From MedicalXpress:
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Some people occasionally hear a low buzzing or humming sound that doesn't have a clear source. An estimated 2–4% of the world's population hear this. Scientists have been trying to figure out for decades where this sound comes from.
Some people find the sound annoying but can live with it. Others can get sick from this low-frequency sound, which is often also experienced as a vibration.
The humming sound isn't easy to hear outdoors, but it often appears indoors—and is most noticeable when you've gone to sleep at night. If you look out the window to see if there is something with a motor in the neighborhood, there's nothing to see.
And others who are in the same place hear nothing.
First discovered in coastal cities
The phenomenon was first recorded and discussed in the city of Bristol, England in the mid-1970s. Suddenly, the Bristol Evening Post began receiving letter after letters from people who heard an inexplicable sound, and wondered where it came from.
One theory was that the humming sound came from large, industrial fans that were located inside the warehouse of a large department store. However, when the warehouse was closed down a few years later, people continued to hear the sound.
Since then, the sound has been recorded in several places in the United Kingdom, mainly in coastal cities such as Hythe, Plymouth, Southampton, Swansea, but also in London.
The sound is called the hum phenomenon, or simply the hum.
In the 1990s, it cropped up in the United States, first in the city of Taos, New Mexico and in the city of Kokomo, Indiana. The phenomenon has since been recorded worldwide: in Canada, Australia, New Zealand, South Africa and several European cities. The sound is typically reported in relatively densely populated areas.
A couple of years ago, people in the Oslo area also reported an unexplained humming sound, according to the Norwegian Broadcasting Corporation (NRK).
Canadian Glen MacPherson began hearing the humming sound when he lived and worked as a teacher on Canada's west coast. When he moved to another city in the same area, the sound disappeared.
He became so interested in the sound phenomenon that he started the interactive World Hum Map [top] and Database Project in 2012, which collects data from places and people where the sound has been noted.
Many different theories
Many different theories have been offered to explain the cause of the phenomenon; everything from acoustic pollution from human-made sources to sounds that nature itself makes—as well as conspiracy theories that the sound is produced by the CIA or even aliens.
There are many human sources of low-frequency sound. These can include ventilation systems, heat pumps, traffic noise, windmills and more. Examples of natural sources include the sound of waves crashing along the coast and wind sweeping through the landscape.
The hum has attracted the interest of hearing and audiology researchers worldwide. Markus Drexl, an NTNU professor, is among this self-selected group.
He and two Ph.D. research fellows and a postdoc have conducted a study of 28 people in Germany who experience hearing an unexplained buzzing or humming. The study is published in the journal PLOS One.
Other explanations for 'the hum'
- Military aircraft and submarines
One theory that has been proposed is that the hum relates to sound waves from US military aircraft that use radio frequencies at the lowest end of the spectrum of sound frequencies to communicate with submarines. These aircraft operate at night, and their movements are top secret. The theory may also explain why many "hum sites" are located on the coast.
- Amorous fish
The Scottish Association for Marine Science has suggested that the noise in the UK coastal town of Hythe could be caused by the mating call of schools of male plainfin midshipman fish (Porichthys notatus). Amorous male fish make loud sounds, sometimes for hours, to attract females.
- Waves, volcanic eruptions, or lightning strikes
In 2015, French researchers suggested that the hum was caused by waves moving along the seafloor. When the waves collide with ridges on the continental shelves, it creates vibrations that are audible to some.
Other researchers have suggested that vibrations caused by volcanic eruptions and earthquakes could be the cause.
Yet another theory points to the lightning strikes that strike Earth every day. Lightning strikes build up a massive electromagnetic charge that creates a resonance between Earth's surface and the ionosphere—much like blowing air over the top of a bottle.
- Sensitive brains
Dr. David Baguley, head of the audiology department at Addenbrooke's Hospital in England, has done extensive research into the phenomenon. He believes it is due to sensitive brains that can pick up ultra-low sound frequencies.
He pointed out that our sense of hearing is greatly affected if we experience a lot of stress, and the brain turns up the volume to detect threatening sounds.
Sounds that can be measured
The researchers tested two hypotheses.
One was that the hum can be measured, both from human-made infrastructure and industry and also from nature itself, which creates low-frequency sounds.
"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.
These sounds have long wavelengths that can travel over great distances.
Extra good hearing?
The first thing the researchers did was test whether the participants had particularly good hearing for low-frequency sounds that are actually known to exist.
Most did not, except for two participants who had better hearing than average 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.
He adds a small caveat: There are differences in hearing thresholds (microstructures) that make it possible for some people to hear sensitively in a very narrow frequency range, for example between 50 and 51 Hertz. These nuances are not captured by conventional hearing tests.
The ear can produce sounds itself
The cochlea in the inner ear itself produces weak sounds with different frequencies, typically between about 500 and 5000 Hertz. These sounds have no function of their own, but are a by-product of a 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.
These particular sounds are called oto-acoustic emissions and can be detected by placing a sensitive microphone in the ear canal. In some people, these spontaneous oto-acoustic emissions can be experienced as troublesome 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.
Sounds that cannot be measured
"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 or ringing in the ears is when you hear a sound in the ear or in the head, which is not caused by an external sound source.
Many people experience tinnitus, either permanently or for shorter periods. These individuals first experience the sounds in their ears as a sound coming from outside.
But as the sound persists, even when they move to other places, they gradually become aware that the source of the sound is not external.
Drexl says that based on what is known about hearing and the tests they conducted on study participants, the best explanation is twofold.
A few people who hear the hum actually have particularly good low-frequency hearing. However, for most people, it may be a form of tinnitus, meaning a sound that originates from inside 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.
Better understanding of the entire auditory system
Markus Drexl became interested in the hum phenomenon because he studies low-frequency sounds.
"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.
Drexl says that over the past decade there has been a growing concern about noise from technical sources in the low-frequency range (between about 20 and 250 Hz) and the infrasound range (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.
More?
Your wish is my demand.
Read the full scientific paper on which the article above is based: titled "On the potential sources of a low-frequency sound percept that only a few people can perceive," it was published March 27, 2026 in PLOS One.
