Audible and Invasive Low-Frequency Humming Sounds;
Their Detection, Measurement, and Possible Causes
by Michael Theroux

The perception of the phenomena of invasive low-frequency humming sounds by persons all over the globe has existed for many centuries. Most often referred to generically as "The Hum," this troublesome noise is described by those who hear it (it is not audible to all people) as a persistent, distant idling diesel engine. These "hums" have been reported in a variety of geographical locations and in some cases a source has been located. The most popular case involving the hum's perception was reported in Taos, New Mexico, and thus the Hum is sometimes called the "Taos Hum." Hums have been reported all over the world, especially in Europe. A Hum on the Big Island of Hawaii, has typically been related to volcanic action, and is heard in locations dozens of miles apart. Oddly, the sensation or hearing of the Hum is most often experienced by men. Typically the Hum is difficult to detect with microphones because of its very low frequency, and its source and nature are very difficult to localize. The Hum is often prefixed with the name of a locality where the problem has been particularly publicized: e.g., the "Bristol Hum" or the "Taos Hum".

Description
The essential description that defines the experience of the Hum is that of a persistent low-frequency sound, often reported as being comparable to that of a distant diesel engine idling, or to some similar low-pitched sound for which obvious sources (household appliances, traffic noise, etc.) have been ruled out.

Other phenomena significantly associated with the Hum, being reported by an large proportion of hearers, is that it is more intense inside buildings as compared with outdoors, that it can be felt through the body, and that the use of earplugs is useless in decreasing the Hum. The Hum is often perceived more intensely during the night.

Some people perceive the Hum continuously, but others perceive it only during certain periods. For some people, the perceived Hum can represent a faint sound and a mild annoyance, while for others who perceive the Hum's sound and/or vibrations more intensely it represents a nuisance that can seriously interfere with daily activities. Common consequences include a lack of sleep, as the Hum can keep some people awake or wake them in the middle of the night. Such cases have given rise to the expression "Hum sufferers."

History
While such hums have been occasionally reported throughout history, it was not until the 1990s that a surge in the frequency of reports of the Hum began. 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, mostly in the United Kingdom but also in other countries such as New Zealand. 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.

Explanations
Some explanations of hums, for which no definitive source has been found, have been put forth. These include:

- Man-made noises
High frequency attenuation of distant industrial sounds, trains (of particular interest to our research), stereo subwoofers from homes and cars, freeway traffic, and even top secret military projects have been suggested as possible explanations of this Hum. As sound moves through the atmosphere or ground, the high frequencies decrease in amplitude more rapidly than the low frequency ones, which subsequently travel greater distances. The low-frequency sounds can be focussed by walls and structural geometry explaining why it is more intense inside buildings. Industrial machinery such as compressors, pumps and fans can also produce similar types of sounds. Although this is one of the explanations that first come to mind, ordinary microphones have failed to detect the Hum (due to their frequency response being narrower than the generated sound of the Hum) and investigations have failed to convincingly trace the Hum to such sources. Some studies in the UK have addressed this issue.

- Infrasound made by geological events
Infrasound from many natural sources, possibly geologic or plate tectonic in nature has been suggested, and in some locales, may actually be the cause. Some scientists state that electrophonic effects may also be caused by lightning strikes, very bright auroras, and earthquakes

- Pulsed microwaves
A phenomenon similar to the microwave auditory effect from pulsed microwave sources, possibly in combination with other factors, has been suggested. The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of audible clicks induced by pulsed/modulated microwave frequencies. The clicks are generated directly inside the human head without the need of any receiving electronic device. The effect was first reported by persons working in the vicinity of radar transponders during World War II. These induced sounds are not audible to other people nearby. The microwave auditory effect was later discovered to be inducible with shorter-wavelength portions of the electromagnetic spectrum. The reported incidence of these noises has steadily increased with the increase in cell phone usage. While these sounds are in no way related to the descriptions of the Hum, various types of electromagnetic sources could involve different physical or physiological mechanisms or a combination thereof.

- Electromagnetic waves caused by meteors
A variant of the audio frequency electromagnetic emissions generated upon the entry of a meteor and its disintegration in the upper atmosphere. The disintegration of larger meteors in the upper atmosphere is known to release megawatts of power in the audio frequency range, primarily through the interaction of the resulting ionization trail with the Earth's magnetic field. For centuries, humans have reported hearing unexplained noises in conjunction with meteors including "thunder-like sounds" at the scene of the Tunguska event on June 30, 1908. Astronomer Edmund Halley collected several such accounts after a widely-observed meteor burned up in the sky over England. The Leonid meteor shower in November 2001 also led to many reports of observers hearing crackling or fizzing noises. Similar observations have been reported by soldiers near the site of nuclear explosions. It is also speculated that the "solar wind" may be causing a similar effect to the "meteor audio effect."

- Extremely low frequency communications systems
Communication systems, such as submarine communications systems that use extremely low frequency (ELF) radio transmissions. Proponents of this theory suggest the transmissions may somehow produce effects either directly or indirectly through mechanisms similar or different of those by which higher frequencies are detected. Powerful VLF waves can induce physical vibrations in objects, which are transmitted to the air as sound waves. On Colin Keay's geophysical electronphonics website he defines the field of geophysical electrophonics as "the production of audible noises of various kinds through direct conversion by transduction of very low frequency electromagnetic energy generated by a number of geophysical phenomena." See December 26, 2008 email from Patrick Bryant at the end of this article.

- Ionospheric heating systems
Large-scale effects of one or several of the ionospheric heating projects in Norway, the U.S. or Russia, such as HAARP in Gakona, Alaska have been suggested. Other than the possible effects from geophysical electrophonics, no direct evidence has been produced.

- Tinnitus
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. Some people who claim to hear the Hum say that it is worse indoors. This would lean towards tinnitus, as tinnitus is generally worse in places with less exterior sound. 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, which only serves to decrease other environmental noise, thus making the Hum more apparent.

Empirical Evidence
In the mid and late 1990s, myself and researcher Gerry Vassilatos undertook a study of the Hum phenomenon. We were both "hearers" of the Hum each in our own locales (myself in Bayside, California - Vassilatos in Staten Island, New York). Our aim was to discover the source of the Hum, to make a recording of it to analyze its properties, and to look into the possibility of developing technology to cancel it out. From my background in pipe organ tuning, and the design and construction of speaker enclosures, I decided to experiment with a type of Hum receiver for recording the sound. The recording apparatus consisted of a tuned bass-reflex port - stopped similar to a organ diapason for tuning puposes, and to ensure a deficiency of upper partials. I then attached the suction cup of a modified telephone pickup coil to the port. A 10ft. cable was run from the trancducer to the recording machine which is a simple portable DC powered deck capable of running at different tape speeds. Tuning of the port was arrived at by formula and empirical experience. The pickup coil is modified with two neodymium-iron-boron magnets and a second coil attached to the rear of the unit. The system was initially amplified in real time, and different tape speeds were used to ensure the recording was picking up the HUM and not artifact from the recording apparatus. The sound on the original recording is exactly as experienced.

Bayside Hum spectrogram (1996)