Earlier this week, a gigantic green fireball was spotted over the Midwest, with sightings reported all the way from Missouri to New York. Tens of thousands of meteorites hurtle toward Earth each day, breaking apart as they enter the atmosphere. The brighter ones, called fireballs, put on spectacular light shows.
They can be noisy, too, as it turns out.
The sounds occur while the meteor is flashing across the sky, and are separate from the sonic boom sometimes heard seconds after the fireball is extinguished. “Cultural factors clearly influenced the way people interpreted the auditory experience,” writes Maria Golia in her 2015 book Meteorite: Nature and Culture. “Gregory of Tours in AD 580 recorded ‘a sound of as many trees crashing to the ground’; Chinese annals (AD 817) note ‘a noise like a flock of cranes in flight’; whereas witnesses of the Peekskill, New York fireball of 1992 heard something ‘like a sparkler.’ Reports from different times and places mention ‘swishing,’ ‘rushing,’ ‘popping,’ ‘buzzing,’ ‘crackling’ or ‘vibrating’ in the air.”
Well into the 20th century, the sounds accompanying meteorite falls were considered auditory illusions and dismissed as the products of “affrighted imaginations,” according to Golia. “One of the few to object was American meteoriticist H.H. Nininger, who thought the sounds should be regarded ‘as a problem in physics rather than psychology.’ ”
Now a team led by Richard Spalding at the Sandia National Labs thinks they have the mystery figured out. In a study published online last week in Scientific Reports, they propose that very brief pulses of bright light associated with fireballs can heat the surfaces of certain transducing (able to convert energy from one form to another) materials, which then “rapidly warm and conduct heat into the nearby air, generating pressure waves...[which] can then manifest as sound to a nearby observer.”
It only happens with fireballs brighter than -12 magnitude, or about the brightness of the full moon. And some materials—leaves, grass, dark clothing and paint, fine hair—make better transducers than others. “It seems significant that people with frizzy hair are reported to be more likely to hear concurrent sound from meteors,” write the authors. “Intuitively, frizzy hair should be a good transducer for two reasons. Hair near the ears will create localized sound pressure, so it is likely to be heard. Also, hair has a large surface-to-volume radio, which maximizes sound creation.”
The so-called photoacoustic effect—light converted to sound—was observed as long ago as 1880, by Alexander Graham Bell. To test their theory, the Sandia researchers exposed several materials, including several types of dark cloth and a synthetic brown wig, to rapidly pulsing light equivalent to what a fireball would produce. They were able to measure faint sounds, “similar in loudness to rustling leaves or faint whispers.”
“Our calculations and experiments are consistent with how observers have described the concurrent sounds associated with fireballs,” they conclude.