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The Hum is a phenomenon, or collection of phenomena, involving a persistent and invasive low-frequency humming, rumbling, or droning noise not audible to all people. Hums have been widely reported by national media in the UK and the United States. The Hum is sometimes prefixed with the name of a locality where the problem has been particularly publicized: e.g., the "Bristol Hum", the "Taos Hum", or the "Bondi Hum".
Data from a Taos Hum study suggests that a minimum of two percent and perhaps as many as 11 percent of the population could detect the Taos Hum and the Daily Telegraph in 1996 likewise reported a figure of two percent of people hearing the Bristol Hum.:575 For those who can hear the Hum it can be a very disturbing phenomenon and it has been linked to at least three suicides in the UK. However, amongst those who cannot hear the hum and some specialists, there has been skepticism about whether it, in fact, exists.
The essential element that defines the Hum is what is perceived as a persistent low-frequency sound, often described as being comparable to that of a distant diesel engine idling, or to some similar low-pitched sound for which obvious sources (e.g., household appliances, traffic noise, etc.) have been ruled out. There are a number of audio reproductions of the Hum available on the web.
Other elements seem to be significantly associated with the Hum, being reported by an important proportion of hearers, but not by all of them. Some people hear the Hum only, or much more, inside buildings as compared with outdoors. Some perceive vibrations that can be felt through the body. Earplugs are reported as not decreasing the Hum.
The Daily Telegraph reported that two percent of people could hear the Bristol Hum. Research into the Taos Hum indicated that between two percent and 11 percent of people could hear the hum, with the actual figure likely being at the lower end of the range. The hum does appear to be geographically focused, i.e. it does appear to be possible for hearers to move away from it; the range of the Taos Hum was reported to be 48 km to 72 km. Women may be more likely to be affected than men. Age does appear to be a factor, with older people being more likely to hear it.:575–576
On November 15, 2006, Dr. Tom Moir of the Massey University in Auckland, New Zealand made a recording of the Auckland Hum and has published it on the university's website. The captured Hum's power spectral density peaks at a frequency of 56 hertz. The Taos Hum was between 40 to 80 hertz. Higher-pitched tones have also been reported; the Hueytown (Alabama) Hum has been compared the sound to that made by a dentist’s drill or the sound made by a fluorescent light bulb near the end of its life.:575
In 2009, the head of audiology at Addenbrooke's Hospital in Cambridge, Dr David Baguley, said that he believed people's problems with hum were based on the physical world about one-third of the time and the other two-thirds stemmed from people focusing too keenly on innocuous background sounds.
Only a handful of articles have been published in the scientific literature including Deming, 2004; Broner, 1978; Cowan, 2003; Fox, 1989; Leventhall, 2003; Mullins & Kelly, 1995, 1998; Hanlon, 1973; Vasudevan & Gordon, 1977; Wilson, 1979.
The World Hum Database and Mapping Project was launched in December 2012, in order to build detailed mappings of hum locations and to provide a database of Hum-related data for professional and independent researchers.
More than 2,000 people reported hearing sounds dating back to the 1940s in the London and Southampton areas of Great Britain. Deming cited Glasgow, Scotland's Sunday Herald 1995 report claiming that the Hum was, "first reported in the late 1950s when people in Britain began to report hearing a most unusual noise—a combination of a humming, droning, and buzzing sound.":573
In Britain, the most famous example was the Bristol Hum that made headlines in the late 1970s.
Since the 1980s, the Hum has been bothering people living in coastal towns in the west coastal area of Scotland including Largs, a coastal town about 31 km west of Glasgow.:574
The University of New Mexico undertook studies of hum sufferers in Taos. One of the researchers reported that the Hum was close to 66 hertz, two octaves below middle C, although it could go as low as the lowest E on a piano.
An ongoing low frequency noise, audible only to some, is thought to originate somewhere near this town and is consequently sometimes known as the Taos Hum. Those who have heard the Hum usually hear it west of Taos near Tres Orejas. The Taos Hum was featured on the TV show Unsolved Mysteries, and it was also briefly mentioned in an episode of The X-Files.
Kokomo, a city of 47,000, allocated $100,000 in 2002 to investigate a hum after nearly 100 complaints were made since 1999. Some sufferers blamed physical symptoms on the hum, including headaches, nausea, diarrhea, fatigue, and joint pain, with one reporting that her health improved when she moved out of the town. In November 2002, Acentech was hired by the Board of Public Works and Safety of the City of Kokomo to investigate the Hum. Following a public meeting held 2 December 2002, Acentech investigation of acoustic sources did not find any conclusive cause and suggested non-acoustic phenomena, such as microwave (radio frequency) hearing, electrosensitivity, chemical sensitivity, hypersensitivity to natural geomagnetic phenomena may cause the "types of symptoms that these people are experiencing."
