Scientists finally think they know the cause of mysterious 'hum' sound heard by people across the globe
For decades, people all over the world have reported hearing a strange, low background tone that nobody else around them seems to notice.
The phenomenon, known simply as 'The Hum', regularly crops up on lists of the world's most baffling unsolved mysteries and has even inspired documentary investigations trying to get to the bottom of it once and for all.
Over the years, theories have ranged from industrial noise pollution to some people having unusually sensitive hearing.
But a new scientific study looking into some of the leading explanations suggests the true source might actually be a lot closer to home than anyone realised, at least for some of the people affected.
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According to a team led by auditory scientist Bonifaz Baumann of the German Center for Vertigo and Balance Disorders (DSGZ), some reports of The Hum could actually be a form of under recognised low-frequency tinnitus.
That doesn't mean every single case shares the same cause, but the findings suggest the source may often be found inside the auditory system rather than out in the world.
"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," said neuroscientist Markus Drexl of DSGZ and the Norwegian University of Science and Technology.
Where did the mysterious 'hum' first come from?
The Hum first entered public discourse back in the 1970s, when residents of Bristol in the UK began complaining about a persistent low-frequency sound in the background of their hearing, sitting at around 50 Hertz.
In the decades since, similar reports have surfaced from all over the world, including Australia, New Zealand and various locations across North America.
People typically describe it as a persistent hum, drone or rumbling sound, one that often comes and goes depending on location, appearing in a bedroom at night but vanishing entirely on a train the next morning.
Despite huge efforts to track down the source, the phenomenon has remained frustratingly difficult to pin down. Some investigations have found no obvious cause whatsoever, while others have identified potential sounds that ultimately failed to fully explain what people were actually experiencing.
"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 explained.
"But it's not so easy to find the source of these sound waves, because it's a struggle to localise low-frequency sounds."
Rather than continuing the hunt for an external source, the researchers instead turned their attention to the people experiencing the sound itself.
They recruited 28 volunteers through a social media campaign, all of whom reported hearing unexplained low-frequency noises, and ran a series of tests to check whether two major theories actually held up.
What did the study find about people who hear the sound?
The first theory being tested was that people who hear The Hum simply have unusually sensitive low-frequency hearing.
The second was that they might actually be hearing sounds produced inside their own bodies, specifically otoacoustic emissions from the tiny hairs inside the cochlea, which are a normal by-product of how the inner ear functions but are usually far too faint to notice.
The first test was a standard hearing exam focused on lower frequency ranges. Most participants had fairly average low-frequency hearing sensitivity, aside from two volunteers, suggesting that exceptionally good hearing doesn't explain most cases.
"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," said Drexl.
Researchers then measured participants' otoacoustic emissions using tiny microphones lowered into the ear canal, a common tool for assessing overall aural health.
Once again, nothing unusual turned up in the recordings, suggesting that for at least some people, The Hum may genuinely be a subjective experience rather than something with a measurable external source.
That points towards tinnitus, a condition involving persistent, subjective perceptions of sound with no identifiable external cause. While tinnitus is most commonly associated with a high-pitched whine, it can occasionally present at much lower frequencies too, and the researchers believe this may be exactly what's happening in some cases of The Hum.
"While not directly tested in this study, low-frequency tinnitus might serve as a good explanation for many, but not all, cases," the researchers concluded.
Crucially, recategorising some cases of The Hum as tinnitus doesn't mean the experience isn't real. Tinnitus is a genuine and well-documented condition, often linked to the brain's auditory pathways and processing centres.
Scientists still don't fully understand what causes it and there's currently no outright cure, but it remains an active area of ongoing research, with established coping strategies and interventions already available to help people manage it.
For those who experience The Hum, the sound is no less real simply because its source may be internal. But if this research holds up, recognising it as a form of tinnitus could finally open the door to treatments and coping strategies that have been largely overlooked until now.
The research has been published in the journal PLOS One.
Thomas Bamford