For years, people around the world have reported being disturbed by a low, persistent droning sound of unknown origin.
Often described as more of a vibration than a specific tone, this sound has become widely known as “The Hum,” a phenomenon that is more frequently reported in certain cities and specific areas than others.
Over time, possible explanations have included industrial ventilation systems, military sonar experiments, and, in some online discussions, extraterrestrial signals, although none of these theories are supported by evidence.
However, in a recent study from the Norwegian University of Science and Technology (NTNU), published in PLOS ONE, researchers now offer evidence that, for most people who experience this phenomenon, the source is probably not external, but instead may actually originate within their own auditory system.
From Bristol to Taos and Beyond
Residents of Bristol, England, first widely reported The Hum in the mid-1970s, when the Bristol Evening Post received several reports from people who claimed they experienced hearing a strange, low sound. People initially thought the industrial fans in an area warehouse were responsible, but the hum persisted after the warehouse shut down.
By the 1990s, reports of a similar phenomenon began surfacing in Taos, New Mexico, where residents described a persistent, diesel engine-like rumble. Both state and federal authorities investigated the sound but could not explain it. The Taos Hum became one of the most studied and widely recognized examples of this phenomenon. Since then, similar reports have come from Canada, Australia, New Zealand, South Africa, and several European countries.
As reports grew, people proposed explanations ranging from ocean-generated infrasound to military sonar and CIA transmitters. Some online discussions have even suggested alien technology as a possible cause. In 2012, Canadian teacher Glen MacPherson, who first experienced The Hum on the west coast of British Columbia, created the World Hum Map and Database Project to document cases worldwide. The project has since collected thousands of entries.
Testing Two Hypotheses
To investigate the phenomenon, Professor Markus Drexl at NTNU recruited 28 people in Germany who all reported hearing a persistent, unexplained humming sound. Clinicians refer to this as a low-frequency sound percept, or LFSP. Most participants described the sound as having a frequency around 50 Hz, about the same as a deep mechanical rumble.
The research team tested two main explanations: whether participants had unusually acute sensitivity to low-frequency sound, or whether some were perceiving sounds generated by their own inner ears.
“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 said. “But it’s not so easy to find the source of these sound waves, because it’s a struggle to localize low-frequency sounds.”
What the Ear Produces on Its Own
The cochlea, a spiral-shaped structure in the inner ear that converts sound waves into neural signals, also actively amplifies incoming sounds. This amplification produces faint sounds emitted from the ear, known as spontaneous otoacoustic emissions (SOAEs).
Most people are not aware of these emissions. In rare cases, people perceive SOAEs as tinnitus. Drexl’s team investigated whether people who hear the Hum might be detecting a low-frequency version of this phenomenon.
The results did not support this idea. The team found no low-frequency SOAEs in any participant. The hypothesis of unusually acute low-frequency hearing was also not supported; while a few participants had slightly better low-frequency hearing than average, most did not.
“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.
When the Sound Has No Source
This left a third group: participants who reported hearing something that no external instrument could detect. “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.
The Hum differs from typical tinnitus because most people who experience it first believe the sound comes from an external source, sometimes spending years searching for its origin before realizing it is internal. The researchers still have not ruled out external causes completely. In some cases, near industrial sites or coastlines, genuine low-frequency sources may exist. However, researchers identified no external source for most participants in the study.
“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.
The Blind Spot in Auditory Research
The study also highlights that the low-frequency range of the auditory system remains poorly understood in hearing science.
“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.
Although the cause of well-known cases like the Taos Hum remains unresolved, the new findings suggest that many reports of unexplained low-frequency sounds may actually originate from within the auditory system rather than from an external source.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.