The Hum has also been heard since before 2003 by residents on Canada's southwest Coast in the region around the city of Vancouver.:43
This phenomenon, first noticed in 2009, has also been reported since 2011 throughout Windsor and Essex County in Ontario, Canada. A 2011 study by Earthquakes Canada indicated that it may be originating from the heavily industrialised Zug Island area on the US side of the Detroit river. A two-hour telephone town hall meeting in 2012 received calls from 13,000 residents, with another 9,000 leaving comments over the next week, although not all of those were from people who could hear the hum.
In 2013 the Canadian Government allocated $60,000 for research by the University of Windsor to determine the source of the noise. As of April 2013[update], a Canadian scientist is using sound-level meters and a portable "pentangular array" of cameras and microphones to try and precisely identify the source of the sound, in order to know who exactly to ask to fix it.
The volunteer Ranchlands Noise Investigation Team began investigating the Ranchlands hum in 2008. Marcia Epstein, an acoustic ecologist at the University of Calgary and a member of the Ranchlands Noise Investigation Team, described the hum as "a concentration of frequencies around 40 hertz, and 40 cycles per second", sometimes described as a "vibrational feeling", affecting "12 to 20 per cent of the community."
The Hum has also frustrated residents in County Kerry, Ireland. This led to it being raised in the Irish Parliament by Michael Healy-Rae, who personally heard the Hum. The official response was described by Healy-Rae as "away with the fairies gobbledygook."
In Pisa, Italy the ISTI institute for Science and Technology of the Italian CNR researchers are reporting of a very disturbing hum especially in the area of the High Performance Computing Laboratory. After some investigations, people from the maintenance department have declared the problem solved. The hum remains and annoyed people remain as well.
Loud sounds were heard in many places of this city, these noises commonly are heard by the night 
Loud industrial sounds were heard mostly in downtown at afternoon.
Some explanations of hums for which no definitive source has been found have been put forth. These include:
A suggested diagnosis of tinnitus, a disturbance of the auditory system, is used by some physicians in response to complaints about The Hum. Tinnitus is generated internally by the auditory and nervous systems, with no external stimulus. However, the theory that the Hum is actually tinnitus fails to explain why the Hum can be heard only at certain geographical locations, to the degree those reports are accurate. There may exist individual differences as to the threshold of perception of acoustic or non-acoustic stimuli, or other normal individual variations that could contribute to the perception of the Hum by some people in the population and not by others.
While the Hum is hypothesized by some to be a form of low frequency tinnitus such as the venous hum, some sufferers claim it is not internal, being worse inside their homes than outside. However, others insist that it is equally bad indoors and outdoors. Some people notice the Hum only at home, while others hear it everywhere they go. Some sufferers report that it is made worse by soundproofing (e.g., double glazing), which serves only to decrease other environmental noise, thus making the Hum more apparent.
People who both suffer from tinnitus and hear the Hum describe them as qualitatively different, and many hum sufferers can find locations where they do not hear the hum at all. An investigation by a team of scientists in Taos dismissed the possibility that the Hum was tinnitus as highly unlikely.
Human ears generate their own noises, called spontaneous otoacoustic emissions, which affects between 38 percent to 60 percent of people, although the majority are unaware of these sounds. The people who hear these sounds typically hear a faint buzzing or ringing, especially if they are otherwise in complete silence. However, these emissions occur with equal frequency across age groups within the population, and the Hum typically occurs in regional clusters and to older people.:575–576 Recordings of sounds that appear to be the Hum, such as that made in Auckland, would indicate that otoacoustic emissions cannot explain all occurrences of the Hum.
On early June, 2008 an article published in Proceedings of the Royal Society announced the location of a "hum hotspot", an "energetic source area stretching from the Labrador Sea to south of Iceland, where wind patterns are especially conducive to generating oppositely traveling waves of same period, and the ocean depth is favourable for efficient microseism generation through the ‘organ pipe’ resonance of the compression waves." Researchers from the USArray Earthscope have tracked down a series of infrasonic humming noises produced by waves crashing together and thence into the ocean floor, off the North-West coast of the USA. Potentially, sound from these collisions could travel to many parts of the globe.
In the case of Kokomo, Indiana, a city with heavy industries, the origin of the hum was thought to have been traced to two sources. The first was a pair of fans in a cooling tower at the local DaimlerChrysler casting plant emitting a 36 Hz tone. The second was an air compressor intake at the Haynes International plant emitting a 10 Hz tone. After those devices were corrected, however, the hum persisted.
A hypothesis put forward by the Scottish Association for Marine Science blames a nocturnal humming sound heard in Hythe, Hampshire in the UK on the male Midshipman fish's mating call. Although this can not be the cause because the Midshipman fish is not native to the Solent.