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The strange hum: Are you among the few who can hear it?
Researchers have spent decades trying to determine where the 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 perceived 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 bed at night.
If you look out the window to see whether something with a motor has appeared in the neighbourhood, there’s nothing to see. And others who are in the same place hear nothing.
The phenomenon was first discovered in coastal cities
The phenomenon was first recorded and discussed in the city of Bristol, England, in the mid-1970s.
Suddenly, the local newspaper Bristol Evening Post began receiving letter after letter from people who heard an inexplicable sound. They wondered where it came from.
One theory was that the humming sound came from large industrial fans inside the warehouse of a large department store. But when the warehouse closed a few years later, people continued to hear the sound.
Since then, the sound has been recorded in several places across the UK, 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 Taos, New Mexico, and in Kokomo, Indiana.
It has now been recorded worldwide
The phenomenon has since been recorded worldwide: Canada, Australia, New Zealand, South Africa, and several European cities. The sound is typically reported in relatively densely populated areas.
A few years ago, people in the Oslo area also reported an unexplained humming sound, according to the Norwegian Broadcasting Corporation (NRK) (link in Norwegian).
Canadian Glen MacPherson began hearing the humming sound while living and working 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 phenomenon that he launched the interactive The World Hum Map and Database Project in 2012. There he collects data from places and people where the sound has been reported.
Many different theories exist
Many different theories have been offered to explain the phenomenon: everything from acoustic pollution from human-made sources to sounds produced by nature itself – 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 include ventilation systems, heat pumps, traffic noise, windmills, and more.
Examples of natural sources include the sounds of waves crashing along the coast and wind sweeping through the landscape.
The Hum has attracted considerable interest from hearing and audiology researchers worldwide. Markus Drexl is one of them. He is a professor at NTNU.
Together with other researchers, he conducted a study of 28 people in Germany who reported hearing an unexplained buzzing or humming.
Are these sounds that can be measured?
The researchers tested two hypotheses.
One was that The Hum could be measured, whether originating from human infrastructure and industry or from natural sources producing 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 localise low-frequency sounds,” says Drexl.
These sounds have long wavelengths that can travel over great distances.
Exceptionally 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.
They did not. The exception was two participants, who 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,” says Drexl.
He adds a small caveat: There are differences in hearing thresholds, also known as microstructures. These make it possible for some people to hear sensitively within a very narrow frequency range, for example between 50 and 51 hertz.
Such subtle differences are not detected by conventional hearing tests.
The ear can produce sounds itself
The cochlea in the inner ear naturally produces weak sounds with different frequencies, typically between about 500 and 5,000 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,” says Drexl.
These are known as otoacoustic emissions, and can be detected by placing a sensitive microphone in the ear canal.
In some people, these spontaneous otoacoustic emissions can be experienced as an annoying ringing in the ears.
“One hypothesis was that the participants in our group could hear otoacoustic emissions at low frequencies. That’s why we tested whether they had them,” says Drexl.
The answer was no.
These are 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,” says Drexl.
Tinnitus, or ringing in the ears, is when you hear a sound in your ear or head that is not caused by an external sound source.
Many people experience tinnitus, either temporarily or permanently. Initially, the sound often seems to come 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.
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,” says Drexl.
We need to understand the entire auditory system
Markus Drexl became interested in The Hum 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 says.
Drexl explains 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 says.
Reference:
Baumann et al. On the potential sources of a low-frequency sound percept that only a few can perceive, PLOS One, 2026. DOI: 10.1371/journal.pone.0326818
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Read the Norwegian version of this article on forskning.no
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