For many years, individuals worldwide have reported detecting a peculiar, low-frequency noise that others nearby do not seem to hear.
This intriguing occurrence is often referred to as ‘The Hum’ and frequently appears on lists highlighting the world’s most puzzling unsolved mysteries. It has even spurred documentary efforts aimed at uncovering its origins.
Throughout the years, numerous theories have been proposed, ranging from industrial noise pollution to the possibility that some individuals possess exceptionally sensitive hearing capabilities.
However, recent scientific investigations into leading hypotheses suggest that for some individuals, the actual source might be more personal than previously thought.
Research conducted by auditory scientist Bonifaz Baumann and his team at the German Center for Vertigo and Balance Disorders (DSGZ) indicates that some instances of The Hum may be a form of low-frequency tinnitus that has been underrecognized.
This does not imply that every case has the same origin, but the research suggests the cause might often reside within the auditory system rather than originating externally.
“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.
The Hum was first brought into public awareness during the 1970s when individuals in Bristol, UK, complained about a continuous low-frequency sound at around 50 Hertz.
In the years since, similar accounts have emerged from various global locations, including Australia, New Zealand, and multiple North American regions.
People typically describe the sound as a persistent hum, drone, or rumbling noise, which sometimes varies with location—present in a bedroom at night but disappearing on a train the next morning.
Despite significant efforts to identify the source, the phenomenon remains elusive. Some investigations have failed to find a clear cause, while others have pointed to potential sources that do not fully account for people’s experiences.
“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.”
Instead of continuing to search for an external source, researchers shifted focus to the individuals reporting the sound.
They engaged 28 volunteers through a social media initiative, all of whom reported unexplained low-frequency noise, conducting tests to evaluate two primary theories.
The first theory examined was whether those who hear The Hum possess exceptionally sensitive low-frequency hearing.
The second theory proposed that individuals might hear sounds generated internally, specifically otoacoustic emissions from cochlear hair cells—a normal auditory by-product too faint for most to notice.
A standard hearing test concentrating on lower frequencies revealed most participants had average hearing sensitivity, with only two exceptions, suggesting that acute low-frequency hearing is not the primary factor.
“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.
Subsequent tests measuring participants’ otoacoustic emissions using specialized microphones found no unusual results, indicating that for some, The Hum may indeed be a subjective experience rather than a measurable external sound.
This points towards tinnitus, a condition involving persistent, subjective sound perceptions without an identifiable external source. While commonly associated with a high-pitched ringing, tinnitus can also manifest at lower frequencies, which researchers believe could explain some instances 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.
Reclassifying some instances of The Hum as tinnitus does not negate the reality of the experience. Tinnitus is a legitimate and well-documented condition, often linked to the brain’s auditory pathways and processing centers.
While the exact causes of tinnitus remain elusive and no definitive cure exists, it is a subject of ongoing research, with already established strategies available to help manage it.
For those affected by The Hum, acknowledging it as a form of tinnitus could provide access to previously overlooked treatments and coping strategies.
The research has been published in the journal PLOS One.