A mysterious low-frequency sound known as The Hum has been reported around the world for decades, leaving researchers searching for its source.
Imagine lying in bed late at night and hearing a faint engine-like drone that seems to come from somewhere outside. You check the windows, listen for traffic, and search for a nearby machine. Nothing. Stranger still, the person next to you hears absolutely nothing.
For decades, people around the world have reported this unsettling experience. Known simply as The Hum, the phenomenon has sparked countless theories, scientific investigations, and even conspiracy claims. Now, researchers say the answer may lie not in the environment, but inside the human hearing system itself.
A Mystery That Refuses to Go Away
Reports of The Hum first gained widespread attention in Bristol, England, during the 1970s, when local newspapers were flooded with letters from residents describing a persistent low-frequency noise.
Although reports come from around the world, The Hum remains surprisingly rare. Surveys suggest only a small percentage of people can hear it, which may explain why the phenomenon often leaves those affected feeling isolated or dismissed.
The sound is typically described as a distant diesel engine, an idling truck, industrial machinery, or a low electrical buzz.
Searching for an Invisible Source
Some suspected industrial equipment, ventilation systems, traffic, electrical infrastructure, or wind turbines. Others pointed to natural sources such as ocean waves, atmospheric conditions, or vibrations traveling through the ground.
The challenge is that low-frequency sounds behave differently from higher-pitched noises. Their long wavelengths allow them to travel great distances, bend around obstacles, and become difficult to pinpoint.
"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," said Markus Drexl, a professor at the Norwegian University of Science and Technology.
Yet despite decades of investigation, no single external source has ever explained all reports of The Hum.
Testing the People Who Hear It
To better understand the phenomenon, Drexl and colleagues studied 28 people in Germany who regularly experienced an unexplained humming or buzzing sound.
The researchers first explored a straightforward possibility: perhaps people who hear The Hum simply possess unusually sensitive hearing at low frequencies.
The results offered only limited support for that idea. Most participants showed normal hearing abilities. Only two demonstrated better-than-average sensitivity at certain low frequencies.
"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 said.
Could the Ear Be Creating the Sound?
The human ear is not completely silent. Deep within the inner ear, the cochlea can generate faint sounds known as otoacoustic emissions. These tiny sounds are a normal byproduct of how the ear amplifies incoming audio signals.
"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 explained.
Because spontaneous otoacoustic emissions can sometimes be perceived as tinnitus, the team wondered whether they might account for reports of The Hum.
Testing found no evidence that this was the primary explanation for the participants in the study.
The Strongest Explanation May Be Tinnitus
"Then there are people who hear something that cannot be measured objectively. We believe people in this category have a form of low-frequency tinnitus," Drexl said.
Most people associate tinnitus with a high-pitched ringing sound, but tinnitus can take many forms. Some people hear buzzing, roaring, clicking, hissing, or low-frequency humming.
Tinnitus is not a disease itself. It is a perception of sound generated somewhere within the auditory system without an external source.
This may explain one of the most puzzling aspects of The Hum. Many people initially believe the sound originates in their surroundings. Only after hearing it repeatedly in different locations do they begin to suspect the source may be internal.
Based on their findings, the researchers propose that The Hum likely does not have a single explanation.
"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," Drexl said.
Why Scientists Are Still Interested
"What we know about the hearing system is mainly based on how we capture and process sound with higher frequencies. We know less about how the auditory system handles and processes low-frequency sound, or infrasound," Drexl said.