Open-shutter photograph of fireball EN09121418 taken by Pavel Spurný at the Czech Fireball Network along with related data regarding sound emitted from fireballs.
Credit: Spalding, R. et al.

 

Meteors emitting sizzling, rustling and hissing sounds as they descend seem contrary to the laws of physics.

What’s up with this observation?

Because sound travels far slower than light, the sounds should arrive several minutes after the meteor hits, rather than accompany or even precede it. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which travel time would be several minutes.

 

New idea

The late Sandia National Laboratories researcher and pioneer Richard Spalding, father of Meteor Allsky Networks.
Credit: Randy Montoya/Sandia National Laboratories

In an article published Feb. 1 in the journal Scientific Reports titled “Photoacoustic Sounds from Meteors,” the late Sandia National Laboratories researcher Richard Spalding and fellow researchers offered up a new idea.

Spalding and his colleagues reasoned that such intense light could suddenly heat the surface of objects many miles away, which in turn heats the surrounding air. This could create sounds near the observer.

Some new experimentally work has demonstrated and analyzed that effect.

Dark cloths and a wig

The investigative researchers found that objects with low conductivity, such as leaves, grass, dark paint and even hair, could rapidly warm and transmit heat into nearby air and generate pressure waves by subtle oscillations that create a variety of sounds.

The experimenters exposed several materials, including dark cloths and a wig, to intense pulsing light akin to that produced by a fireball. The process produced faint sounds similar to rustling leaves or faint whispers. Computer models bear out the results.

The process is called photoacoustic coupling.

Acoustic waves

According to a Sandia news release, sounds concurrent with a meteor’s arrival “must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer and transduced into acoustic waves,” according to the article in Scientific Reports. “A succession of light-pulse-produced pressure waves can then manifest as sound to a nearby observer.”

A less extreme version of the photoacoustic effect had been observed in 1880 by Alexander Graham Bell.

At that time, Bell tested the possibilities of light for long-distance phone transmissions and he intermittently interrupted sunlight shining on a variety of materials and noted the sounds produced.

Resources

Along with Spalding, the other scientists engaged in the research:

Sandia National Laboratories, Albuquerque, New Mexico – John Tencer, William Sweatt, Benjamin Conley, Roy Hogan, Mark Boslough & GiGi Gonzales

Astronomical Institute, Czech Academy of Sciences, Ondřejov, Czech Republic – Pavel Spurný

To read the full paper, “Photoacoustic Sounds from Meteors” go to:

http://www.nature.com/articles/srep41251

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