Home » hum research » The Physics of the Hum: A Primer on Propagation of Very Low Frequency (VLF) Radio Waves for the Layperson – Part One

The Physics of the Hum: A Primer on Propagation of Very Low Frequency (VLF) Radio Waves for the Layperson – Part One

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Follow World Hum Map and Database Project on WordPress.com

(Note to working scientists and those with scientific credentials: I realize that I have glossed over and simplified some of this subject matter. This is not a scientific journal article, but rather an attempt to bring everyday people to the point where they can follow what I am proposing. If anybody would like to pursue the details of this with me, by all means contact me).

After I first noticed the Hum, a web search quickly led me to David Deming’s 2004 paper, “The Hum: An Anomalous Sound Heard Around the World”. My first reaction – fascination and relief – was similar to many people around the world who write to me when they find the Hum Map (www.thehum.info). In his paper,  Deming lays out the logic and evidence that suggest the Hum could be caused by Very Low Frequency (VLF) radio waves used for naval communication. Building on that work, I propose a mechanism that explains the sometimes elusive behaviour of the Hum Phenomenon.

But many of us do not have strong backgrounds in physics or biology, and therefore I am presenting a series of brief crash courses on some basic topics so that greater numbers of people – hearers in particular – can understand what I am proposing. In this post, we will look at how VLF radio waves travel.

Read the primer on EM radiation if you have not done so. We are not discussing low frequency sounds here; that is a completely different type of energy.

FM radio stations fade out quickly as we move away from the transmitter. VLF energy, however, can travel tremendous distances – right around the planet, in fact. That is why the world’s major powers use VLF radio for communication. But VLF radio has another attractive property: an extremely large “skin depth”, which means that VLF radio waves can penetrate materials, such as ocean water, to a much greater distance than radio waves with higher frequencies, such as AM radio, microwaves, and so on. This is why VLF is used to communicate with submerged submarines. VLF radio waves can also penetrate earth as deeply as 150 m. I’ve written previously about the irony of the tinfoil hat being the icon for clinical paranoia, because microwaves have a very small skin depth and a thin layer of foil can, indeed, block and reflect microwave energy (which is why we don’t put aluminum foil in the microwave oven).

VLF waves travel (propagate) in three ways. First, and less important here, is the direct wave that travels line-of-sight distances directly to the receiver. Second is the “ground wave”, which runs between the surface of the Earth and the bottom layer of the ionosphere (an electrically charged layer in the atmosphere). The third is the “skywave”, in which the VLF wave bounces from the ground to the ionosphere and back to the ground, sometimes making multiple “hops” around the globe.

The Ground Wave. Within 100 to 300 km (60 to 180 miles) of the transmitter, the ground wave provides most of the energy of a received VLF signal. The strength of the signal can be sharply affected by mountain ranges, mineral deposits in the ground, ice fields, etc. (http://www.navy-radio.com/manuals/0101-1xx/0101_113-02.pdf). Because the signal spreads out in all directions, the ground wave signal becomes weaker as you move away from the transmitter. But something interesting happens beyond a certain distance from the transmitter: the waves start converging (coming together) and refocus on the opposite side of the planet, at a place called the “antipodal point”. This was first tested in 1925 by Tremellen, Eckersley, and Lunnon (http://ia801505.us.archive.org/5/items/noteonantipodalf182wait/noteonantipodalf182wait.pdf). Let’s take a specific example. The VLF transmitter at Cutler, Maine was, and could still be, one of the most powerful transmitters on the planet, on any frequency. The antipodal point for Cutler Maine is located in the Southern Ocean between Australia and Antarctica. The signal strength at that location coming from VLF Cutler will be much stronger than in, say, South America, which is thousands of kilometres closer to the transmitter. Contact me if you would like further references in this area. To find your antipodal point, click here.

The Skywave. First some basics about the Earth’s atmosphere – specifically, the layers that comprise the ionosphere. The ionosphere is a layer of electrically charged particles, created mainly by the impact of Ultraviolet (UV) radiation from the sun that strikes the atmosphere. Because the particles have an electric charge, they affect the propagation of radio waves. The ionosphere changes at night because the night side of the Earth is not being bombarded with the solar “wind” of UV and x-ray radiation. During a typical 24 hour cycle, layers will predictably form, disappear, and change in height. The ionosphere also changes with the seasons, as regions of the planet experience changing levels of solar radiation. The daytime and nighttime layers of the ionosphere are shown in the diagram below.

