Do you ever hear a low, persistent buzzing noise that nobody else seems to notice? Congratulations - you're part of an exclusive club that includes an estimated 2 to 4 percent of the world's population. For decades, researchers have been trying to figure out what causes this phenomenon, known simply as "The Hum."
For some, The Hum is a mild annoyance. For others, it's a full-body experience that can cause physical discomfort or illness, like a vibrating ghost that only you can feel. The sound is most noticeable indoors, especially at night when you're trying to sleep. You might look outside for a truck or a machine, but no - it's just the void humming back at you. And to make it weirder, other people in the same room might hear nothing at all.
The Hum first made headlines in Bristol, England, in the mid-1970s, when the Bristol Evening Post received a flood of letters from residents asking, "What is that noise?" One theory pointed to industrial fans in a department store warehouse, but the reports continued even after the warehouse closed. Because of course they did. Similar reports popped up in coastal UK towns like Hythe, Plymouth, Southampton, and Swansea, as well as London. In the 1990s, the phenomenon crossed the Atlantic, showing up in Taos, New Mexico, and Kokomo, Indiana. Since then, it has been documented in Canada, Australia, New Zealand, South Africa, and several European cities - mostly in densely populated areas, because even mysteries prefer company.
Canadian teacher Glen MacPherson first heard The Hum while living on the west coast of Canada. After moving to another city in the same region, it disappeared. His curiosity led him to launch The World Hum Map and Database Project in 2012, where people can report their own hum experiences. Because if you can't beat the hum, map it.
Proposed explanations for The Hum range from the plausible (acoustic pollution from heat pumps, road traffic, or windmills) to the fun (CIA mind games, aliens). Nature also contributes low-frequency sounds from ocean waves and wind. But a new study from the Norwegian University of Science and Technology (NTNU) suggests the answer may be less exciting - and more inside your head.
Led by Professor Markus Drexl, the researchers studied 28 people in Germany who reported hearing an unexplained hum. They tested two main hypotheses: first, that The Hum is an external sound that can be measured, and second, that it's generated internally. Low-frequency sounds have long wavelengths, making them hard to trace - so even if it's external, good luck finding the source.
Most participants did not have exceptional hearing for low frequencies. Only two showed better-than-average sensitivity at certain 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. However, he noted that standard hearing tests might miss tiny variations in hearing thresholds - like being able to hear a very specific 50-51 Hz hum that no one else can.
The second hypothesis involved oto-acoustic emissions: faint sounds naturally produced by the cochlea in the inner ear, usually between 500 and 5000 Hertz. Most people don't hear these, but some can - and when they do, it may be perceived as distressing tinnitus. The researchers tested for these emissions but found they didn't explain the low-frequency hum for most participants.
That leaves low-frequency 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. Tinnitus is the perception of sound with no external source - like a phantom ringtone that never stops. People often initially think the sound is coming from their surroundings, but when it persists in different locations, they realize it's all in their head. Literally.
Based on the study, Drexl believes the most likely explanation has two parts: a small number of people genuinely have unusually sensitive low-frequency hearing, but for most, The Hum is a type of tinnitus originating within the auditory system. "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," he said.
Drexl became interested in The Hum through his research on low-frequency sound, noting that our understanding of hearing is biased toward higher frequencies. "We know less about how the auditory system handles and processes low-frequency sound, or infrasound," he said. With growing concern about noise from technology - especially low-frequency and infrasound - understanding how we perceive these sounds is increasingly important. "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 them," he added.
So, the next time you hear a mysterious hum, just remember: it's probably your ears playing tricks on you. Which is either a relief or deeply disappointing, depending on how much you were hoping for aliens.