SIGNALS

Unfortunately, your house is on fire. Which of the following should you do?
(a) Call 911.
(b) Grab your fire extinguisher.
(c) Grab your garden hose.
(d) Crank up the bass.
The answer, of course, is (d).

Your home theater has a subwoofer and hopefully it provides you with kick-ass bass music frequencies and floor-shaking sound effects. But thundering low frequencies, while certainly entertaining, can also be used for more practical purposes. For example, at least according to some enterprising university students, low frequencies can be used to extinguish flames.

Consider this paper authored by Adam Friedman at the University of Maryland. Download it using the link or if you're short on time, here's a quick video that shows two George Mason University students, Seth Robertson and Viet Tran, dousing a flame using a woofer.

The mechanism at work is probably pretty simple. Sound propagates as longitudinal waves; changes in pressure locally move air molecules forward and back along the axis of travel. Regions of pressure compression alternate with regions of rarefaction (picture a sine wave with its high and low portions representing the former and the latter). The lower the frequency, the larger the physical sizes of those regions, and the greater the amplitude, the greater the local displacements.

Now imagine directing a loud, low frequency waveform at a flame. The push and pull of the air molecules would agitate the air molecules around the flame source. A sufficient agitation could temporarily disrupt the supply of air needed to keep the flame going. The fire goes out. Apparently, frequencies between 30 and 60 Hz do the trick and periodic waveforms are best; music is too inconsistent.

While sound may not be the most efficient means to fight fire, I can imagine some interesting applications. For example, a system could be built into the hood over a stove to protect against grease fires. And, because you just know that the government loves to research weird stuff, a DARPA project explored similar fire suppression mechanisms, probably in an extremely costly way.

Legal Disclaimer: You should not try to extinguish fire with your subwoofer. Or a woofer, midrange, or tweeter. Do not play music or watch movies while your house burns. In the event of fire, do not try to rescue your subwoofer. At least not until you've carried your LP collection to safety.