Do you ever lie awake at night when the house is quiet, only to notice a low hum that seems to come from nowhere?
If so, you're not alone. Between 2 and 4% of the world's population report hearing this mysterious sound, often described as a buzzing or vibration. It's called The Hum, and for decades, scientists, engineers, and ordinary people have been trying to figure out what it is.
A new study in PLOS One suggests two possible reasons for the phenomenon: either people have an unusually sharp sensitivity to low-frequency sounds, or they are hearing faint sounds made by their own inner ear, normal by-products of how the ear amplifies sound.
The phenomenon began to attract attention in the 1970s, when readers flooded a local newspaper with letters from Bristol, England, and residents claimed they could hear an unidentifiable noise. At first, an industrial fan was blamed, but the hum remained when a possible warehouse location closed down.
Since then, reports have surfaced across the UK: Hythe, Plymouth, Southampton, Swansea, even London, and later worldwide, from Taos, New Mexico, to Kokomo, Indiana, and as far afield as Canada, Australia, South Africa, and Norway.
Some people shrug it off. Others find it unbearable, even debilitating. And here's the strange part: often, one person hears it clearly while their neighbor hears nothing at all.
Canadian teacher Glen MacPherson became so intrigued after experiencing The Hum himself that he launched the World Hum Map and Database Project in 2012, inviting people everywhere to log their experiences. The map now shows clusters of reports in densely populated areas, suggesting that human-made infrastructure may play a role.
But what does science say? Markus Drexl, professor at NTNU, has studied the phenomenon with colleagues in Germany. They tested 28 people who reported hearing unexplained humming.
"We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don't hear them. But it's not so easy to find the source of these sound waves, because it's a struggle to localize low-frequency sounds," Drexl explains. These sounds have long wavelengths that can travel great distances, making them difficult to pin down.
The team tested whether participants had unusually good low-frequency hearing. Most did not, though two showed heightened sensitivity at certain frequencies (50 Hz, for example) based on a frequency-matching test.
"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 says.
He added that subtle differences in hearing thresholds and microstructures might allow some individuals to detect narrow frequency ranges that standard tests miss.
Another hypothesis was that people were hearing oto-acoustic emission, tiny sounds produced by the inner ear itself.
"Most of us don't hear these sounds. However, a few people can actually hear the sounds that the ear itself produces. And these sounds can be measured objectively," Drexl notes. But testing ruled this out for their group.
So what remains? Drexl believes many cases may be a form of low-frequency tinnitus.
"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," he says.
That doesn't mean The Hum is constantly beaming "in your head." Certain low-frequency sounds, ventilation systems, wind turbines (where permitted), traffic, and waves are real and measurable. However, the line between subjective noise and external perception is a thin one.
Drexl emphasizes that science still knows far more about how we process higher frequencies than low ones. "If we want to conduct a thorough assessment of low-frequency sounds and infrasound, we first need a better understanding of how sensory systems process low-frequency sound and infrasound," he said.
The next time you hear that eerie hum, you're tuning into one of science's longest-lasting puzzles. Is outside a world of hidden vibrations? Or is it your own auditory system generating noise that only you can hear? The answer for now is both, and the search continues.
This research was published in PLOS One
Source: Norwegian SciTech News
Fact-checked by Mike McRae
