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The hum is a low rumble continually present in the ground even when there are no earthquakes happening, but is detectable only by very sensitive seismometers. Its frequency is near 10 millihertz, below the range of human hearing.
Webb has applied old work on ocean waves to predict what sort of background noise would be made by waves moving over the shallow ocean floor. He found his prediction closely matched the spectrum of the Earth's hum.
He says the hum is caused by the combination of two waves of the same frequency travelling in opposite directions. The waves alternately cancel out and amplify each other so that the sea surface goes from wavy to flat to wavy. This creates a standing wave that "goes thump, thump, thump on the ocean floor at twice the frequency of the waves you started off with, driving the hum", says Webb.