Strange as it may seem, the Earth’s atmosphere rings out in a chorus of frequencies just below the reach of the human ear. Although we cannot hear these “infrasonic” waves — which have frequencies ranging from 0.01–10 Hz — we know they exist from acoustic recordings around the globe. About ten years ago, however, researchers discovered another type of infrasonic background noise of 3-7 mHz believed to originate from the solid Earth itself. Scientists have struggled to pinpoint the exact origin and mechanism of this intriguing phenomenon, known as the Earth’s "hum". Now, a pair of physicists at the University of California claim to have the most convincing physical evidence yet. Using data from a wide-spread array of seismic recorders, Peter Bromirski and Peter Gerstoft have located the Pacific coast of Central America as the dominate source of background noise, with the western coast of Europe acting as the main secondary supply.
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