If you are constantly plagued by a bizarre humming noise, scientists now have an explanation for it. This low-frequency sound, often experienced as a vibration, is heard by millions of people across the world.
It is not easy to hear outdoors but often appears indoors, especially when lying in bed at night. While it may sound like there is a car outside, there is nothing to see – and others in the same place might not hear anything at all.
Theories about the cause of this phenomenon range from acoustic 'pollution' originating from human sources to sounds from nature and even noises produced by the ear itself.
'We know that there are people who hear low-frequency sounds that can actually be measured, even if other people don't hear them,' said Professor Markus Drexl from the Norwegian University of Science and Technology. 'But it is not so easy to find the source of these sound waves, because it is a struggle to localize low-frequency sounds.'
After exploring the phenomenon, his team concluded that the distinctive hum can be attributed to two different things – either having very good low-frequency hearing or suffering from a form of tinnitus.
The History of The Hum
The phenomenon was first recorded and discussed in Bristol in the mid-1970s, when a local newspaper began receiving letters from residents who reported hearing an inexplicable sound. One theory was that the humming sound came from large industrial fans located inside a warehouse of a large department store. However, when the warehouse was closed down a few years later, people continued to hear the sound.
Since then, the sound has been recorded in several places in the UK, mainly in coastal cities such as Hythe, Plymouth, Southampton, and Swansea – but also in London. The sound is called The Hum phenomenon, or simply The Hum.
In the 1990s, it cropped up in the US, first in Taos, New Mexico, and in Kokomo, Indiana. The phenomenon has since been recorded worldwide – in Canada, Australia, New Zealand, South Africa, and several European cities. The sound is typically reported in relatively densely populated areas. A couple of years ago, people in the Oslo area also reported an unexplained humming sound, according to the Norwegian Broadcasting Corporation (NRK).
Possible Explanations
Extra Good Hearing?
The first thing researchers did was test whether participants had particularly good hearing for low-frequency sounds that are actually known to exist. Most did not, except for two participants who had better hearing than average at certain low frequencies. '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,' Professor Drexl said.
The Ear Producing Sounds?
The cochlea in the inner ear itself produces weak sounds with different frequencies, typically between about 500 and 5000 Hertz. These sounds have no function of their own but are a by-product of the body's process of making certain sounds louder. 'Most of us don't hear these sounds,' Professor Drexl said. 'However, a few people can actually hear the sounds that the ear itself produces. And these sounds can be measured objectively.'
These particular sounds are called oto-acoustic emissions and can be detected by placing a sensitive microphone in the ear canal. In some people, these sounds can be experienced as troublesome tinnitus. 'One hypothesis was that the participants in our group could hear oto-acoustic emissions at low frequencies – that is why we tested whether they had them,' Professor Drexl added. However, the study, published in the journal PLOS One, revealed none of them did.
Sounds That Cannot Be Measured
'Then there are people who hear something that cannot be measured objectively,' the professor said. 'We believe people in this category have a form of low-frequency tinnitus.' Tinnitus, or ringing in the ears, is when you hear a sound in the ear or in the head that is not caused by an external sound source. Many people experience tinnitus, either permanently or for shorter periods. These individuals first experience the sounds in their ears as a sound coming from outside.
So, What Is It?
Professor Drexl said that based on what is known about hearing and the tests they conducted on study participants, the best explanation is twofold. A few people who hear The Hum actually have particularly good low-frequency hearing. However, for most people, it may be a form of tinnitus. '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.
Sources of Low-Frequency Sound
Human sources: ventilation systems, heat pumps, traffic noise, windmills. Natural sources: waves crashing along the coast, wind sweeping through landscape.
