t0aj15,
I believe I got it from my days as a neurophysiologist, but I also believe you can find it in pretty much any high school physiology textbook. As a matter of fact, I believe you can probably find it in the paperwork that comes with almost any pair of quality earphones when you buy them. On average, the human ear's hearing range is from 20 - 20,000 Hz, or 20Hz to 20kHz. People vary, but that's the average range.
Now, while you can hear sounds down to 20Hz, you can
feel them down into the single digits. However, just because your ear is capable of hearing a given sound as low as 20Hz, it doesn't automatically mean your guitar string is capable of generating sounds as low as that. On a standard-tuned guitar, the low E string, as I mentioned previously, produces a fundamental tone of right around 82.4Hz. That's the lowest tone it is physically capable of generating. By definition, anything below that is considered a resonant undertone, a/k/a a harmonic undertone. Resonant undertones can certainly be felt by a human being all the way down to single digits, as also previously mentioned.
However, in order for a resonant undertone to be perceived as actual sound instead of just a felt vibration, a few things have to be in place. First, the mass of the item generating the originating fundamental tone has to be sufficiently large such that any undertones generated are of sufficient amplitude to be perceived by the human ear. Vibrating guitar strings, unamplified, do not have sufficient mass for this.
Second, the power transformer and speakers must both have sufficient dynamic range such that the undertones in question are capable of being generated as sound without dipping below the low threshold of the dynamic range. This is almost never a factor with the power transformers used in guitar amps, but is regularly an issue with typical guitar cabinet speakers. If the undertone is lower than the speaker's low threshold, it will be manifested by the speaker as destructive vibration, damaging or even destroying the speaker, especially over time. If the undertone is above that threshold, it will be generated as sound. If the natural unaltered amplitude (i.e., volume) of the undertone is too small, as it typically is in a guitar string, the amp's tone circuitry and power transformer must "step in" to boost that undertone's amplitude to a level where it can be heard by the human ear. This is something almost all amps do quite well.
Third, and regardless of the other factors, the undertone generated by the combination of the guitar string, power transformer and speakers must have a frequency greater than the low threshold of 20Hz, on average, of the human ear. Otherwise, and even if is above the low threshold of a given speaker, it will be perceived as a felt vibration rather than as sound by the human listener.
If the undertone is higher than the human ear's low threshold but lower than the speaker's low threshold, then even though the human ear is still capable of hearing it, it will be experienced as noise/harmonic distortion because the speaker itself is not capable of generating that frequency as an actual tone. Such an undertone will also cause varying degrees of damage to the speaker.
Guitar speakers' dynamic ranges vary from type to type, but typical 12 inch speakers naturally emphasize the guitar's midrange of frequencies due to their size. Though usually technically a full-range speaker, the size of the transformer and cone and the cone's materials naturally favor about a three-octave range situated in the middle of the spectrum. If you want high-fidelity reproduction of tones lower than the aforementioned 82.4Hz, you are more likely to succeed if you use a dedicated woofer or sub-woofer designed to handle such frequencies without being damaged.
Doing so doesn't automatically guarantee that you will be able to actually hear specific undertones in the signal, though. The undertones still have to be above 20Hz (the human ear's lower threshold) and they have to have sufficient amplitude. The amplitude is almost never a problem, since as mentioned, the power transformer of a typical guitar amp is more than capable of providing sufficient signal strength.
It should be noted, however, that even with a sub-woofer capable of safely reproduding it, and even with a transformer capable of generating sufficient amplitude, the insufficient mass of the guitar string itself means that most of the amplitude of that undertone relative to the fundamental tone will be "artificially" provided by the amp's tone circuitry and the power transformer, meaning an unavoidable introduction of at least a minimal level of added harmonic distortion. There's a good chance that absolute purity of the undertone won't be a significant criterion, but some distortion is unavoidable.
Well....that's it. As for "buy[ing] a word of it", I don't suggest you buy it from me. Verify for yourself every single little point I've made by checking it out on your own. Since you're already not accepting what I'm writing in this thread, and this post is only more of my writing, I suggest you verify anything and everything you care to through independent sources. Cheers.