What causes the Hum?
Most researchers investigating the Hum express some confidence that the phenomenon is real, and not the result of mass hysteria or hearers' hypochondria (or extraterrestrials beaming signals to Earth from their spaceships).
As in the case of the Kokomo Hum, industrial equipment is usually the first suspected source of the Hum. In one instance, Leventhall was able to trace the noise to a neighboring building's central heating unit.
Other suspected sources include high-pressure gas lines, electrical power lines, wireless communication devices or other sources. But only in a few cases has a Hum been linked to a mechanical or electrical source.
There's some speculation that the Hum could be the result of low-frequency electromagnetic radiation, audible only to some people. And there are verified cases in which individuals have particular sensitivities to signals outside the normal range of human hearing.
Medical experts are quick to point out that tinnitus (the perception of sound when no external noise is present) is a likely cause, but repeated testing has found that many hearers have normal hearing and no occurrences of tinnitus.
Environmental factors have also been blamed, including seismic activity such as microseisms — very faint, low-frequency earth tremors that can be generated by the action of ocean waves.
Other hypotheses, including military experiments and submarine communications, have yet to bear any fruit. For now, hearers of the Hum have to resort to white-noise machines and other devices to reduce or eliminate the annoying noise.
Leventhall, who recommends that some hearers turn to cognitive-behavioral therapy to relieve the symptoms caused by the Hum, isn't confident that the puzzle will be solved anytime soon.
"It's been a mystery for 40 years, so it may well remain one for a lot longer," Leventhall told the BBC.
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