This is part of a crash course in physics that will get you ready for discussing theories surrounding the Hum. If you took high school physics, then you might know this already.
Electromagnetic radiation, which includes visible light, microwaves, infrared, x-rays, radio waves, etc., travels as two connected waves, one of them electric, the other magnetic. You can visualize it like this:
Electromagnetic (EM) energy travels at the speed of light, which is about 300, 000 km/s or about 186, 000 miles per second. The word frequency refers to the vibration of the wave; that is, how many times per second the wave is oscillating, or vibrating. Frequency is measured in Hertz, abbreviated as Hz. For example, if you are listening to an FM radio station at 92.3 MHz (M means Mega, or million), this means that the radio waves are vibrating/oscillating at 92.3 million times per second. The wavelength (the distance between the wave peaks) in this case would be about 3.2 metres (about 10 feet 6 inches). The higher the frequency, the shorter the wavelength, and visa-versa. Your wireless computer network might use a frequency of 2.5 GHz (2.5 billion vibrations per second), and the wavelength would be about 10.2 cm or about 4 inches.
Here is a chart that shows you the various frequencies and wavelengths of the different types of EM energy:
VLF (Very Low Frequency) radio waves have frequencies between 3 kHz to 30 kHz (3000 Hertz through 30000 Hertz). These wavelengths can be as long as 100 km. It is this type of EM signal which I suspect might play a major role in the Hum.
Interesting things can happen when two waves collide. In the picture below, we see waves spreading out from two point sources. Notice the patterns created:
This is an example of an “interference pattern”. In some places, you will experience a much stronger wave, and in some places a very weak or flat signal.
Keep the above in mind when you read the section on VLF propagation.