Earth, air, fire and water.

So many possible combinations. So many possible stories.

One story we all know is that air feeds fire while earth and water smother it. Natural antagonists. A fair fight – two against two.

Turns out it’s not that simple.

Viet Tran and Seth Robertson are students in the electrical engineering program at George Mason University. For their senior project they designed and built a device that extinguishes fire using low frequency sound. In early February they uploaded a video that demonstrates their fire extinguisher in action. In late March the news media found it and their story went viral.

Tran and Robertson didn’t discover that low frequency sound can extinguish fires. DARPA (Defense Advanced Research Projects Agency) administered a program from 2008 to 2011 that explored how sound can be used to put out a fire. Here’s a video showing their fire extinguisher.

Tran and Robertson’s achievement, and it’s a valuable one, was to turn DARPA’s unwieldy sound production system into a portable unit that may well be the first step in adapting this technology for use outside the lab.

How does it work? Sound is a wave that moves through fluid (air) much like waves in the ocean move through water. The wave doesn’t carry the fluid along with it, it moves and disturbs the fluid as it passes through. Low frequency sounds make bigger waves and disturb more air than high frequency sounds. If the low frequency sound is tuned correctly, it can disturb enough air to put out a fire.

Wait a minute. Isn’t air supposed to feed fire? How can pushing more air at the fire put it out? DARPA and the two students both used a flammable liquid to test their devices. When the liquid burns, combustion takes place at the boundary layer between the liquid and the surrounding air. Disturbing the air around the fire with a low frequency sound has two effects on the boundary layer that contribute to putting the fire out. First, the air movement thins the layer which decreases the density of the flame and makes the fire easier to extinguish. Second, the surface of the liquid is disturbed which increases its surface area.  This has the effect of spreading the heat out over a larger area which lowers the temperature over any part of that area. In other words, the low frequency sound both thins and lowers the temperature of the burning surface thus making the fire easier to extinguish.

Wait another minute. We’re talking about how fire can be affected by low frequency sound. That’s the bass! Bass isn’t supposed to put your fire out, it’s supposed to light you up!

When faced with a conundrum like this it’s time to turn to the experts. Who knows more about earth, air, fire and water than Earth Wind and Fire? Their bass player was Vernon White. What does his bass have to say? Click Play.

You groovin’? Does that bass sound like a fire extinguisher or a fire starter to you?

Fire and air.

Light my fire / Smite my fire.

Two sides of the same coin.

Love the bass.

(This one is for my friend and brother Bob Strawn who turned me on to Viet Tran and Seth Robertson’s acoustic fire extinguisher and who also loves the bass.)