Early Monday morning, a bright green fireball streaked across the American midwest at 38,000 miles per hour until it exploded above Lake Michigan. The 600-pound meteor hissed low-frequency sounds that were recorded hundreds of miles away in Manitoba, Canada. But according to a new study, it’s likely that people on the ground also heard the sizzling chunk of rock before it exploded in the sky — especially if those people had frizzy, thin hair.
In a recent paper published in Scientific Reports, a team of researchers from Sandia National Laboratories explain why people hear the “popping, hissing, and faint rustling sounds” of meteors even though they stand miles below the upper atmosphere, where meteors pass through. What people hear, the researchers write, is actually the light generated by very bright meteors.
The meteor generates pulses of light that are so bright that they rapidly heat the air around them in waves. This sudden temperature change creates waves of pressure, which then shoot off in a chain reaction towards the listener and are converted into acoustic waves when they smash into natural “dielectric transducers” — dark-colored, heat-absorbing objects on Earth — thereby producing sound.
The presence of certain transducers can increase the likelihood of hearing a meteor. These are categorized as either “half-space” transducers — like dark wood, asphalt, and dark paint — and “fibrous” transducers, like hair, dark clothing, pine needles, and dry leaves. When the air surrounding these transducers is rapidly heated from the meteor’s pulse of light, pressure waves are generated and are converted to sound.
“For fireballs, the sound pressure waves track the time history of the illumination, and the amplitude depends on the irradiance,” the team writes. “Also important to the generation of sound are the thermal conductivity, specific heat, and density of both the dielectric solid and the air as well as the light penetration depth into the solid.”
The researchers came to this conclusion by examining photographs of different fireballs and plotting meteor light intensity against the sensitivity of the human ear. Normally, a person’s hearing is most sensitive above a few hundred Hertz, but the light signal from the meteor was maximized way below 100 Hertz — and people still reported that they heard sounds when these meteors streaked by. “Despite this mismatch, photoacoustic sound from fireballs is occasionally heard,” they write. The researchers then tested different transducer materials to see whether their properties would affect the intensity of the sound emitted.
One of the special transducers was hair, which they determined by experimenting with a fake wig. Frizzy hair is an especially good transducer because it has a large surface-to-volume ratio and covers the ears, which localizes sound pressure.
The Sandia researchers named their theory the “photoacoustic hypothesis,” and they say that this is the first time an experiment has explained how a meteor’s light energy is converted to audible sound. An observant person in a quiet environment has a good chance of hearing a meteor’s light as it blazes in the sky, they reason — especially if that person has frizzy hair.