# Why are VLF EM waves labeled as "sonic" in ARRL charts?

In the ARRL Ham Radio License Manual (4th Edition) an Amateur Allocation chart labels VLF with "Audible Range":

                    Very Low Frequency (VLF)
0Hz             10Hz                  10kHz          100kHz
Activities                      [   Audible Range   ]
<- Infra-sonics -> <-     Sonics      -> <- Ultra-sonics ->


Is there a reason this is included? The other activities listed are referencing EM waves: Microwaves (in the photo), Infrared, and X-rays. I agree that if this were an acoustic wave these frequencies would be in the audible spectrum.

The wikipedia page for VLF amateur use mentions QRSS, MFSK, and coherent BPSK as the modes used. I was at first thinking the carrier wave with no modulation could be associated with sound somehow. Maybe submarines (not amateur!) interpret the carrier wave as a continuous tone as it relates to some existing system. Maybe it means the mathematics of digital audio can be applied here, so traditional sound cards can be used as receivers.

Or maybe it is just a reference to something more familiar, like comparing a wavelength to the size of the Eiffel Tower, and I'm reading into it too much. Are there some audio applications with VLF?

• Hello Jared, and welcome to ham.stackexchange.com! Apr 1 at 2:40
• Your final paragraph is correct. Electromagnetic and sound waves are very different things, being generated, propagated and detected by different means. Apr 1 at 12:53
• @BrianK1LI is spot-on: antennas vs. microphones. IMHO, mentioning Infra-sonics, Sonics, and Ultra-sonics in that chart is utterly confusing. They should have been left out of the chart, and a separate explanation like your last sentence should have been included. Apr 1 at 15:38

You're reading into it too much. It just means signals that, if you were to receive them directly, with no demodulation or frequency mixing, they would fall into (or near) the audible frequency range. And indeed, some people do receive these kinds of signals using a loop antenna connected directly to a PC sound card's line-in.

• "receive them directly" could be either a microphone or an antenna, depending on whether the signal originated from a mechanical speaker or an RF transmitting antenna (which could include lightning). One is propagated mechanically through the air as sound waves; the other by means of EM propagation as you describe. Apr 1 at 15:42

When VLF EM waves interact with the material world, they can not only induce voltages and currents, but mechanical displacements or pressure waves than are in the range of human hearing.

Lots of people used to be annoyed by CRT monitor/TV flyback hum, and horizontal scan rate is VLF. And Van Eck remote mirroring (spying) shows that these CRT signals are indeed RF EM.

• Hmmm ... can you explain how a distant VLF EM wave could cause a mechanical displacement/pressure wave in a "material"? Maybe I'm dense. ;-) Apr 1 at 22:03
• I didn’t say anything about distance or power level. But I have heard power transformers hum. Apr 2 at 0:13
• May I suggest expanding this answer with exactly that point, to make it clear what phenomena you're referring to? Perhaps another sentence like "For example, power transformers audibly hum, acting as transducers for VLF waves at 50 or 60 Hz." Apr 7 at 21:34
• By flyback "hum", I assume you mean 15.75 kHz (yes, I was also annoyed when I had younger ears! ;-) In the old TVs, that was propagated both by 1. sound waves (generally not an EMF, because you don't hear it through wall) and 2. RF (EMF, not sound) which could be picked up with a VLF received at a much greater distance. Apr 7 at 22:14
• Respectfully, I think that you have the terms confused. The audible hum that we hear near a power transformer or older TV set is not EM in nature. Our ears do not pick up EM fields. Apr 7 at 22:20