Have you ever seen a super-bright meteor and at the same time heard hissing or rustling sounds? Some people claim to hear the sizzle of a bright fireball, comparing it to the sound of frying bacon. I’ve never heard any sounds associated with bright fireballs. Then again, I’ve never seen one that’s approached the moon or sun in brilliance. That’s apparently what you need to break the “sound barrier.”
If you think about it for a second, there’s no way you can simultaneously watch a bright meteor and hear the sound it’s making as it tears through the atmosphere. While the light travels to your eyes in a tiny fraction of a second, sound travels much more slowly, requiring at least several minutes to arrive from even the closest fireball.
So how might we explain that hissing and sizzling? Meteors obviously give off lots of visible light, but they also emit very low frequency (VLF) radio waves. While these are still beyond the range of human hearing, they travel at the speed of light toward the observer and interact with familiar materials. Plant foliage such as pine needles, thin wires, aluminum foil and even dry, frizzy hair can act as a “transducer,” a physical object that can be set to vibrating by radio waves.
Their vibrations create reverberations in the air that we hear as the buzzing or rustling of brilliant fireballs. The phenomenon is called electrophonics and it may also explain why the aurora is audible to some people — though not me of course despite having stood under some monster displays! Still, laboratory experiments have proven that VLF radio waves beamed at a variety of different objects produce sounds that were heard easily.
Now, new research by Richard Spalding and colleagues out of Sandia National Laboratories in Albuquerque, New Mexico, offers an alternative explanation for why we may sometimes hear meteors. He discovered that fireballs have strong, short-lived brightness fluctuations (on the order of milliseconds) that can rapidly warm common materials like hair, clothing, and leaves. These in turn heat the surrounding air, creating small pressure waves that our ears pick up as popping and hissing sounds. The fast-flashing flares have been recorded in nearly all bolides (large, exploding meteors) observed by the Czech Fireball Network.
This isn’t the first time that light has been observed to affect sound, called the photoacoustic effect. Telephone inventor Alexander Graham Bell and colleagues observed it in 1880 when they heard a tone in certain materials by alternately exposing them to and then blocking them from sunlight. For a wonderful demonstration on how to transmit sound using sunlight, learn how a Photophone works in this youtube video.
While the strongest signals produced by materials touched by a fireball’s light are of low frequency and not especially easy to hear, they still fall within the range of human hearing. The best materials for producing photo-acoustical sound are “dark paint, fine hair, leaves, grass, and dark clothing,” according to the researchers, who tested them using white LED lights inside a plastic dome.
Their conclusion was this: An observant person in a quiet environment surrounded by the right materials could hear photo-acoustically induced sound from a magnitude −12 or brighter fireball (one day before full-moon-brightness or brighter), assuming it emits light that can be converted into sound by those materials.
While it’s hard to believe that light from a meteor can heat objects at a distance, the experiments indicate that it’s a real possibility. Fascinating stuff! If you’d like to know more, check out the scientific paper. And now I’ve got to ask — have you ever heard a bright meteor?