VLF_transmission

You will notice from the diagram above that the skywave will take a longer path to get to a receiver than the ground wave. That means that if you are in a location where you can receive both the skywave and the ground wave, you will receive two identical signals, slightly out of synch with each other. As explained in the primer on EM radiation, this creates what is called an “interference pattern”. I am simplifying this (and what is above) considerably, but in an interference pattern, there can be destructive interference (“dead zones”) and constructive interference (“hot spots).

But it gets more complicated. Suppose you are in, say, Portland, Oregon, relatively close to the massive Jim Creek VLF transmitter in Washington. The ground wave rushes toward you, but there is another ground wave that takes the long way around the planet in the opposite direction (great circle route) and strikes your location from the opposite direction. This can create what is called a “standing wave”, which oscillates in a particular location. http://nvlpubs.nist.gov/nistpubs/jres/68D/jresv68Dn1p27_A1b.pdf

Even professional physicists write about the massive complexity of this topic due to the fact that the models used to discuss this topic are just that – models, and the real world doesn’t always behave in the way that our equations might predict. The main idea from the above is that you could be located in a place that has very high levels of VLF energy, and yet be hundreds or even thousands of kilometers away from any transmitter. Also, local geography and geology cause variations in the strength of a received VLF signal. Changing your location by a little as a few dozen kilometers will change the signal strength, especially where oceans meet mountains and valleys/fjords. VLF travels differently at night from during the day, and differently during summer from in winter.


17 Comments

  1. Xabier says:

    I just completed the survey. Looking forward news on the Hum. I started hearing it in October 2013.

  2. Kristen moy says:

    I started hearing the hum April 2, 2014. I can only hope it will stop.

  3. Andy says:

    High frequency sky wave and ground wave causing new low frequency signals? Absolutely. But these frequencies are apparent to a stationary observer as a result of changing propagation distances and would never create a constant hum like this. Not ever. The transmitter(s) and receiver/observer would need to be moving with respect to each other (constantly and along the same vector in relation to the radio waves).

    • But there is a physically dynamic component: David Deming’s seminal paper on the topic suggested that the airborne and constantly circling aircraft of the TACAMO system and their high power VLF transmissions were to blame.

      • Andy says:

        That would be a circular path more than a straight line then. ie the frequency would be far more dynamic than what we experience. A constant drone.
        Interesting comment though. High power VLF from aircraft doesn’t sound like an easy thing to do.

      • I’m wondering about your science on this one. Feel free to have an expert comment, but standing EM waves, for example, as well as interference patterns, can be created by two static sources. Why do you think there needs to be motion involved?

      • Andy says:

        No worries. Standing waves are simply that. they don’t move. They have wavelength in a sense (as seen in the interference pattern) but no frequency because they are standing still (frequency is proportional to the speed past the observer divided by the wavelength). The frequency only becomes apparent when the observer moves through them or they, past the observer. For standing waves to be moving past a stationary observer, at least one of the sources must either be moving or changing frequency. In the latter case, The frequency (and possibly shape) of the standing wave would also change steadily. This is not the case.

        These interference patterns occur in both radio and acoustics, creating locations where the signal may be strong, weak (nulled) or somewhere in between, depending upon the summing effects of the original waves at that location. The signal strength at this location remains steady. To get a hum (cycling between strong and weak), the location must be continually changing or the propagation paths must be changing with respect to each other. Further more – for a continuous hum, the effective movement must be in a straight line (vectorially adding or reducing the distance). Varying propagation distance is unlikely to be a factor because the ionoshphere refracts signals in a far less mechanical manner to what we observe with the hum. This is almost a proof that sky waves and their resulting interference patters are not responsible.

        Without knowing what causes the hum, I can’t declare something to be impossible.. but this really sounds like the wrong suspect to me.

      • Here is an example: Standing waves in VLF transmissions produced by geographical discontinuities.
        R. BARR, Geophysical Observatory, Physics and Engineering Laboratory, DSIR, Christchurch, New Zealand
        and P. 0. HELM, Prime Minister’s Department, Wellington, New Zealand.

