TOKYO —

For a select segment of the global population, a persistent, low-frequency hum or buzz intrudes upon their daily lives, a sound that defies easy explanation. This phenomenon, often referred to as "The Hum," affects an estimated 2-4 percent of people worldwide, presenting a unique auditory challenge that has perplexed researchers for decades. While for some it is a mere annoyance, for others, the pervasive low-frequency vibrations can manifest as physical discomfort, illness, and a disorienting sense of an internal tremor. The enigmatic nature of The Hum is amplified by its elusive characteristics: it is often more noticeable indoors, particularly during the quietude of night, and bewilderingly, may be entirely inaudible to others sharing the same space.

The Genesis of a Global Enigma: From Bristol to the World

The documented history of The Hum’s widespread recognition begins in the mid-1970s in Bristol, England. A surge of letters to the Bristol Evening Post from bewildered residents described an inexplicable sound, prompting an investigation into its origin. One early theory pointed to industrial fans within a large department store warehouse. However, this explanation proved insufficient as the reports persisted even after the warehouse ceased operations years later, suggesting a more complex or widespread cause.

The phenomenon soon began to surface in other parts of the United Kingdom. Coastal communities such as Hythe, Plymouth, Southampton, and Swansea, along with the bustling metropolis of London, reported similar experiences. This collective phenomenon became colloquially known as "The Hum."

The 1990s saw reports of The Hum emerge across the Atlantic, with initial accounts originating from Taos, New Mexico, and Kokomo, Indiana, in the United States. Since then, the phenomenon has been documented in a diverse range of countries, including Canada, Australia, New Zealand, South Africa, and various European cities. Interestingly, many of these reported occurrences tend to be concentrated in relatively densely populated urban and suburban areas, hinting at potential correlations with human activity or infrastructure. More recently, in the early 2020s, residents around Oslo, Norway, also reported experiencing an unexplained humming noise, as documented by the Norwegian Broadcasting Corporation (NRK).

A pivotal figure in the ongoing study of The Hum is Canadian teacher Glen MacPherson. His personal encounter with the sound while living on Canada’s west coast, only to have it disappear upon relocating within the same region, ignited his curiosity. This personal quest for understanding evolved into a significant academic endeavor. In 2012, MacPherson launched The World Hum Map and Database Project, an interactive platform dedicated to collecting and mapping reports of The Hum from individuals globally. This project has become an invaluable resource, compiling vast amounts of anecdotal evidence and geographical data, providing researchers with a broader scope of the phenomenon’s reach and characteristics.

Unraveling the Source: A Spectrum of Potential Explanations

The mystery of The Hum has spurred a wide array of proposed explanations, ranging from the mundane to the extraordinary. These suggestions encompass acoustic pollution generated by human activities, naturally occurring environmental sounds, and even more speculative theories involving clandestine government operations or extraterrestrial origins.

Technological sources are frequently implicated. A multitude of modern infrastructure and machinery can produce low-frequency noise, including ventilation systems, HVAC units, road traffic, and wind turbines. The subtle vibrations and droning sounds generated by these elements can, under certain conditions, propagate and become perceptible to sensitive individuals.

However, the natural world also presents potential culprits. Environmental sounds within the low-frequency spectrum can be generated by sources such as ocean waves crashing against shorelines, the movement of wind across landscapes, and seismic activity. These natural forces can create infrasound (frequencies below 20 Hz) and low-frequency sound (between 20 and 250 Hz), which, due to their long wavelengths, can travel considerable distances and penetrate structures.

Scientific Inquiry: Norway’s Contribution to Understanding The Hum

The persistent nature and widespread reports of The Hum have attracted the attention of hearing specialists and audiology researchers worldwide. Among them is Professor Markus Drexl from the Norwegian University of Science and Technology (NTNU). Recognizing the need for rigorous scientific investigation, Professor Drexl, along with two PhD research fellows and a postdoctoral researcher, embarked on a study examining 28 individuals in Germany who reported experiencing an unexplained hum or buzz.

The research team focused on two primary hypotheses to explain the phenomenon.

Hypothesis 1: An Externally Produced, Measurable Sound

The first hypothesis posited that The Hum might be an externally generated sound that could be objectively measured. This external source could stem from industrial processes, widespread infrastructure, or natural phenomena capable of producing low-frequency acoustic 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," Professor Drexl explained. "But it’s not so easy to find the source of these sound waves, because it’s a struggle to localize low-frequency sounds." The inherent challenge lies in the long wavelengths of low-frequency sound, which allow them to travel extensive distances, making precise source identification exceptionally difficult.

