Do you sometimes notice a deep buzzing or humming noise that seems to have no identifiable source? If so, you may be among the estimated 2-4 percent of people worldwide who experience this unusual sound. Researchers have spent decades trying to determine where it originates.
For some, the noise is irritating but manageable. For others, the low-frequency sound can cause physical discomfort or illness and may also feel like a vibration moving through the body.
The hum can be difficult to detect outdoors. It is more commonly noticed inside buildings, especially at night when people are trying to sleep. Someone may look outside for a nearby engine or machine, only to find nothing that could explain the noise.
Even more puzzling, other people in the same room may hear nothing at all.
Reports of The Hum Begin in England
The phenomenon first gained widespread attention in Bristol, England, during the mid-1970s. The Bristol Evening Post began receiving a stream of letters from residents who described hearing an unexplained sound and wanted to know what was producing it.
One proposed explanation pointed to large industrial fans operating inside a department store warehouse. Yet the reports continued even after the warehouse closed several years later.
Similar accounts later emerged elsewhere in the United Kingdom, particularly in coastal communities including Hythe, Plymouth, Southampton and Swansea. Reports also came from London.
The mysterious sound became known as The Hum phenomenon, or simply The Hum.
During the 1990s, reports began appearing in the United States, initially in Taos, New Mexico and Kokomo, Indiana. The phenomenon has since been documented in Canada, Australia, New Zealand, South Africa and several European cities. Most reports tend to come from relatively densely populated areas.
According to the Norwegian Broadcasting Corporation (NRK), residents around Oslo also reported an unexplained humming noise a couple of years ago.
Canadian teacher Glen MacPherson first heard The Hum while living and working on the west coast of Canada. After moving to another city in the same region, he could no longer hear it.
His curiosity eventually led him to establish the interactive The World Hum Map and Database Project in 2012. The project gathers reports and location data from people who say they have experienced the sound.
Possible Sources Range From Machines to Nature
Researchers and the public have proposed numerous explanations for The Hum. Suggested causes include acoustic pollution from human activity, naturally occurring environmental sounds and conspiracy theories involving the CIA or even aliens.
Many technologies can generate low-frequency noise, including ventilation systems, heat pumps, road traffic and windmills. Nature also produces sounds in this range through sources such as ocean waves striking the shore and wind moving across the landscape.
The mystery has drawn the attention of hearing specialists and audiology researchers around the world. One of them is Markus Drexl, a professor at the Norwegian University of Science and Technology (NTNU).
Drexl, two PhD research fellows and a postdoc studied 28 people in Germany who reported hearing an unexplained hum or buzz.
Testing Whether the Hum Comes From Outside
The researchers examined two main hypotheses.
The first was that The Hum might be an externally produced sound that could be measured. Such noise could come from industry, infrastructure or natural processes that generate low-frequency waves.
“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,” Drexl said.
Low-frequency sound waves have long wavelengths, which allows them to travel across considerable distances. That makes their source especially difficult to pinpoint.
Most Participants Did Not Have Exceptional Hearing
The team first investigated whether the participants were unusually sensitive to known low-frequency sounds.
Most showed no exceptional ability in this range. Only two people had 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.
However, he noted an important limitation. Tiny variations in hearing thresholds (microstructures) may allow some people to detect sound within an extremely narrow frequency range, such as between 50 and 51 Hertz. Standard hearing tests are not designed to capture differences this precise.
Could the Inner Ear Be Producing the Noise?
The cochlea inside the inner ear naturally creates faint sounds at different frequencies, generally between about 500 and 5000 Hertz. These noises do not serve a direct purpose. Instead, they are a by-product of the ear’s physiological sound amplification process.
“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 said.
Known as oto-acoustic emissions, these sounds can be detected by placing a sensitive microphone inside the ear canal. In certain individuals, spontaneous oto-acoustic emissions may be perceived as distressing tinnitus.
“One hypothesis was that the participants in our group could hear oto-acoustic emissions at low frequencies. That’s why we tested whether they had them,” says Drexl.
But… the answer was no.
Low-Frequency Tinnitus May Explain Many Cases
“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, often described as ringing in the ears, occurs when someone perceives a sound inside the ear or head even though no external source is producing it.
Tinnitus may be temporary or persistent. People often initially interpret the sound as something coming from their surroundings.
When the noise continues after they change locations, however, they may eventually realize that it is not being generated by anything nearby.
Based on current knowledge of hearing and the results of the participant tests, Drexl believes the most likely explanation has two parts.
A small number of people who hear The Hum may genuinely have unusually sensitive low-frequency hearing. For most, however, the experience may be a type of tinnitus in which the sound begins 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 of hearing pulsations of low-frequency sound perceptions,” he said.
Scientists Need a Better Picture of Low-Frequency Hearing
Drexl became interested in The Hum through his research into low-frequency sound.
“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,” he said.
Concern about noise from technological sources has increased during the past decade, particularly for sounds in the low-frequency range (between about 20 and 250 Hz) and infrasound (below 20 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,” he said.