In our most recent Darklore release (Volume 9), I wrote about the history of the scientific anomaly/controversy known as “electrophonic meteors” (my full article, “Rocks In Your Head”, is available at the Darklore website). In short, throughout history, people have reported hearing meteors at the same time as seeing them – despite this appearing to be impossible, given any sound originating from meteors should be delayed by a quite long period as they are generally many miles distant.
For a couple of centuries, respected astronomers rubbished such reports, but in recent decades the phenomenon has become more accepted, with some scientists suggesting that the sounds were caused by radio frequency emissions, possibly from the plasma of the meteor’s fireball. Now, a new experiment – published in Nature, no less – has suggested that the mechanism creating the sounds is photoacoustic coupling:
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 observers1–3. 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.
…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.
…[T]he most efficient light-to-sound transducer materials have high absorption coefficients, so the light is absorbed near the surface. They also have low thermal inertia characterized by low conductivity, which minimizes heat flow, and low volumetric heat capacity, which maximizes the temperature rise. This combination of properties is found in most dark-colored dielectric materials. Likely candidates for producing photoacoustic sound are dark paint, fine hair, leaves, grass, and dark clothing – all of which we tested.
Our test setup consisted of a 10 cm square white-light LED array producing a peak flux of E=5W/m2 on the test sample, the sample, and a scientific grade laboratory microphone. The setup was placed inside a plastic dome located in an anechoic chamber. Outside, we located a signal generator and linear amplifier to drive the LEDs and a spectrum analyzer to record the signal from the microphone.
Their testing was successful in producing sounds via photo-acoustic coupling (see their recording of the song “Greensleeves” being transmitted in this way), leading the researchers to conclude that their “calculations and experiments are consistent with how observers have described the concurrent sounds
associated with fireballs”.
Further reading: Rocks In Your Head – The Strange History of a Scientific Anomaly