Investigating Auditory Sensitivity: Do Hum-Hearers Possess Super-Hearing?

A crucial aspect of the first hypothesis involved investigating whether individuals who hear The Hum possess an unusually heightened sensitivity to low-frequency sounds. The research team conducted comprehensive hearing tests on the 28 participants.

The results indicated that the majority of participants did not exhibit exceptional hearing abilities within the low-frequency range. Only two individuals demonstrated hearing thresholds that were better than average at specific 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," Professor Drexl stated.

However, Professor Drexl acknowledged a significant limitation in standard hearing tests. He noted that subtle variations in hearing thresholds, often referred to as "microstructures," could allow some individuals to detect sounds within extremely narrow frequency bands, such as between 50 and 51 Hertz. Current audiometric testing protocols are not designed to capture such minute distinctions. This suggests that while statistically uncommon, a subset of individuals might possess a genuinely heightened sensitivity to specific low-frequency ranges that standard tests overlook.

Hypothesis 2: The Inner Ear as a Source of the Sound

The second hypothesis explored the possibility that the sound originates not from an external source, but from within the auditory system itself. The human cochlea, the spiral cavity of the inner ear, naturally generates faint sounds at various frequencies, typically between 500 and 5000 Hertz. These sounds, known as oto-acoustic emissions, are not consciously perceived and do not serve a direct communicative purpose; rather, they are a byproduct of the ear’s physiological sound amplification mechanisms.

"Most of us don’t hear these sounds," Professor Drexl elaborated. "However, a few people can actually hear the sounds that the ear itself produces. And these sounds can be measured objectively."

Oto-acoustic emissions can be detected using sensitive microphones placed within the ear canal. In certain individuals, spontaneous oto-acoustic emissions can manifest as a form of distressing tinnitus, or ringing in the ears.

"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," Professor Drexl said.

The results of these tests were definitive: the participants did not exhibit measurable oto-acoustic emissions at the low frequencies they reported hearing. This finding led the researchers to tentatively set aside the oto-acoustic emission hypothesis as a primary explanation for this specific group.

The Emerging Consensus: Low-Frequency Tinnitus as a Leading Explanation

Following the exclusion of oto-acoustic emissions as a direct cause for the studied group, Professor Drexl and his team turned their attention to another potential internal source: tinnitus.

"Then there are people who hear something that cannot be measured objectively," Professor Drexl stated. "We believe people in this category have a form of low-frequency tinnitus."

Tinnitus, commonly characterized as a ringing or buzzing in the ears, occurs when an individual perceives sound in the absence of any external auditory stimulus. It can be a temporary or persistent condition. Initially, individuals experiencing tinnitus often attribute the sound to their external environment, searching for a tangible source. However, when the perceived sound continues even after changing locations or eliminating potential external sources, the realization that the sound is originating internally can gradually dawn.

Based on their research and existing knowledge of auditory physiology, Professor Drexl’s team has proposed a two-part explanation for The Hum phenomenon. For a small subset of individuals, genuinely heightened sensitivity to low-frequency sounds may indeed be the cause. However, for the majority of those who report experiencing The Hum, the phenomenon is likely a manifestation of low-frequency tinnitus. In these cases, the perceived sound originates within the auditory system itself.

"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," Professor Drexl concluded. This suggests that while external sources may account for some instances of The Hum, a significant portion of cases may be attributed to internal auditory processing anomalies.

The Path Forward: A Deeper Understanding of Low-Frequency Hearing

Professor Drexl’s engagement with The Hum stemmed from his broader research into the complexities of low-frequency sound perception. He highlights a critical gap in current scientific understanding: "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."

In recent years, concerns regarding noise pollution from technological sources, particularly in the low-frequency and infrasound ranges, have escalated. This heightened awareness underscores the urgency for more comprehensive research.

"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," Professor Drexl emphasized. Advancing our comprehension of how humans perceive and process these lower auditory frequencies is crucial for accurately diagnosing conditions like low-frequency tinnitus and for developing effective mitigation strategies for noise pollution. The ongoing mystery of The Hum serves as a compelling catalyst for this vital scientific endeavor, promising to illuminate a lesser-understood aspect of human hearing and its potential environmental influences.