For more than half a century, people across the world have reported a strange low-frequency sound often described as a distant, steady rumble. A new study published in PLOS One suggests that many of these experiences may not originate in the environment at all, but could be linked to internal processes within the human auditory system.
The phenomenon first gained attention in the 1970s in Bristol, UK, where residents complained of a persistent low drone, most noticeable at night. Despite repeated reports and public concern, no clear external source was ever identified at the time.
Since then, similar accounts have appeared across Europe and North America. The Guardian reports that hundreds of cases have been logged through global databases tracking the phenomenon, even though the “Hum” remains difficult to measure reliably in controlled conditions.
A Long Trail Of Similar Reports
After the early Bristol cases, similar descriptions emerged in other parts of the United Kingdom, including Hythe, Plymouth, Southampton, and Swansea. The reports tended to follow the same pattern: a low, continuous sound that seemed to appear without warning and without an identifiable origin.
Later, the phenomenon was also reported much farther away, including in Taos, New Mexico, and Kokomo, Indiana. This geographic spread helped push the Hum from a localised curiosity into a broader scientific question, since it appeared in environments with no obvious shared physical source.
The World Hum Database Project, which has collected testimonies since 2012, continues to gather similar accounts. The descriptions are often consistent: a deep, monotonous drone that is hard to locate and sometimes more noticeable in quiet, indoor settings.
No Clear External Explanation Yet
Over the decades, researchers have explored several possible environmental causes. One early idea from the 1970s suggested that interactions between fast-moving jet streams and slower air masses could generate low-frequency vibrations. More recently, in 2015, French researchers proposed that continuous ocean wave activity might play a role.
The Guardian notes that these hypotheses face a shared problem: very low-frequency sound is notoriously difficult to isolate in real-world environments. Because of its long wavelength, it can blend into background noise and resist precise localisation, making it hard to confirm a single source.
Despite these efforts, no external explanation has consistently matched all reported cases. The phenomenon continues to appear irregularly, both in intensity and location.
Study Points to Internal Sound Activity
A recent study published in PLOS One takes a different approach by focusing on people who report hearing the sound. Researchers led by Markus Drexl at the Norwegian University of Science and Technology examined 28 participants in Germany who described low-frequency sound percepts (LFSPs).
Only two participants showed unusually strong sensitivity to low-frequency sound during testing, suggesting that heightened external hearing is unlikely to explain most cases in the group.
The researchers also investigatedotoacoustic emissions, which are faint sounds generated by the inner ear itself. While these emissions can sometimes be detected under specific conditions, the study found no consistent link between them and the participants’ reported experiences.
In comments shared alongside the study, Drexl explained that:
“We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don’t hear them,” he added in a release published by the Norwegian SciTech News that, “it’s not so easy to find the source of these sound waves, because it’s a struggle to localize low-frequency sounds.”
A Working Hypothesis of Low-Frequency Tinnitus
With external sources and otoacoustic emissions largely ruled out for the participants studied, the researchers point toward low-frequency tinnitus as a likely explanation for many cases. Unlike the more familiar high-pitched form, this version can present as a deep hum or vibration-like sensation, sometimes perceived as external despite being internally generated.
“What we know about the hearing system is mainly based on how we capture and process sound with higher frequencies,” he said. “We know less about how the auditory system handles and processes low-frequency sound, or infrasound.”
The researchers do not present the issue as fully resolved. While internal auditory mechanisms may account for a significant portion of reported cases, the long history and wide distribution of the Hum suggest that more than one explanation could still be involved.
