Audible Meteors
Meteors are spectacular celestial events in all aspects. They tear through our atmosphere at high velocities, burning off layers that have formed over the centuries. They might not be big, but they certainly are very potent.
However, large meteoroids might damage upon entering the atmosphere. The bigger the rock, the bigger the impact. Consequently, that impact might even produce significant sounds.
Scientists think that every meteoroid that enters the atmosphere produces sound. Even amateurs agree – it is a logical conclusion if we take into account other similar examples, i.e. lightning. The sound boom or a major crack in the air is generally a natural part of the atmosphere penetration. However, there is more to this than meets the eye, or ear, if you will.
Fireballs
Large meteoroids that burn brighter than -4 magnitude are fireballs. Sometimes, people confuse fireballs with bolides, but the seemingly different concepts refer to the same thing (in most cases). To shine so brightly, the nucleus must be larger than a grain. Therefore, fireballs, as bodies of greater size, most certainly produce effective sounds.
The earliest fireball record from 1676 states that the body was followed by “cannon-like, rattling” sounds and a “hissing” noise. First of all, fireballs must fulfill certain conditions to be heard and seen at the same time. The fireball must enter the atmosphere low enough and have a duration so that both visual and audio experiences can simultaneously act.
It is not strange to experience an inversion of sound and view. Even if you would expect to hear a sound at the same time as you spot it. The atmosphere and terrain refract and reflect sound so your location (as the observer) is important. Therefore, the conditions are as follows: low angle entrance, at least 10 cm diameter, deep plunge in the air, and asteroidal type. Cometary material is often fragile, so it quickly vaporizes in the upper layers of the atmosphere. The conditions that we just listed often accompany a magnitude of -8.
Searching for Explanations
However, it is important to note that in most cases, not all conditions are fulfilled. Nevertheless, the sound was present. But, here is the trick – in most cases, the sound waves reached the observers after several minutes, while they were able to spot the object instantly.
The explanation for this lies in the electromagnetic waves. Havey Nininger was the first one to suggest that meteors and fireballs produce electromagnetic waves in 1934. The idea was completely dismissed until Keay started investigating the phenomenon in 1980. Keay proposed that plasma trails of big fireballs can generate ELF and VLF radio emissions in the range of 1 to 10 kHz. Even though his fellow scientists accepted the proposal several years later, one problem remained. Humans cannot register electrophonic sounds.
But, other objects can convert ELF and VLF emissions into sounds that we can hear. Cars, buildings, and similar objects can produce pressure oscillations after radioactive heating. In other words, they produce audible sounds. The sound that we hear is barely more than a whisper, though. Maybe that is why some observers report sounds, and others do not.
The theory of photoacoustic effects (produced by radiative heating of materials) was first tested in 1880 by Alexander Graham Bell. However, this is the first time we have combined photoacoustic effects with meteor and fireball observations. The study that connected the two notions was published in Nature: Scientific Reports by Sandia National Laboratories. The team of scientists that worked on this phenomenon provided elaborate evidence to test and prove their theory. Among other sources, they included the Perseid observation documents provided by Czech Fireball Network. The European Fireball Network is one of the several organizations that closely monitors and observes fireballs and meteors in Europe.
Learn More
If you want to explore the fireball phenomenon further, you can visit the American Meteor Society which has a special section dedicated primarily to fireballs. Another amazing source is the International Meteor Organization (IMO) which has an excellent program for fireball observations. If you want to explore the latest fireball events, you can even browse their fireball reports page that features all fireball sightings for 2019.