The World’s Most Mysterious Hum May Finally Have an Answer
For decades, people around the world have reported hearing a persistent, low-frequency humming or buzzing sound that no one else can hear and no one can explain. Known simply as “The Hum,” the phenomenon first surfaced in Bristol, England, in the mid-1970s, when dozens of residents wrote to the local newspaper describing a droning noise that seemed to come from everywhere and nowhere at once. Similar reports followed in the 1990s from Taos, New Mexico, and Kokomo, Indiana, and later from cities across Canada, Australia, and Europe.
Now, researchers at the Norwegian University of Science and Technology (NTNU) believe they have identified the most likely explanation. In a study published in PLOS One, they argue that for the vast majority of people who hear the hum, the source is not an external sound at all, it is low-frequency tinnitus.
Testing the Two Leading Hypotheses
The phenomenon affects an estimated 2 to 4 percent of the population worldwide. Over the years, researchers have proposed two main explanations: that some people have extraordinarily sensitive hearing that picks up very low-frequency sounds others cannot detect, or that the sound originates within the ear itself through a process called spontaneous otoacoustic emissions.
To test both hypotheses, NTNU professor Markus Drexl and his team recruited 28 participants in Germany who reported hearing an unexplained hum or buzz. Each participant underwent a battery of hearing tests designed to measure sensitivity to low-frequency and infrasound-range signals.
The results were decisive for the first hypothesis. Only 2 of the 28 participants showed better-than-average hearing 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.
The second hypothesis fared no better. The human cochlea naturally produces faint sounds called spontaneous otoacoustic emissions as a byproduct of its amplification mechanism. These sounds can be measured with a sensitive microphone placed in the ear canal. None of the participants had measurable low-frequency otoacoustic emissions.
The Likely Answer: Low-Frequency Tinnitus
After ruling out both external sensitivity and cochlear emissions, the researchers concluded that the most plausible explanation is low-frequency tinnitus, a form of the condition in which people perceive sound without any external acoustic source. Tinnitus is most commonly described as a ringing in the ears, but it can take many forms, including low-frequency humming, buzzing, or pulsations.
“Then there are people who hear something that cannot be measured objectively,” Drexl said. “We believe people in this category have a form of low-frequency tinnitus.”
The researchers note that the sound often seems external at first. People may search their homes for faulty electrical equipment, investigate nearby factories or ventilation systems, or complain to neighbors before realizing the sound follows them wherever they go. That persistent sense of an external source is what has made the hum so mysterious and so resistant to explanation.
A small minority of cases may still involve genuinely sensitive hearing or actual external sound sources. Low-frequency sound waves have long wavelengths, travel long distances, and are notoriously difficult to localize, making it possible that some people are hearing real environmental sounds at the threshold of human perception. But the NTNU team’s data suggests these cases are the exception, not the rule.
A Gap in Scientific Knowledge
One reason the mystery has persisted so long is that scientists know far more about high-frequency hearing than they do about how the auditory system processes low-frequency sound and infrasound. Standard hearing tests are not designed to capture the full range of low-frequency perception, and they may miss subtle differences in sensitivity within narrow frequency bands, for example, the ability to detect a tone at 50 Hz but not at 51 Hz.
“If we want to conduct a thorough assessment of low-frequency sounds and infrasound, we first need a better understanding of how sensory systems process low-frequency sound and infrasound,” Drexl said.
The researchers acknowledge the limitations of their study. The sample size of 28 participants is small, and the geographic scope is limited to Germany. But the findings represent the first systematic experimental test of the leading hypotheses about the hum, and they point toward a clear direction for future research.
For the millions of people who hear a sound no one else can, the answer may be frustratingly anticlimactic: it is not the world humming, but the brain.
Sources
[1] Norwegian University of Science and Technology. “The mysterious hum heard around the world may finally have an explanation.” ScienceDaily, 19 July 2026. https://www.sciencedaily.com/releases/2026/07/260719040002.htm
[2] Baumann, Voss, Jurado, Drexl. “Low-frequency tinnitus as a cause of unexplained humming perceptions.” PLOS One 21(3): e0326818 (2026). DOI: 10.1371/journal.pone.0326818