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Scientists believe they may finally understand the mysterious low-frequency sound known as 'The Hum', more than 50 years after it first began troubling people around the world.
The Hum first surfaced in the 1970s, when the Bristol Evening Post was flooded with letters from locals troubled by a strange, unexplained noise.
Coastal towns like Southampton, Swansea and Plymouth started reporting it, then London, before cases surfaced across the US in the 1990s. Since then, people in Canada, Australia, New Zealand, South Africa and Norway have all described the same low buzzing, and one American town became so associated with the phenomenon it was branded 'haunted'.
Over the years the blame has been pinned on everything from technology and traffic to natural sounds like wind and waves, and even alien activity.
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The breakthrough came from Markus Drexl, a professor at the Norwegian University of Science and Technology, whose team studied 28 people in Germany who live with the noise. As reported by Science Daily, the researchers tested two possibilities, either that hearers have exceptional low-frequency perception, or that measurable sounds are being produced inside their own ears.
"We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don't hear them," Drexl said.
"But it's not so easy to find the source of these sound waves, because it's a struggle to localise low-frequency sounds."
Weighing up both ideas, the team revealed the phenomenon to be low-frequency tinnitus, though a small number may simply have unusually sensitive hearing.
"Based on our results, although we haven't ruled out cases of physical external sound sources, we suggest that subjective tinnitus in the low-frequency range is often the cause of hearing pulsations of low-frequency sound perceptions," added Drexl.
A tiny portion of The Hum's hearers may boast unusually sensitive low-frequency hearing, though.
Tinnitus is a condition in which someone hears noise in one or both ears, or the head, with no external source. Around two to four percent of the global population experiences it, the study noted.
"For some, the noise is irritating but manageable," the study read. "For others, the low-frequency sound can cause physical discomfort or illness and may also feel like a vibration moving through the body."
It can present as a faint ringing or hum, or as hissing, whistling and roaring.
But the answer is not yet definitive, the researchers suggested.
"Even though the group we tested was small, it still means that the hypothesis of having especially good hearing for low-frequency sounds does not hold for most people," Drexl concluded.
