Listening for hidden fires.

The snap and crackle of a moonlit campfire can gently set a scene for romance. But the subtle strains and creaks of a house catching fire certainly do not.

Either way, the distinct sounds of combustion have led scientists to a new way to detect hidden fires: Listen for them. Acoustic sensors can be tuned to catch the unique vibrations of materials about to burst into flames.

William Grosshandler and Margaret Jackson, at the Building and Fire Research Laboratory of the National Institute of Standards and Technology in Gaithersburg, Md., have tested sound sensors, called piezoelectric transducers, in experimental fires. The sensors are able to detect the specific acoustic signals of typical housing materials that are about to burst into flames. Using different types of wood, plastic, aluminum, and gypsum board, the researchers have found distinct acoustic signatures associated with the rapid heating of these materials.

Grosshandler calls this fire detection method "a viable but undeveloped concept."

A smoldering fire or overloaded electrical circuit creates heat, which causes surrounding materials to expand. That stress produces sound, at frequencies up to 500 kilohertz, which can signal a serious overheating event even before actual ignition, the researchers say.

The detection technique has many potential advantages, they add. It can scan a large area and is unaffected by the presence of people or machinery, both of which can sometimes give false clues of fire. The sounds of thermal expansion spread more quickly than combustion products or infrared radiation (heat). And an acoustic sensor may serve in an integrated, intelligent fire-detection system, which can locate and analyze hidden hot spots in a building.