Do you sometimes notice a deep humming or buzzing sound that seems to have no identifiable source? If so, you may be among the estimated 2 to 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, low-frequency sound may 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 most commonly noticed inside buildings, especially at night when people are trying to sleep. Someone may look outside for a nearby engine or machine, but find nothing that could explain the noise.
Even more disconcerting is that other people in the same room may not hear anything at all.
The Hum reports begin in England
The phenomenon first came to attention in Bristol, England, in the mid-1970s. The Bristol Evening Post began receiving a stream of letters from residents describing hearing an unexplained sound and wanting to know what was causing it.
One proposed explanation pointed to large industrial fans operating inside a department store warehouse. However, the reports continued even after the warehouse closed several years later.
Similar accounts later emerged in other parts of the United Kingdom, particularly in coastal communities such as 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 to appear 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 Norwegian Radio Television (NRK), residents around Oslo also reported an unexplained humming sound a couple of years ago.
Canadian professor 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, she could no longer hear him.
His curiosity eventually led him to establish the interactive project The World Hum Map and Database Project in 2012. The project collects 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 noise pollution caused by human activity, natural ambient 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 hitting the coast and wind moving across the landscape.
The mystery has caught the attention of hearing specialists and audiology researchers around the world. One of them is Markus Drexl, professor at the Norwegian University of Science and Technology (NTNU).
Drexl, two doctoral research fellows and a postdoc studied 28 people in Germany who reported hearing an unexplained humming or buzzing sound.
Test if the hum is coming from outside
The researchers examined two main hypotheses.
The first was that The Hum could 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 that easy to find the source of these sound waves, because it’s difficult to locate low-frequency sounds,” Drexl said.
Low-frequency sound waves have long wavelengths, allowing them to travel considerable distances. That makes its source especially difficult to identify.
Most participants did not have exceptional hearing
The team first investigated whether the participants were unusually sensitive to familiar low-frequency sounds.
Most did not show any exceptional abilities at this range. Only two people had better than average hearing at certain low frequencies.
“Although the group we tested was small, that means that the hypothesis of having especially good hearing for low-frequency sounds does not hold true for most people,” Drexl said.
However, he pointed out an important limitation. Small variations in hearing thresholds (microstructures) can allow some people to detect sounds within an extremely narrow frequency range, such as between 50 and 51 Hertz. Standard hearing tests are not designed to capture differences as accurately.
Could it be the inner ear that produces the noise?
The cochlea within the inner ear naturally creates weak sounds at different frequencies, usually between 500 and 5000 Hertz. These noises have no direct purpose. Rather, they are a byproduct of the ear’s physiological sound amplification process.
“Most of us do not hear these sounds. However, some people can actually hear the sounds produced by the ear itself. And these sounds can be measured objectively,” Drexl said.
These sounds, known as otoacoustic emissions, can be detected by placing a sensitive microphone inside the ear canal. In certain people, spontaneous otoacoustic emissions may be perceived as distressing tinnitus.
“One hypothesis was that participants in our group could hear otoacoustic emissions at low frequencies. That’s why we checked to see if they had them,” says Drexl.
But… the answer was no.
Low-frequency tinnitus can explain many cases
“Then there are people who hear something that cannot be objectively measured. We believe that people in this category have a low-frequency form of tinnitus,” Drexl said.
Tinnitus, often described as ringing in the ears, occurs when someone hears a sound inside the ear or head even though it is not produced by an external source.
Tinnitus can be temporary or persistent. At first, people often interpret sound as something coming from their environment.
However, when the noise continues after changing locations, they may eventually realize that it is not being generated by anything nearby.
Based on current knowledge about hearing and the participants’ test results, Drexl believes the most likely explanation is two-fold.
A small number of people who listen to The Hum may have unusually sensitive low-frequency hearing. However, for most, the experience may be a type of tinnitus in which the sound begins within the auditory system.
“Based on our results, although we have not ruled out cases of external physical sound sources, we suggest that subjective tinnitus in the low-frequency range is often the cause of auditory 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 on low-frequency sound.
“What we know about the auditory system is primarily 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 over the last decade, particularly for sounds in the low frequency range (between approximately 20 and 250 Hz) and infrasound (below 20 Hz).
“If we want to conduct a comprehensive evaluation of low-frequency sounds and infrasound, we first need a better understanding of how sensory systems process low-frequency sounds and infrasound,” he said.