        Abstract-It has generally been accepted that the fields constituting the waveguide modes reflected from natural disontinuities in the lower boundary of the earth-ionosphere waveguide have a negligible amplitude compared to the fields constituting the incident modes at VLF (Kirchoff’s approximation). Recent measurements, in the region of the South Island of New Zealand, show cases where such reflections play a significant role in the determination of VLF field strength and phase.

  4. jimvandamme says:

    Would a beat frequency between VLF stations be a possible cause? That is, if say Lualualei and Jim Creek were broadcasting 2 MW at 100 Hz apart, the resultant beat could be detected by a nonlinear system (oxide/metal). Unlikely, because they are kilohertz apart to avoid interference with each others’ signals.

    The other possibility is that people are “hearing” the modulating waveform, which is in the low audio range. I did some preliminary design work on an antenna tuner for the VLF transmitters; the idea was to match the antenna dyamically, synchronized with the modulation. I don’t remember the exact waveform, but it was something like 200 Hz spacing at a 20 Hz rate (don’t quote me, I’ll look it up later).

    I found that I can pick up Cutler, Maine, from my house in central NY, using a Heathkit receiver hooked up to my VHF TV antenna. I’ve picked it up 180 feet underground in a mine with a loop antenna.

    • George G. says:

      Jim,
      I am very interested in your last paragraph! Please give more detail, specifically on Cutler TX frequency and your loop antenna circumference/orientation( horizontal or vertical)

      Cheers,

      G.
      P.S. Is Cutler a commercial station, beacon, AM, FM, etc. etc. Any info is welcome.

  5. George G. says:

    Moreover, at 180 feet underground, DID YOU HEAR THE HUM??? Or, are you talking about ‘Cutler?’
    Is the mine ‘active’ or dead??? Are there cables running vertically ground to 180 ft. below?? Is there a lid to this shaft?? Is your antenna be magnetic loop????? If you get another opportunity, could you please orientate your antenna through V to H and note Sig. strength, as well as direction?

    Your access to this mine may be a valuable tool, please keep us all informed.

    G.

  6. jimvandamme says:

    Sorry, I didn’t get your messages because I hadn’t checked off the reply notification button below. I haven’t been “silenced by the authorities”.

    I have some tinnitus, but don’t hear The Hum. I’m just interested…

    The Wiki article at https://en.wikipedia.org/wiki/VLF_Transmitter_Cutler tells you what a non-engineer needs to know about the Cutler VLF station. The site is pretty impressive with its large transmitter components and antenna. I worked at the USAF High Power Laboratory at Griffiss AFB for some years, doing similar R&D.

    We worked in a zinc mine in Gouverneur NY developing below-ground imaging techniques for ground penetrating radar. One experiment was determining the attenuation of the rock & soil at VLF. I built a loop antenna out of a 20 foot or so chunk of 50 pair telephone cable; I spliced the pairs in series to make a 100 turn coil that we laid on the floor of the mine. It was pretty quiet down there (no detected signals from LF up through microwaves) and we had to use expensive well-shielded inverters to power our instruments and find any signals. I transmitted using a repurposed 20 KHz switching power supply on the surface; the receiver was 180 feet down a 5 turn spiral mine entrance for large ore haulers. I don’t recall the 20 KHz attenuation numbers; I could also pick up the “local” Loran station (Caribou, ME, I think). You get a lot of attenuation through wet soil, so underground is a natural electrically quiet place.

    Unfortunately the zinc mine was closed; but there are lots of similar places. The fields inside my house are pretty low (cell, TV, radio reception is terrible without outside antenna) because the roof is steel and the walls have reflective aluminum insulation.

    It’s simple to use a small ferrite loop antenna to receive VLF, although exact quantitative measurements of field strength require a calibrated antenna. Finding polarization and direction of arrival is simple; you just turn the loopstick.

    • George G. says:

      Thank you Jim.

      Your 100 turn vertical (magnetic) loop at 180 feet below ground is indeed a gift to this observer.
      Thank you for sharing the data, you have saved me a lot of hard work.

