Millions of people worldwide report hearing a persistent, low-frequency humming sound, a phenomenon that scientists now believe can be attributed to two primary factors: exceptionally acute low-frequency hearing or a form of tinnitus. This sound, often perceived as a vibration, is typically more noticeable indoors and can be particularly distinct at night.
The 'Hum' phenomenon, as it is known, has been documented in various locations, with early reports emerging in Bristol, England, in the mid-1970s. Despite earlier theories suggesting industrial sources like large ventilation fans, the sound persisted even after potential origins were removed, indicating a more complex cause.
Over the decades, the Hum has been reported globally, appearing in the United States in places like Taos, New Mexico, and Kokomo, Indiana, during the 1990s. It has since been heard in Canada, Australia, New Zealand, South Africa, and several European cities, often in densely populated areas.
Recent research led by Professor Markus Drexl at the Norwegian University of Science and Technology explored the phenomenon. Initial investigations tested whether participants possessed unusually good low-frequency hearing. While a few individuals did demonstrate superior hearing at certain low frequencies, this did not explain the experience for the majority of those studied.
Another line of inquiry examined sounds produced by the inner ear itself, known as oto-acoustic emissions. These are byproduct sounds generated by the cochlea, typically between 500 and 5000 Hertz, which most people do not perceive. The study tested if participants could hear these emissions at low frequencies, but the results indicated that none of them did.
Professor Drexl concluded that for many individuals experiencing the Hum, the most likely explanation is a form of subjective, low-frequency tinnitus. Tinnitus is characterized by the perception of sound without an external source, and in these cases, the internal sound is experienced as an external hum.
While the study did not entirely rule out the possibility of external physical sound sources contributing to the Hum in some instances, the findings strongly suggest that in many cases, the phenomenon is a manifestation of how some individuals' auditory systems process internal sounds.