Do you sometimes notice a deep buzzing or humming sound that you can’t pinpoint the source of? If so, you may be one of the estimated 2-4% of people worldwide who experience this unusual sound. Researchers have spent decades trying to determine its origins.
For some people, noise is irritating but manageable. For others, infrasound can cause physical discomfort or illness, and can also feel like vibrations traveling through the body.
Hum can be difficult to detect outdoors. They are often seen inside buildings, especially at night when people are trying to sleep. If someone were to go outside and look for nearby engines or machinery, they might not find anything that could explain the noise.
Even more inexplicably, other people in the room may not be able to hear anything.
Reports on The Ham begin in the UK
The phenomenon first gained widespread attention in Bristol, England, in the mid-1970s. The Bristol Evening Post began receiving letters from residents who said they had heard unexplained sounds and wanted to know what was making them.
One proposed explanation pointed to a large industrial fan operating inside a department store warehouse. However, reports continued even after the warehouse was closed several years later.
Similar reports later emerged in other parts of the UK, particularly in coastal areas such as Hythe, Plymouth, Southampton and Swansea. There were also reports from London.
This mysterious sound became known as the hum phenomenon, or simply hum.
It first began to be reported in the United States in the 1990s in Taos, New Mexico, and Kokomo, Indiana. The phenomenon has since been recorded in Canada, Australia, New Zealand, South Africa, and several European cities. Most reports tend to come from relatively densely populated areas.
According to the Norwegian Broadcasting Corporation (NRK), residents around Oslo also reported an unexplained hum several years ago.
Canadian teacher Glenn McPherson first heard The Ham while living and working on Canada’s west coast. After moving to another city in the same area, I no longer hear that sound.
His curiosity eventually led him to found the interactive The World Hum Map and Database Project in 2012. The project collects reports and location data from people who say they have experienced the sounds.
Possible sources range from mechanical to natural
Researchers and the public have proposed many explanations for The Ham. Possible causes include acoustic pollution from human activities, naturally occurring environmental sounds, and conspiracy theories involving the CIA or even aliens.
Many technologies can generate low-frequency noise, such as ventilation systems, heat pumps, road traffic, and wind turbines. Nature also generates this range of sounds through sources such as waves crashing on the shore and wind blowing across the landscape.
This mystery has captured the attention of audiologists and hearing researchers around the world. One of them is Markus Drexl, a professor at the Norwegian University of Science and Technology (NTNU).
Drexl, two postdocs, and a postdoc studied 28 people in Germany who reported hearing unexplained humming or buzzing sounds.
Test if the hum comes from outside
The researchers considered two main hypotheses.
The first was that hum could be an externally generated, measurable sound. Such noise can originate from industries, infrastructure, or natural phenomena that generate low frequencies.
“We know that some people hear measurable low-frequency sounds that other people can’t hear. But finding the source of these sound waves isn’t so easy because low-frequency sounds are difficult to locate,” Drexl said.
Low-frequency sound waves have long wavelengths, so they can travel great distances. This makes it especially difficult to identify its source.
Most participants did not have special hearing
The researchers first investigated whether participants were unusually sensitive to known low-frequency sounds.
Most showed no special abilities in this range. Only two people had better than average hearing in specific bass frequencies.
“Although the group we tested was small, that still means that the hypothesis that low-frequency sound hearing is particularly good is not true for most people,” Drexl said.
However, he noted that there are important limitations. Slight variations in hearing thresholds (microstructures) may allow some people to detect sounds within a very narrow frequency range, such as 50 to 51 hertz. Standard hearing tests are not designed to pick up differences this accurately.
Could your inner ear be producing the noise?
The cochlea in the inner ear naturally produces faint sounds at a variety of frequencies, usually around 500 to 5000 hertz. These noises serve no direct purpose. Instead, they are a byproduct of the ear’s physiological sound amplification process.
“Most of us don’t hear these sounds, but a small number of people can actually hear the sounds their ears themselves make, and these sounds can be objectively measured,” Drexl says.
These sounds, known as otoacoustic emissions, can be detected by placing a sensitive microphone inside the ear canal. In certain people, spontaneous otoacoustic emissions may be perceived as painful tinnitus.
“One of the hypotheses was that the participants in our group could hear low-frequency otoacoustic emissions. That’s why we tested whether they had otoacoustic emissions,” Drexl says.
But…the answer was no.
Often can be explained by low frequency tinnitus
“Some people hear something that cannot be measured objectively. We believe that people in this category have a type of low-frequency tinnitus,” Drexl said.
Tinnitus, often described as ringing in the ears, occurs when a person perceives a sound in their ears or head, even though there is no external sound source.
Tinnitus can be temporary or persistent. People often initially interpret the sound as coming from their surroundings.
However, if the noise persists even after changing locations, you may eventually realize that the noise is not from your immediate vicinity.
Based on current knowledge about hearing and the results of participant testing, Drexl believes the most likely explanation has two parts.
The few people who hear a hum may indeed have unusually sensitive low-frequency hearing. But for most people, the experience is likely a type of tinnitus, where the sound starts within the auditory system.
“Based on our results, we do not rule out the case of physical external sound sources, but we believe that subjective tinnitus in the low-frequency range is often responsible for auditory pulsations in the perception of low-frequency sounds,” he said.
Scientists need a better understanding of low-frequency hearing
Drexl became interested in The Ham through his research into infrasound.
“What we know about the auditory system is largely based on how it perceives and processes high-frequency sounds. We know less about how the auditory system processes and processes low-frequency sounds, or infrasound,” he says.
Over the past decade, there has been growing concern about noise from technological sources, especially in the low frequency range (approximately 20-250 Hz) and infrasound (below 20 Hz).
“If we want to conduct a thorough assessment of infrasound and infrasound, we first need to better understand how sensory systems process infrasound and infrasound,” he said.
