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Future Cities blog #4 Can sound stop forest fires?

#4 Can sound stop forest fires?

This is Charlotte and Manne Friman. Spouses and acousticians at ÅF! We usually work in Stockholm, but during two months we have relocated to USA. We left the cold fall in Sweden for a warmer one in Los Angeles, where we continue our ongoing assignments. Doesn’t it sound nice to work in a warmer climate?

Charlotte Friman och Manne FrimanHowever, the climate changes have made a frightening impact on the temperatures in California. At the moment there are ongoing forest fires with several causalities, homes ruined and even a burned down town. There have been a long drought in California which have made the landscape dry and frail to forest fires.  Due to the new climate, the critical season for fires in California is now all year round. 90 % of the fires are started by humans, intentional and unintentional.

Challenges of fire-fighting

The Woolsey fire, which is actively burning north of Los Angeles, is in steep terrain making it inaccessible for firetrucks. Firefighters have to walk in with all the water they are able carry, around 10-20 liters, in addition to their heavy equipment. The fire-fighting is hard work! The fire was 0 % contained for several days and strong winds made the fire spread rapidly.

Swedish firefighters faced similar challenges in Sweden this summer. Inaccessible fires in forests and terrains called for alternative fire-fighting methods. The Swedish air force fought fire with fire with JAS-Gripen airplanes. They dropped a bomb on the fire when it was closing in on nearby communities. The shock wave from the bomb extinguished the fires by putting out the oxygen for a short period of time at the front of the wave.

Brace for change

During our stay here in California, forest fires have occupied our attention. Perhaps the future holds new means of fire-fighting, methods that can reduce or even stop fires from spreading and minimize the risk of casualties among brave firemen and citizens. We don’t know what the future fire-fighting looks like, but we do know that the future city needs to brace for change! The forest fires in California threaten densely populated areas and large cities and we see the same thing happening in other areas of the world.

Two students at the George Mason University have made an invention to extinguish fires with sound. They have used low frequency sound, between 30-60 Hz, to separate the oxygen from the fire with success. Low frequency sound can be heard over long distances and as the sound wave propagates it displaces the oxygen. Doing this continuously will move the oxygen back and forth near the flame and extinguish the fire. This works because the air velocity near the flame increases by the sound, which makes the flames boundary layer thin. This is where the combustion occurs, making it thinner weakens the flame and eventually stops it.

Sound as fire extinguisher

With directional loudspeakers the invention can be mounted to a drone and target forest fires from above. A lot of research and testing is still needed, and there are no established method in how the drone and loudspeaker will persist the heat, deliver enough sound strength and douse a flame over a larger area. Sound doesn’t have the cooling effect that water does, but it sure is a lot lighter. Machines that produce shock waves and drones with loudspeakers might be a first defense against forest fires which could facilitate the rest of the fire-fighting.

Acousticians could develop acoustic lenses that focus the sound energy, resonance inducing techniques at specific frequencies to increase the strength, amplitude modulation to increase an ongoing wave like an inverse active noise control that increases the sound instead of decreasing like the noise reducing headphone does.

The use of drones might look different in the future and perhaps they will serve as an eye in the sky with heat sensors, warning us of potential fires. A possible futuristic scenario could be fire-fighting drones equipped with directional loudspeakers to react on small fires. An UFO-looking example called Firesound, a fire fightning saucer illustrated by Charles Bombardier explains our thoughts:
http://www.bbc.com/future/story/20160415-a-flying-saucer-that-fights-fire-with-sound

As acousticians we don’t understand the complexity of forest fires or fire-fighting. This is food for thought on a subject heavily affecting us and the millions of people in California at the moment.