Skulls in the Stars

A friend of mine forwarded me a link (h/t “JakeRyker”) about extinguishing fire with sound waves that appeared in Scientific American. As noted in the article, it has been known since experiments by John Tyndall in 1857 that flames could be extinguished by sound, but evidently the exact mechanism is still somewhat elusive. This is not necessarily surprising; combustion is a complicated physical and chemical process. What is more surprising, though, is that the author of the article and the researchers suggest that sound might be used as a fire extinguisher to put out significant flames. A few observations below the fold…

Mythbusters actually investigated a more specific version of this phenomena, in studying whether a flame can be extinguished by the power of a human voice alone (full episode here). In the early 1900s, a naturalist named Charles Kellogg toured the country demonstrating his amazing bird call imitations and campaigning for the protection of the California redwoods. He also supposedly could put out fires with his bird calls! The Mythbusters demonstrated that this specific claim was rather unlikely, because effective fire extinguishing occurs from low frequency sound, not high frequency.

The Scientific American article is curious because it is a recent article but seems to be based only on work that was done in 2004, which doesn’t exactly make it breaking news!

Even more curious, perhaps, are the observations made in the rather old experiment. The experimenters tested a range of frequencies between 5 and 500 Hz and found that ~50 Hz is the ‘sweet spot’ for snuffing a flame. At first glance, one might suspect that fire damping arises simply from the low pressure part of the wave. Sound waves, like any other types of waves, consist of ‘ups’ and ‘downs’ of some quantity. For sound, the ‘ups’ and ‘downs’ are in pressure. A lower frequency wave will have longer periods of low pressure (read: less oxygen), and it would seem that lower is always better. This is apparently not the case, though, as the article implies that frequencies lower than 50 Hz are again less effective at flame retarding.

What is going on then? It’s not entirely clear. The article suggests that the ‘pressure drop’, i.e. the gradient of the pressure is what causes the extinction, but it doesn’t give any sort of explanation beyond that.

It’s interesting to note that the Mythbusters’ experiments agreed with the work done by the University of West Georgia students: the ‘Busters found that their (relatively large) flame was extinguished at 55 Hz. However, their experiments also suggest that sound will not be a particularly practical method of extinguishing: they needed sounds on the order of 149 dB (comparable to a military jet takeoff and just below the threshold of eardrum rupturing) to snuff the flame!

While I’m thinking of sound and fire, it’s worth posting a video of a nice physics demonstration called the Ruben’s tube (h/t again “JakeRyker”):

[youtube=http://www.youtube.com/watch?v=HpovwbPGEoo&rel=1]

The video is pretty much self-explanatory.  Sounds of different frequencies set up different standing wave patterns (patterns of high and low pressure), and these patterns result in a higher or lower flame intensity, respectively.