Concurrent sound associated with very bright meteors manifests itself as popping, hissing, and faint rustling sounds occurring simultaneously with the arrival of the light from the meteor1,2,3,4,5,6,7. Concurrent sound occasionally is generated by fireballs8 with apparent magnitude (visual brightness) as low8,9 as −9, and numerous occurrences have been documented1,2 with apparent magnitudes of −11 to −13. These sounds cannot be attributed to direct acoustic propagation from the upper atmosphere for which the travel time would be several minutes. Concurrent sounds 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. Prior to now, the means by which energy from meteors could be propagated to Earth and then converted into audible sound has not been adequately explained and validated by experiment. Here we present observational data, experimental results, and numerical models in support of photoacoustic coupling as the mechanism. Recent photometric measurements of fireballs reveal strong millisecond flares and significant brightness oscillations at frequencies of 40 Hz and higher7,8. Experiments and models show that strongly modulated light at these frequencies and light intensity on Earth from −12 apparent magnitude meteors (same as full moon illumination ~10−3 W/m2) can radiatively heat common dielectric materials like hair, cloth, paint, etc. This heating can produce small pressure oscillations in the air adjacent to the absorber. These can be loud enough to be audible (~25 dB SPL). A previous hypothesis of coupling to natural antennas from RF radiation generated by plasma oscillations1,2 does not seem to be adequately supported by observational evidence of radio waves emanating from meteors12,13,14,15. The photoacoustic hypothesis seems to better explain this longstanding mystery about the generation of concurrent sounds by fireballs. However, it is possible that both mechanisms contribute to the observed audio signal.

Strong, millisecond-duration flares have been recorded in nearly all bolides observed by the Czech Fireball Network10,11. The meteors of interest typically have initial speeds below 40 km/s and burn durations longer than 2 s. These optical pulse trains, if converted to sound, often have time characteristics consistent with the popping, swishing, or sizzling noises reported by observers1,2,3. We suggest that each pulse of light can heat the surfaces of natural dielectric transducers. The surfaces rapidly warm and conduct heat into the nearby air, generating pressure waves. A succession of light-pulse-produced pressure waves can then manifest as sound to a nearby observer.

The photoacoustic effect was observed in 1880 by Alexander Graham Bell and colleagues who heard a tone when they illuminated certain dielectric materials with sunlight modulated with a chopper wheel16. In 1976 Rosencwaig & Gersho invented Photo-Acoustic Spectroscopy17 (PAS) and provided the first detailed understanding of the physics.

For fireballs, the sound pressure waves track the time history of the illumination, and the amplitude depend on the irradiance. 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.

Figure 1a is an open-shutter photograph of fireball EN091214 taken December 9, 2014. Figure 1b is its intensity-time history as recorded by the Czech Fireball Network18. The fireball’s average apparent magnitude was reported as -15, about ten times brighter than the full moon. Concurrent sounds from this early-evening fireball were heard by people in several nearby locations. Figure 1c shows the Fourier transform of the light intensity, along with the normalized sensitivity of the human ear. We plot these curves together to show that the observer’s hearing is most sensitive above a few hundred Hertz while the signal from the fireball light is maximized below 100 Hz. Despite this mismatch, photoacoustic sound from fireballs is occasionally heard.