      A 16 metre 10 turn vertical loop at the surface here picks up man-made noise only, no natural signals. A horizontal 7.5 metre 55 turn loop however is rich in natural information. Mounted on a pivot, it is also highly directional, as you can imagine.

      If I can ask another question; Did you ever encounter spurious transients during your tour underground??? (Specifically negative-potential spikes)

      Cheers for now, and thank you for your reply,

      G.

  7. jimvandamme says:

    Have you looked into whistlers? They are caused by lightning impulses reflecting between the poles. They’re in the audio band; you hook up an audio amp and record them directly (no demodulation), but you have to be far far away from manmade RFI (power lines mostly) and use battery powered gear. Neat stuff.

    If you’re finding spikes, they might be caused by a number of things, possibly your equipment or static charges. Down in the mine, we were pretty well “grounded”. And standing in puddles of water.

  8. George G. says:

    Yep, for several years I have been listening to whistlers. I built my workshop specifically for such tasks. Most of my equipment is DC powered, the high gain amps I made can pick the pulse from an electric fence several kilometres away. Mains EMR is a total drag, I switch in filters as required, and using directional loops I can work around most man-made EMI.

    The Hum does not bother me much, my wife however is tormented by it. As a favor to her I have tried to pinpoint its source. As a hearer myself, I worked around 30HZ (approx) and assumed it is electromagnetic in nature. I have been unable to detect it with my antenna array. When I read you worked with VLF radio gear deep in a mine my ears pricked up. The loop antenna you described 180 ft. below the surface orientated horizontally (E Field) grabbed my attention. I expected a vertical H field would surely be detected by such a loop. You seem certain that no such signal was present during your time in the mine. Notwithstanding the Hum’s not constant, but undergoes peaks and dips over a period of so far undefined time, and hoping that your monitoring period/s were sufficient enough to cover the lull/peak times of the Hum, I must admit the Hum may not be electromagnetic after all. In which case I no longer care to study it. I am a little bothered about one thing. You stated your receiver was geared to 20KHZ. Does this mean you were not looking for VLF/EFL??? More specifically, did you ever couple the loop directly into an audio amp??? I am guessing the answer is no. It seems you were operating a radio receiver, in which case you would have had very little chance of detecting an approx. 30HZ unmodulated sinewave. Would you agree?

    Regarding transients. Since retirement, my main interest has been observing negative-going-transients from the earth which precede major seismic events, usually within 72 hours of the transient. I am very familiar with ‘spikes’, man-made or natural. I just wondered if you ever encountered any negative-going voltage anomalies, sudden rises in DC earth currents, etc.
    (Not telluric currents) Again, I am guessing you were probably not looking out for such phenomena.

    Best regards,

    G.

    • jimvandamme says:

      We were not looking for 30 Hz, or any outside signals for that matter. There was a big power line (kilovolts) near the mine, and a high line (4160?) feeding the mine operations, so we just wanted to avoid/filter that all out. Our project was to do 3D imaging of below ground structures (bunkers, hideouts, that sort of thing) using ground penetrating radar. We were able to make images of the 15 foot wide mine tunnels down to 200 feet in quartzite. To get that resolution we needed to go into the HF bands so we were interested in attenuation in the rock. We used mostly bow tie antennas which were planar, wideband, easy to move (sort of).

      We used a lot of (expensive) portable spectrum analyzers, but for the GPR receivers we used homebuilt 12 bit A/Ds at either a low IF (kilohertz) or baseband I&Q. We’d fill up a hard drive then munch on the data with Matlab for days afterwards to get a 3D image.

      We also looked at detection and location of signals from equipment (computers, inverters, AC stuff) in buried bunkers (this was all military oriented of course). So we always tried to reject sky waves or any naturally occurring interference (QRM is the term I guess).

      The negative spikes idea is intriguing. I did build a couple of active dipole antennas. Transmit was always a big (up to 50 feet) stationary bowtie for efficiency and good match, but we had to move the Rx to many points in a grid to form an image. The active ants were only a few feet long so easy to move. I noticed a lot of low frequency noise which limited the performance. I had a crude 1MHz filter to roll off the AM brodcast, and I ran the antenna off a 9V battery and coupled the output through a transformer, but we still picked up noise. I thought it might be a nearby power line, or static caused by walking around with the antenna. Never figured it out.

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