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The Sound of Earth

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Mother Nature is always speaking; she speaks in a language only the sincere listener can understand. Actually, she is not only speaking, she is singing also. In some places on Earth, the sand “sings” as it falls down dunes, making a low sound that lies within the bottom half of a cello’s musical range. Those eerie sounds have frightened travelers for millennia.

Sand dunes sing when the sand slides down their sides; people can set the sand in motion themselves. Sometimes the wind can create sand avalanches, creating a sudden, booming chorus. Scientists previously thought the sound happened because avalanching sand created vibrations in the more stable under-layers of the dunes. However, in 2009, University of France researchers found that the avalanche of sand itself sings, not the dunes.

To study this phenomenon, physicist Simon Dagois–Bohy and his fellow researchers at Paris Diderot University in France, recorded two different dunes: one near Tarfaya, a port town in southwestern Morocco; and one near Al-Askharah, a coastal town in southeastern Oman. In Morocco, the sands consistently produced a note at 105 hertz. The Omani sands also sang, but sometimes belted out a cacophony of almost every possible frequency 90–150 hertz.

Although the Omani dunes are somewhat sloppy singers, the researchers identified some tones that were slightly stronger than others. The scientists also observed that sand grains from the Omani dune came in a much wider range of sizes than their Moroccan counterparts. The Omani dune grains were 150–310 microns, while the Moroccan dune grains were only 150–170 microns. As a result, Dagois-Bohy and his colleagues brought grains from the Omani dune back to the lab.

They first ran the mix of Omani sands down a constructed incline, recording its sound and measuring the sand vibrations. They then used a sieve to isolate the sand grains between 200–250 microns, and ran that sand down the same slope. They compared the sound of the isolated sands with the sound of the mixed-size control and found that, while the grains of a broad size range sang noisily, the sands of a narrow size range sang a clear note at about 90 hertz, much like the Moroccan sands do naturally. This suggested that grain size is an important factor in what tone the dunes produce.

The research team suggests that the grain size affects the purity of tones generated by the dunes; when grain size varies, the streams of sand flow at varied speeds, producing a wider range of notes. When the grains of sand are all of the same size, the streams of sand within the avalanche move at more consistent speeds, causing the sound to narrow in on specific tones. However, scientists still do not know how the erratic motion of flowing grains translates into sounds coherent enough to resemble musical notes.

“The study attempts, and I think succeeds in many ways, to solve the problem of what the mechanism is” that translates tumbling sand into a song, said Tom Patitsas, a theoretical physicist at Laurentian University in Sudbury, Ontario, who did not participate in the study. Patitsas said the theory behind the sound still requires more elaboration to explain why, for example, the flowing sand still needs a thin layer of stationary sand underneath it to make a sound. He suggests the sliding sand resonates with similar-sized grains beneath the avalanche. Those buried grains may lie in chain-like patterns that intensify the resonance. “Once you have this resonance, the amplitude of the vibration will be large,” Patitsas said.

In addition to the eerie sound of sand, the sky sometimes emits strange sounds too! Since 2008, or maybe earlier, people all around the world have heard an extremely loud sound coming from the sky; the spooky sound is like someone blowing a trumpet. Dozens of people from across the globe have posted videos of these bizarre sounds, the origin of which is not yet fully understood.

As it is the nature of ordinary people, many chose to assume fantastic reasons, ranging from alien lifeforms to end of the world. However, scientists have analyzed records of these sounds and found that most of their spectrum lies within the infrasound range; what people hear is only a small fraction of the actual power of these sounds. In geophysics, they are called acoustic-gravity waves; they are formed in the upper atmosphere, at the atmosphere-ionosphere boundary in particular.

According to scientists, the source of such powerful and immense acoustic-gravity waves must be very large-scale energy processes. Such processes include powerful solar flares and huge energy flows generated by them, rushing towards Earth’s surface and destabilizing the magnetosphere, ionosphere, and upper atmosphere.

Given the surge in solar activity as manifested in the higher number and energy of solar flares since mid-2011, we can assume that there is a high probability of impact of the substantial increase in solar activity on the generation of the unusual humming coming from the sky. The observed increase in solar activity is fully consistent with the forecast of the International Committee GEOCHANGE published in the Committee’s Report in June 2010.

Another possible cause of these sounds may lie at the Earth’s core. The acceleration of the drift of the Earth’s North Magnetic Pole, which increased more than fivefold between 1998 and 2003, and is at the same level today, points to intensification of energy processes in the Earth’s core, since it is processed in the inner and outer core that form the Earth’s geomagnetic field.

Intensification of the energy processes in the Earth’s core can modulate the geomagnetic field, which, through a chain of physical processes at the ionosphere, generates acoustic-gravity waves the audible range of which has been heard by people in the form of a frightening low-frequency sound in different parts of our planet.

The more we observe nature, the more it interests us with its phenomena. The next time you hear those bizarre sounds, do not panic, it is the Earth amusing its inhabitants by its natural orchestra.

References
bookofresearch.com
dailymail.co.uk
livescience.com
sciencedaily.com
smithsonianmag.com


*Published in SCIplanetEarth Sciences (Spring 2017)

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