Have you ever encountered a deep buzzing or humming sound with no clear source? You might be among the 2-4% of individuals globally who experience this perplexing phenomenon. Researchers have dedicated decades to unraveling its origins.
For some, this noise is merely an annoyance, while for others, infrasound can lead to physical discomfort or even illness, often manifesting as vibrations felt throughout the body.
The hum is often challenging to detect outdoors; it tends to be more prevalent indoors, particularly at night when individuals are attempting to sleep. Even after searching for nearby engines or machinery, one might find no explanation for the noise.
Even more baffling is that other people in the same room might not hear anything at all.
Origins of The Hum Phenomenon in the UK
The hum first gained significant attention in Bristol, England, during the mid-1970s. The Bristol Evening Post started receiving letters from residents reporting mysterious sounds and seeking answers.
One proposed cause was a large industrial fan in a department store warehouse, but reports persisted even after the warehouse closed years later.
Subsequent reports emerged from other UK locales, particularly coastal towns like Hythe, Plymouth, Southampton, and Swansea, as well as London.
This puzzling sound became known as the “hum phenomenon,” or simply “the hum.”
Reports of the phenomenon appeared in the United States during the 1990s in places like Taos, New Mexico, and Kokomo, Indiana. It has since been documented in Canada, Australia, New Zealand, South Africa, and across Europe, usually in densely populated areas.
In a recent report by the Norwegian Broadcasting Corporation (NRK), residents near Oslo also reported hearing an unexplained hum.
Canadian teacher Glenn McPherson first experienced the hum while living on Canada’s west coast. After relocating to a different city nearby, he no longer heard the sound.
This experience inspired him to create the interactive The World Hum Map and Database Project in 2012, which gathers reports and location data from individuals who have encountered these sounds.
Possible Sources of the Hum: Mechanical to Natural
Researchers and the public have proposed numerous explanations for the hum. Potential sources include acoustic pollution from human activities, naturally occurring environmental sounds, and even conspiracy theories involving organizations like the CIA or extraterrestrial beings.
Various technologies generate low-frequency noise, including ventilation systems, heat pumps, road traffic, and wind turbines. Mother Nature also produces sounds in this frequency range, such as waves crashing on the shore and windy landscapes.
This mystery has caught the attention of audiologists and hearing researchers worldwide. One notable researcher is Markus Drexl, a professor at the Norwegian University of Science and Technology (NTNU).
Drexl and his team studied 28 individuals in Germany who reported hearing unexplained humming or buzzing sounds.
Investigating Outside Sources of the Hum
The researchers considered two primary hypotheses.
The first suggests that the hum could be an externally generated, measurable sound originating from industries, infrastructure, or natural phenomena producing low frequencies.
Drexl noted, “Some people can hear measurable low-frequency sounds that others cannot. However, identifying the source of these sound waves is challenging since low-frequency sounds are difficult to pinpoint.”
Due to long wavelengths, low-frequency sound waves can travel vast distances, complicating source identification.
Most Participants Showed No Special Hearing Abilities
The study first explored whether participants had heightened sensitivity to low-frequency sounds.
Most revealed no unusual abilities in this range, with only two demonstrating above-average hearing at specific bass frequencies.
Drexl stated, “While our group was small, it indicates the idea that low-frequency sound perception is unique to certain individuals isn’t valid for most.” He also pointed out important limitations, noting that slight variations in hearing thresholds may allow some individuals to detect sounds within very narrow frequency ranges, such as between 50 and 51 hertz. Standard hearing tests typically do not measure such precise differences.
Could Your Inner Ear Be the Sound Source?
The cochlea in the inner ear naturally produces faint sounds at various frequencies, generally around 500 to 5000 hertz. These noises are byproducts of the ear’s sound amplification process, not intentional emissions.
Drexl stated, “While most people don’t hear these sounds, a few can perceive the sounds their ears generate, which can be objectively measured.” These sounds, known as otoacoustic emissions, can be detected using a sensitive microphone in the ear canal. In some cases, spontaneous otoacoustic emissions may manifest as painful tinnitus.
One hypothesis was that the study participants could hear low-frequency otoacoustic emissions. However, testing confirmed otherwise.
Low-Frequency Tinnitus May Explain the Experience
Drexl explained, “Some individuals perceive sounds that cannot be measured objectively. We believe that this category corresponds to a type of low-frequency tinnitus.” Tinnitus, often described as ringing in the ears, occurs when a person hears sounds in their head or ears without any external source present.
This condition can be temporary or chronic, and individuals may initially attribute the sound to their environment. However, if the noise continues when changing locations, they may realize it’s not originating from their surroundings.
Based on current understanding and participant testing results, Drexl posits a two-part explanation. Some individuals who hear a hum may indeed have an unusual sensitivity to low-frequency sounds, but for the majority, the sensation likely stems from tinnitus originating in the auditory system.
“While we don’t dismiss the possibility of physical external sound sources, we believe subjective tinnitus in the low-frequency range often accounts for perceived auditory pulsations,” he stated.
Further Research Needed on Low-Frequency Hearing
Drexl’s interest in the hum developed during his research into infrasound.
“Our knowledge of the auditory system mostly focuses on high-frequency sounds. We understand far less about how it processes low-frequency sounds or infrasound,” he noted.
In recent years, there has been increasing concern regarding noise from technological sources, especially within the low-frequency range (approximately 20-250 Hz) and infrasound (below 20 Hz).
“To conduct a comprehensive assessment of infrasound, we need to better understand how sensory systems process these frequencies,” he concluded.
Source: www.sciencedaily.com