In my view there are four hypotheses for the source of the Worldwide Hum that survive trivial scrutiny. So I start with VLF radio waves. What I present below is my best guess about what is happening and how radio theory suggests a specially designed Faraday-type unit – what I call the Deming Box – that can test the VLF hypothesis.
The interaction of VLF and ELF electromagnetic (EM) radiation with human tissue is being actively and intensely researched. If we go back to 2002 we can see an example from Panagopoulos, Margaritis, and Karabarbounis of one workable theory to explain how pulsed radio waves at lower frequencies can activate human nerve cells. For more than 50 years, increasing numbers of powerful VLF transmitters (mobile, stationary, and airborne), have been in operation. The VLF radio spectrum comprises frequencies between 3 kHz and 30 kHz. By line of sight, ground wave, sky wave, antipodal focusing, and geomagnetic coupling, the surface of planet Earth is riddled with zones of modulated high intensity VLF EM radiation. A small proportion of people – I now estimate no more than four percent of the adult population – have auditory systems that are sensitive to lower sound frequencies and the type of biological radio wave activation described in the above paper. They may be able to detect the Hum in many places on Earth solely by interaction with VLF radio, while there may be another group of people who need some extant sub-audible low frequency sound or infrasound at certain frequencies in order to create sufficient auditory activation that, together with VLF radio energy, would be interpreted as sound. Sources of industrial infrasound, such as mining, hydro-electric projects, windmills, high pressure gas pipelines, and massive construction projects may or may not be prerequisite or aggravating factors in some settings. When there is strong but sub-audible infrasound such as from a large industrial site, it may take little VLF exposure in order to activate the auditory system. High levels of ambient noise during the day from traffic, industry, mechanical devices, and other people, often mask the Hum and explain why the Hum is stronger at night when society has quieted down somewhat, and why the Hum can become very loud in sound-reduced rooms. If the Hum is rooted in VLF energy, it is therefore affected by the behaviour of the Earth’s magnetic field and by the height and layers of the ionosphere, solar activity, the time of day, and the season. The Hum also is rooted in the particular radio frequencies that governments use for communication. Therefore, when a powerful VLF transmitter suddenly stops broadcasting or changes frequency, this will cause a simultaneous change in the Hum at multiple locations across the planet. During a big solar storm, anything could happen. The Hum can also slowly drift over an area as do the entry and exit points for geomagnetic conjugate magnetic field lines.