A mysterious low-frequency sound called "The Hum" has been reported in many places around the world for decades. People who hear it often describe it as the deep roar of a diesel engine or an idling truck in the distance, but it has always been difficult to find a clear source of the sound. Recently, a study led by the Norwegian University of Science and Technology (NTNU) gave a new explanation: in many cases, this kind of buzzing is likely not to come from the environment, but from a kind of low-frequency tinnitus in the human auditory system itself.
The "buzzing" phenomenon first attracted widespread public attention when the local media in Bristol, England, received a large number of letters of complaint in the 1970s. Residents generally claimed that they would hear continuous low-frequency noise in the dead of night and that it was difficult to determine the direction. Since then, similar reports have appeared in other parts of the UK, North America, Australia, New Zealand, South Africa and some European countries. Among them, the case in Taos, New Mexico, the United States, is the most famous. Local residents have complained about the "underground engine-like roar" for many years, so much so that scientists were specially invited to investigate.
It is worth noting that although reports of "hearing buzzing" have spread all over the world, the proportion of people who are actually affected is very low. Most surveys show that only a few people claim to continuously hear this low-frequency sound, which has also caused many people involved to feel suspected or marginalized. They often describe this sound as the hum of distant diesel engines, industrial equipment, and low-frequency currents, and it is most obvious indoors at night when the background noise is extremely low.
There has been a lot of speculation surrounding the cause of the hum for a long time. Industrial equipment, ventilation systems, road traffic, power infrastructure, wind turbines, and natural factors such as waves, special atmospheric conditions, and ground vibrations have all been listed as "suspects." The problem is further complicated by the physical properties of low-frequency sound waves: they have long wavelengths, travel long distances, and can travel around obstacles, making it difficult to pinpoint the source of the sound through conventional methods.
Markus Drexl, a professor at the Norwegian University of Science and Technology, pointed out that there are indeed some people who can hear objectively measurable low-frequency sounds, but it is extremely challenging to find the specific source of these sound waves, which opens up the imagination for "unidentifiable sounds." In the absence of a visible external source, conspiracy theories and supernatural claims ranging from "secret government projects" and "military experiments" to "extraterrestrial activity" have continued to emerge, but so far no single external noise source has been able to explain all "buzzing" reports.
In order to study this phenomenon more systematically, Drexel's team recruited 28 subjects in Germany who had long felt unknown low-frequency buzzing or humming, and investigated possible causes one by one. The researchers first tested a seemingly straightforward hypothesis: whether these people are more sensitive to low-frequency hearing than the average person. However, the test results provided only limited support for this hypothesis - the vast majority of participants' hearing was within the normal range in conventional tests, and only two showed above-average sensitivity in specific low-frequency bands.
Drexel said that even with a limited sample size, this result is enough to show that "excellent hearing at low frequencies" does not explain the majority of buzz cases. However, he also reminded that conventional hearing tests often sample in a wider frequency band and may miss abnormally sensitive individuals in a very narrow frequency window. Therefore, the possibility of "supersensitivity in small frequency bands" cannot be completely ruled out.
The team then turned to a more unexpected direction: whether the ears themselves were the source of the sound. The human ear is not absolutely silent. Extremely weak "otoacoustic emissions" are produced deep in the cochlea of the inner ear, which is a normal by-product of the cochlea's process of amplifying external sounds. Most people are never aware of these small, self-generated sounds, but a few can actually hear them, and these acoustic signals can be measured objectively with specialized equipment.
Because spontaneous otoacoustic emissions can in some cases be subjectively perceived as tinnitus, the research team hypothesized that this might explain some reports of buzzing. However, in this sample, the detection of otoacoustic emissions found no evidence that it was the main cause, and this path has been temporarily ruled out as a universal explanation.
After excluding the two main causes of "common low-frequency hearing hypersensitivity" and "measurable otoacoustic emissions", the research focused on another path: low-frequency tinnitus. Drexel said some people hear sounds that cannot be detected by any objective acoustic means and are likely to suffer from a form of subjective low-frequency tinnitus, a persistent sound perception generated internally by the auditory system in the absence of an external sound source.
The public usually associates tinnitus with high-frequency, sharp "rings," but tinnitus can actually manifest in a variety of ways, ranging from high-pitched ringing to buzzing, roaring, hissing, clicking, or even extremely low-frequency muffled or "engine-like buzzing." Medically speaking, tinnitus itself is not an independent disease, but an "auditory hallucination" sound experience produced by the auditory system or related neural circuits in the absence of external stimulation.
This explanation corresponds to the most puzzling point of the buzzing phenomenon: many affected people initially believed that the sound came from the surrounding environment. Later, after changing residences, regions and even countries many times, "the sound always followed them", they began to suspect that the problem might lie in their own auditory system. Based on the research data, the team believes that the "buzzing around the world" is most likely not caused by a single cause, but by the superposition of different mechanisms: part of it is low-frequency environmental noise that does exist in reality and is only noticed by a few people, and the other part is low-frequency tinnitus that is mistaken for external sound sources.
The Drexel team wrote in the conclusion of the study that, without completely ruling out the physical external sound source in some cases, they tend to believe that subjective low-frequency tinnitus is in many cases the root cause of the low-frequency pulsating buzzing that people feel. In other words, for many "buzz listeners", the "external noise" they are looking for may always come from their own auditory system.
Drexel's interest in buzz phenomena stems from his broader background in studying low-frequency sounds. He pointed out that most theories and data on human auditory mechanisms are based on the study of mid- and high-frequency sound processing. Regarding the perception and processing mechanism of low-frequency sounds and even infrasound (below 20 Hz), humans currently have far less details than the high-frequency part.
In the past decade, society's concern about the impact of low-frequency industrial noise and infrasound has increased significantly. From large machinery and equipment, wind power facilities to building electromechanical systems, related disputes and health concerns have continued to increase. Drexel emphasized that if these low-frequency and infrasound sources are to be scientifically evaluated, the first prerequisite is a deeper understanding of how the human sensory system works in this frequency band, including the mechanical properties of the cochlea, central nervous system coding, and the interaction with emotion and attention.
According to reports, this work was published in a paper titled "Potential Sources of Low-frequency Sound Perception Perceivable by a Few People" on March 27, 2026, and was co-signed by Bonifaz Baumann, Andrej Voss, Carlos Jurado and Markus Drexl. The overall conclusion of the paper is: the so-called "low-frequency buzzing heard by only a few people" often points to subjective tinnitus rather than mysterious external noise in a statistical sense. The research also provides new ideas for the future formulation of low-frequency sound exposure, hearing health and noise standards, prompting policymakers and the engineering community to consider both objective acoustic testing and subjective tinnitus screening when dealing with low-frequency noise complaints.