For a segment of the global population, a persistent, low-frequency auditory sensation, often described as a deep hum or pervasive buzz, has long been a source of bewilderment and, for some, significant distress. This elusive sound, which appears to emanate from no discernible external source, has perplexed individuals and scientific communities for decades, with estimates suggesting that between two and four percent of people worldwide experience this phenomenon. While for some the auditory intrusion is merely an annoyance, for others it can manifest as tangible physical discomfort, a disquieting sensation of vibration, or even lead to debilitating health symptoms. The very nature of this low-frequency sound makes it particularly insidious; it tends to be more pronounced indoors and is often most noticeable during periods of quiet, such as at night, disrupting sleep and prompting futile searches for nearby industrial machinery or traffic. Compounding the mystery is the fact that individuals in close proximity, even within the same room, may remain entirely oblivious to the sound, highlighting a deeply personal and selective auditory experience.
The earliest documented widespread accounts of this peculiar auditory event emerged from Bristol, England, in the mid-1970s. Local newspapers, notably the Bristol Evening Post, became a conduit for numerous letters from residents detailing their encounters with this unexplained noise, seeking an origin for their auditory torment. Initial hypotheses often pointed towards large industrial ventilation systems, such as those found in department store warehouses. However, reports persisted and even escalated after the suspected sources ceased operations, deepening the enigma. Similar reports soon began to surface across the United Kingdom, particularly in coastal towns like Hythe, Plymouth, Southampton, and Swansea, as well as in the bustling metropolis of London. This recurring auditory anomaly gradually became recognized and discussed globally as "The Hum" phenomenon.
The scope of The Hum’s reported occurrences expanded significantly throughout the 1990s, with early significant reports originating from Taos, New Mexico, and Kokomo, Indiana, in the United States. Since then, anecdotal evidence and systematic data collection have pointed to its presence in diverse geographical locations, including Canada, Australia, New Zealand, South Africa, and numerous European nations. Intriguingly, a common thread among many reported instances is the tendency for The Hum to be perceived in relatively densely populated urban or suburban areas. More recently, reports from the vicinity of Oslo, Norway, have further underscored the ongoing nature of this global auditory puzzle. The personal journey of Glen MacPherson, a Canadian educator who first experienced The Hum while living on the country’s west coast, only to have it vanish upon relocating within the same region, exemplifies the localized and sometimes transient nature of the phenomenon. His persistent curiosity led him to establish "The World Hum Map and Database Project" in 2012, an initiative dedicated to collecting and mapping the geographical data and personal testimonies of individuals who have encountered this perplexing sound.
Over the years, a wide array of potential explanations has been put forth by both the scientific community and the public. These proposed causes span a broad spectrum, from acoustic pollution generated by human technological endeavors and naturally occurring environmental sounds to more speculative, even conspiratorial, theories involving clandestine government operations or extraterrestrial influences. The sheer variety of proposed sources underscores the difficulty in pinpointing a single, unified cause. Technologies such as ventilation systems, heat pumps, vehicular traffic, and wind turbines are known producers of low-frequency noise. Concurrently, natural phenomena like the rhythmic crashing of ocean waves against shorelines and the movement of wind across varied landscapes also generate sounds within this same frequency range.
The persistent mystery of The Hum has attracted the dedicated attention of hearing specialists and audiology researchers worldwide. Among these dedicated investigators is Professor Markus Drexl of the Norwegian University of Science and Technology (NTNU). Professor Drexl, collaborating with two doctoral research fellows and a postdoctoral researcher, embarked on a focused study examining 28 individuals in Germany who reported experiencing an unexplained humming or buzzing sound. Their research aimed to systematically evaluate prevailing hypotheses regarding the origin of these perceived sounds.
A primary avenue of investigation for Drexl and his team was to determine if The Hum represented an externally generated sound that could be objectively measured. This hypothesis posited that the sound might originate from industrial activities, infrastructural elements, or natural processes that produce low-frequency acoustic waves. As Professor Drexl explained, "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." The inherent characteristics of low-frequency sound waves, namely their extended wavelengths, enable them to travel significant distances, thereby complicating the process of pinpointing their exact origin.
The research team initially sought to ascertain whether the participants possessed an unusual sensitivity to known low-frequency sounds. Their findings indicated that the majority of the participants did not exhibit heightened auditory acuity in this specific frequency range. Only two individuals demonstrated hearing capabilities that were demonstrably superior to the average at certain low frequencies. Professor Drexl commented on this aspect of the study: "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 critical limitation in standard hearing assessments: the potential for minute variations in hearing thresholds, sometimes referred to as "microstructures," which could enable certain individuals to detect sounds within extremely narrow frequency bands, such as between 50 and 51 Hertz. Standard audiological tests are not typically calibrated to capture such fine distinctions.
A compelling alternative hypothesis explored by the researchers delved into the possibility of the auditory system itself generating the perceived sound. The cochlea, a vital component of the inner ear, is known to produce faint, intrinsic sounds at various frequencies, typically ranging from approximately 500 to 5000 Hertz. These internally generated sounds are not believed to serve a direct functional purpose but rather arise as a byproduct of the ear’s inherent sound amplification mechanisms. While most individuals are not consciously aware of these subtle sounds, a subset of the population can perceive them. These internally produced sounds, known as oto-acoustic emissions, can be objectively measured using highly sensitive microphones placed within the ear canal. In some individuals, spontaneous oto-acoustic emissions can be experienced as a distressing form of tinnitus. Professor Drexl elaborated on this line of inquiry: "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." However, the results of this specific test were negative; the participants did not exhibit the expected oto-acoustic emissions at low frequencies.
This led the research team to consider another significant possibility: that a form of low-frequency tinnitus might be responsible for a substantial proportion of these perceived hums. Tinnitus, commonly characterized as a ringing in the ears, involves the perception of sound in the absence of any external auditory stimulus. It can be a transient or persistent condition. Individuals experiencing tinnitus often initially attribute the perceived sound to their external environment. However, when the sound persists even after relocating to different environments, they may eventually conclude that the origin is internal. Based on the current understanding of human hearing and the empirical data gathered from their participant group, Professor Drexl posits a dual explanation for The Hum phenomenon. He suggests that while a small number of individuals may indeed possess exceptionally sensitive low-frequency hearing, the experience for the majority is likely a manifestation of tinnitus originating within the auditory system itself. He concluded, "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’s engagement with The Hum phenomenon stemmed directly from his broader research interests in the complexities of low-frequency sound perception. He notes a significant gap in our understanding of the human auditory system’s processing of these lower frequencies: "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 in recent decades, particularly within the low-frequency range (approximately 20 to 250 Hz) and infrasound (below 20 Hz), has amplified the importance of this research area. To adequately address and assess the potential impacts of these pervasive low-frequency sounds and infrasound, a more comprehensive understanding of how human sensory systems process these frequencies is a critical prerequisite.
About the Author
