, to be used in situations such as large forest fires or urban blazes, thereby improving safety for firefighters.

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