For decades, a peculiar auditory experience, colloquially known as "The Hum," has perplexed individuals and scientists alike across the globe, manifesting as a persistent, low-frequency drone with no discernible external origin. This widespread phenomenon, affecting an estimated 2-4% of the world’s population, has prompted extensive research into its potential causes, with a recent scientific inquiry suggesting that the source of this mysterious sound may lie not in the external environment, but within the human auditory system itself. While some affected individuals find the persistent noise merely an annoyance, others report significant physical discomfort, feelings of vibration, and even illness, underscoring the profound impact this auditory anomaly can have.
The experience of The Hum is often characterized by its insidious nature; it is frequently more noticeable indoors and particularly during periods of quiet, such as at night, when individuals are attempting to rest. A common scenario involves someone searching for a logical explanation, such as nearby machinery or industrial activity, only to find no external source to account for the persistent sound. Adding to the mystery, the experience is not universal even among those in close proximity; individuals in the same room may perceive the sound while others remain oblivious.
The documented history of The Hum traces its emergence into public consciousness to Bristol, England, in the mid-1970s. Local media outlets began receiving a deluge of correspondence from residents describing an inexplicable buzzing or humming sensation. Initial theories pointed towards the operation of large industrial fans within a department store’s warehouse facilities. However, the reports persisted even after the closure of the warehouse, highlighting the elusive nature of the phenomenon. Following these early accounts in Bristol, similar reports began to surface across other regions of the United Kingdom, including coastal towns such as Hythe, Plymouth, and Southampton, as well as London. It was during this period that the perplexing auditory occurrence became widely recognized and labeled as "The Hum."
The phenomenon’s reach extended globally throughout the 1990s, with initial reports emerging from the United States in locations like Taos, New Mexico, and Kokomo, Indiana. Since then, accounts of The Hum have been documented in various countries, including Canada, Australia, New Zealand, South Africa, and several European nations, with a tendency for reports to originate from areas with higher population densities. More recently, reports have also emerged from regions such as Norway, where residents around Oslo have described hearing an unexplained humming sound. The subjective and often localized nature of the experience led to the establishment of an interactive global database by Canadian teacher Glen MacPherson. His project, "The World Hum Map and Database," initiated in 2012, systematically collects eyewitness accounts and geographical data from individuals reporting their experiences with The Hum, aiming to map the prevalence and potential distribution of this auditory anomaly.
Over the years, a multitude of explanations have been put forth by both the scientific community and the public, ranging from man-made acoustic pollution to natural environmental sounds and even speculative theories involving covert government projects or extraterrestrial activity. Potential sources of low-frequency noise are diverse and include commonplace technologies such as ventilation systems, heat pumps, road traffic, and wind turbines. Nature itself also generates sounds within this frequency range, with phenomena like the rhythmic crashing of ocean waves against shorelines and the movement of wind across landscapes contributing to the ambient sonic environment.
The persistent enigma of The Hum has captured the attention of hearing specialists and audiology researchers worldwide, prompting rigorous scientific investigation. Among these researchers is Professor Markus Drexl from the Norwegian University of Science and Technology (NTNU). Professor Drexl, alongside a team of doctoral candidates and a postdoctoral researcher, embarked on a study focusing on 28 individuals in Germany who reported experiencing an unexplained hum or buzz. The primary objective of this research was to systematically evaluate the most plausible hypotheses regarding the origin of The Hum.
The scientific investigation centered on two principal hypotheses. The first posited that The Hum might be an externally generated sound, detectable through objective measurement. Such external sounds could originate from industrial operations, widespread infrastructure, or natural processes that produce low-frequency acoustic waves capable of propagating over significant distances. As Professor Drexl noted, "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." This difficulty in localization stems from the physical properties of low-frequency sound waves, which possess long wavelengths, enabling them to travel far, thereby obscuring their precise point of origin.
A crucial aspect of the research involved assessing whether individuals experiencing The Hum possessed unusually acute hearing capabilities, specifically within the low-frequency spectrum. The team conducted a series of tests to determine if the participants exhibited a heightened sensitivity to known low-frequency sounds compared to the general population. The results indicated that the majority of participants did not demonstrate exceptional hearing abilities in this particular frequency range. Only two individuals out of the tested group showed hearing thresholds that were better than average at specific low frequencies. Professor Drexl elaborated on these findings, stating, "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." However, he acknowledged a significant caveat: the potential for minute variations in hearing sensitivity, often referred to as "microstructures," might enable certain individuals to perceive sounds within extremely narrow frequency bands, such as a range as precise as 50 to 51 Hertz. Standard audiological evaluations are typically not designed to detect such fine-grained differences in hearing thresholds.
The research then pivoted to explore the possibility that the source of The Hum might originate internally, within the human auditory system itself. The inner ear, specifically the cochlea, is known to produce faint sounds as a byproduct of its physiological sound amplification processes. These naturally occurring sounds, known as spontaneous oto-acoustic emissions, typically occur at frequencies between approximately 500 and 5000 Hertz and are generally imperceptible to most individuals. However, in some cases, these internal auditory emissions can be detected objectively using sensitive microphones placed within the ear canal. For a subset of the population, these spontaneous oto-acoustic emissions can manifest as a form of distressing tinnitus. Professor Drexl explained this possibility: "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." The researchers tested the hypothesis that participants might be perceiving low-frequency oto-acoustic emissions, but the results were negative; the participants did not exhibit these specific internal sounds.
This led the research team to consider another internal auditory phenomenon: low-frequency tinnitus. Tinnitus, commonly described as a ringing in the ears, is the perception of sound in the absence of any external acoustic stimulus. It can be a temporary or persistent condition, and individuals often initially attribute the perceived sound to their surroundings. However, when the sound continues regardless of environmental changes, the realization dawns that the origin is internal. Professor Drexl and his colleagues propose that for a significant portion of individuals experiencing The Hum, the phenomenon may represent a form of low-frequency tinnitus. This internal auditory perception, originating within the auditory system, may be the underlying cause of the reported pulsations and low-frequency sound perceptions.
Based on the current understanding of human hearing and the outcomes of their investigations, Professor Drexl suggests a dual explanation for The Hum. While a small fraction of individuals may indeed possess an exceptionally sensitive low-frequency hearing ability, the more prevalent cause, according to their findings, is likely a form of subjective tinnitus that begins within 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 concluded.
Professor Drexl’s engagement with The Hum stemmed from his broader research interests in the complexities of low-frequency sound perception. He highlights a significant gap in our current scientific knowledge: "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." The increasing prevalence of noise from technological sources, particularly in the low-frequency range (defined as between approximately 20 and 250 Hertz) and infrasound (below 20 Hertz), underscores the urgency of this research gap. Professor Drexl emphasizes the need for a more comprehensive understanding of how sensory systems process these lower frequencies. "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 stated, underscoring the foundational nature of this research for future studies and potential interventions.
About the Author
