Summary: A precision hearing and audiology study unmasked the biological mechanisms behind “The Hum”, an enigmatic, low-frequency buzzing sound reported by citizens in densely populated and coastal cities worldwide. The investigation tested whether the phenomenon stems from hyper-acute hearing thresholds, internal ear emissions, or an unmeasurable auditory glitch.
The findings reveal that while a minute fraction of the population possesses extraordinary low-frequency hearing, the vast majority of people who hear the noise are actually experiencing a rare, low-frequency form of subjective tinnitus generated entirely within their own auditory systems.
Key Facts
- The Global Enigma of The Hum: First recorded in Bristol, England, in the mid-1970s, “The Hum” has since cropped up in coastal and densely populated cities worldwide, spanning the UK, the United States, Canada, Australia, and Norway. Described as a low-frequency vibration most noticeable indoors at night, the sound routinely induces sickness or sleep disruption in subset populations, while others in the exact same room hear absolutely nothing.
- The NTNU Investigation Framework: To separate environmental infrastructure pollution from internal biological anomalies, Professor Markus Drexl of NTNU, alongside an international team of PhD research fellows and a postdoc, conducted a specialized study of 28 individuals in Germany who experience the persistent, unexplained humming.
- Dismantling the Hyper-Hearing Theory: The team first tested whether sufferers possessed uniquely sensitive hearing thresholds for known, external low-frequency sound waves. Except for two outliers who exhibited better-than-average baseline processing at certain low frequencies, the vast majority of participants possessed entirely normal hearing, disproving the theory that “Hum” perceivers simply have exceptional low-frequency hearing.
- The Oto-Acoustic Emission Check: Human ears are naturally equipped with a cochlea that produces weak, objective sounds between 500 and 5,000 Hertz as a biological byproduct of our inner sound-amplification process. The researchers tested whether participants were suffering from spontaneous, low-frequency oto-acoustic emissions that could be measured via sensitive ear-canal microphones—but the tests came back completely negative.
- The Subjective Tinnitus Verdict: Bypassing external acoustic pollution, Professor Drexl’s team concluded that for the vast majority of sufferers, the noise is an unmeasurable, entirely internal phenomenon. It represents a subjective, low-frequency variation of tinnitus, an internal phantom pulse generated by the auditory system itself, which individuals initially mistake for an external motor or environmental vibration before realizing it travels with them wherever they go.
- The Infrasound Knowledge Gap: While public concern grows regarding technical low-frequency noise (20–250 Hz) and infrasound (below 20 Hz) generated by windmills, traffic, and heat pumps, modern audiology still lacks a complete map of how the brain processes these sounds. Drexl emphasizes that uncovering the true nature of The Hum requires a deep re-evaluation of how our sensory systems handle the lowest thresholds of the acoustic spectrum.
Source: NTNU
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 neighbourhood, 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 The World Hum Map 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 sounds 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, a professor at the Norwegian University of Science and Technology (NTNU), is among this self-selected group.
He and two PhD research fellows and a postdoc have conducted a study of 28 people in Germany who experience hearing an unexplained buzzing or humming.
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.
Must understand 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.
Key Questions Answered:
A: Because the quiet background of an indoor bedroom at night strips away sensory distractions, leaving the brain alone with its internal noise. If the hum is caused by low-frequency subjective tinnitus, the absolute silence of a bedroom removes environmental audio masking, forcing the auditory system’s internal phantom frequencies to sound intensely loud, vibrant, and distressing.
A: Yes, the inner ear naturally manufactures its own weak acoustic signals as a biological byproduct. The cochlea produces weak sounds called oto-acoustic emissions during its normal process of amplifying incoming sound. While most people never notice them, these internal echoes can occasionally bleed into a person’s awareness, though testing proved they are not the cause of the global low-frequency “Hum” phenomenon.
A: By tracking whether the sound follows the individuals when they change environments. True infrastructure noise, like ventilation, heat pumps, or windmills, can be measured with microphones and stays anchored to the source area. If the sound waves cannot be recorded and continue to buzz inside a person’s head after they move to an entirely different city, it stands as a definitive diagnostic signature of low-frequency tinnitus.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this auditory neuroscience and tinnitus research news
Author: Nancy Bazilchuk
Source: NTNU
Contact: Nancy Bazilchuk – NTNU
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“On the potential sources of a low-frequency sound percept that only a few can perceive” by Bonifaz Baumann, Andrej Voss, Carlos Jurado, and Markus Drexl. PLOS One
DOI:10.1371/journal.pone.0326818
Abstract
On the potential sources of a low-frequency sound percept that only a few can perceive
A small percentage of the general population reports almost constant humming or pulsing low-frequency sound percepts (LFSPs), while others in their vicinity, such as family members, often do not perceive these sounds. The origin of these LFSPs remains to be elucidated and may, or may not, be related to external sound sources.
The underlying causes of these perceptions could also be subjective and belong to the tinnitus family, especially in cases where no external sound source sufficiently explaining the LFSP can be found.
The present study puts forth two hypotheses to explain the phenomenon, based on both subjective and objective auditory phenomena: an unusually high auditory sensitivity to low-frequency sound, and hearing one’s own low-frequency spontaneous otoacoustic emissions (SOAEs), sounds actively produced by the inner ear as a normal, physiological by-product of cochlear amplification.
The present study employed high-resolution, low-frequency hearing threshold measurements and SOAE measurements in 28 individuals with LFSPs and in control groups devoid of LFSPs. LFSP complainants self-reported hearing an LFSP at a median frequency of 50 Hz, obtained with a frequency-matching procedure.
With a few clear exceptions, complainants most often did not present unusually sensitive low-frequency hearing thresholds. Furthermore, hearing threshold microstructure was comparable to that of the control group.
In addition, no SOAEs in the low-frequency range could be measured. Based on our results, while cases of hearing physical external sound sources are not ruled out, we suggest that subjective tinnitus in the low-frequency range is often the reason behind hearing these LFSPs.
