High-pressure gas pipelines Industrial-facilities mechanical engineer Steve Kohlhase spent $30,000 on legal fees and equipment related to his independent investigation of the low-frequency hum. Garret Harkawiks' 2019 documentary film
Doom Vibrations focused on Kohlhase's ten year journey to figure out what was causing the noise, and his theory behind it. In all reported cases Kohlhase studied, he said that the locations were along high-pressure gas pipelines, or at least in close proximity to them. Baguley said for example the noise can be attributed to environmental causes, such as industrial machinery at a nearby factory or an industrial fan. After those devices were corrected, however, reports of the hum persisted. Three hums have been linked to mechanical sources. The
West Seattle Hum was traced to a
vacuum pump used by CalPortland to offload cargo from ships. After CalPortland replaced the silencers on the machine, reports of the hum ceased. Likewise, the
Wellington Hum is thought to have been due to the
diesel generator on a visiting ship. A 35 Hz hum in
Windsor, Ontario, is thought to have originated from a
steelworks on the industrial zone of
Zug Island near
Detroit, with reports of the noise ceasing after the
U.S. Steel plant there ceased operations in April 2020. One hum in
Myrtle Beach, South Carolina, was suspected of originating at a
Santee Cooper substation almost two miles away from the home of a couple who first reported it. The substation is home to the state's largest
transformer. One local couple sued the
power company for the disruption the hum was causing them. The hum was louder inside their house than out, in part, they believed, because their house vibrated in
resonance to the 60 Hz hum. In the lawsuit they claimed that the volume of the hum was measured at up to 64.1 dB in the couple's home. Some researchers speculate that the
very low frequency radio waves or
extremely low frequency radio waves of the military
TACAMO system, used by aircraft to
communicate with submarines, might be the source for the hum. David Deming observes that the difficulty of locating a source of the hum could be attributed to its broadcast from moving aircraft in this fashion, although he notes that there have never been any reports of the Hum around the
U.S. Navy's stationary broadcast stations at
Cutler, Maine, and
Jim Creek, Washington. Deming considers it significant that the Hum "avoids publicity", often subsiding in response to an increase in local press coverage, and speculates that this may be a sign that the source is
anthropogenic in nature. Tinnitus is generated internally by the auditory and
nervous systems, with no external stimulus. While the Hum is hypothesized by some to be a form of low frequency tinnitus Long-term use of
ibuprofen is associated with increased risk of developing
hearing damage. It was suspected to have relationship to the hum.
Spontaneous otoacoustic emissions Human ears generate their own noises, called
spontaneous otoacoustic emissions (SOAE). Various studies have shown that 38 to 60 percent of adults with normal hearing have them, although the majority are unaware of these sounds. The people who do hear these sounds typically hear a faint hissing (
cicada-like sound), buzzing or ringing, especially if they are otherwise in complete silence. Hence, researchers who looked at the Taos Hum considered otoacoustic emissions as a possibility.
Jet stream Philip Dickinson suggested at an
Institute of Biology conference in 1973 that the 30- to 40-Hz hum could be a result of the
jet stream shearing against slower-moving air and possibly being amplified by
power line posts, some of which were shown to vibrate, or by rooms which had a corresponding
resonant frequency. Geoff Leventhall of the Chelsea College Acoustics Group dismissed this suggestion as "absolute nonsense". A hum previously reported in
Sausalito, California, was traced to the mating call of male midshipman fish. However, in that case the hum was resonating through
houseboat hulls and affecting the people living on those boats. In the West Seattle case, the University of Washington researcher determined that it would be impossible for any resonating hum, transmitted via tanker or boat hulls, to be transmitted very far inland, certainly not far enough to account for the reports. The
Scottish Association for Marine Science hypothesised that the nocturnal humming sound heard in
Hythe, Hampshire, could be produced by a similar "sonic" fish. The council believed this to be unlikely, since such fish are not commonly found in inshore waters of the UK. As of February 2014, although the source had still not been located, the Hythe hum had been recorded. ==Treatment==