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.
Other elements seem to be significantly associated with the Hum, being reported by an important proportion of hearers, but not by all of them. Many people hear the Hum only, or much more, inside buildings as compared with outdoors. Many also perceive vibrations that can be felt through the body. Earplugs are reported as not decreasing the Hum. The Hum is often perceived more intensely during the night.
In the Unsolved Mysteries segment called "Mystery Hum", a tape re-creation of the Taos Hum was used for this segment. Robert Stack reported that one of the "Hum sufferers" created the audio tape, mainly for the purpose in that particular segment. This was done since their audio equipment didn't pick up low-frequency sounds very well, and so that the show's viewers and other non-"Hum sufferers" would get an idea of what the actual auditory phenomenon sounded like.
On 15 November 2006, Dr. Tom Moir of the University of Massey 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. 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.
It was during the 1990s that the Hum phenomenon began to be reported in North America and to be known to the American public, when a study by the University of New Mexico and the complaints from many citizens living near the town of Taos, New Mexico, caught the attention of the media. However, in the 1970s and 1980s, a similar phenomenon had been the object of complaints from citizens, of media reports and of studies. It is difficult to tell if the Hum reported in those earlier cases and the Hum that began to be increasingly reported in North America in the 1990s should be considered identical or of different natures. During the last decade, the Hum phenomenon has been reported in many other cities and regions in North America and Europe and in some other regions of the world.
Some possible explanations:
Some explanations of hums for which no definitive source has been found have been put forth. These include:
Generated by the body, the auditory or the nervous system, with no external stimulus. However, the theory that the Hum is actually tinnitus fails to explain why the Hum can only be heard 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 fact that some people in the population perceive the Hum and others do not.
While hypothesized 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. More mystery is added as some only notice the Hum at home, while others hear it everywhere they go. Some reports indicate that it is made worse by attempted soundproofing (e.g., double glazing), which only serves to decrease other environmental noise, thus making the Hum more apparent. Tinnitus is also generally worse in places with less exterior sound.
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.
Spontaneous otoacoustic emissions
Human ears generate their own noises, called spontaneous otoacoustic emissions, which about 30% of people hear. The people that hear these sounds typically hear a faint buzzing or ringing, especially if they are otherwise in complete silence, but most people don't notice them at all.
Colliding ocean 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.