Are we hearing a subharmonic? Friends and I hear a note that is lower than my lowest unfretted string on my standard acoustic guitar with standard tuning when I use the following very simple, easy method, pictured here:
As seen in the picture, a guitar pick is placed on top of the bass E string and under the next string (A string) in a manner similar to the way folks usually store a pick near the top of the fingerboard. I wedge the pick in between the E and A strings so that the edge of the pick rests on the fingerboard and the A string pushes down on the pick. The pick pushes down on the E string in this manner. (If the guitar action is too high to get the wedge affect, a piece of light cardboard can be placed under the edge of the pick that contacts the fingerboard, in order to shim it.) Then the E string is gently stroked or plucked with my finger. We hear a very low note that is much lower than the E unfretted string would make.
The resultant note is constant frequency, with unremarkable amplitude falloff in a fraction of a second or so. It does not warble or "beat", as far as we can tell. It sounds an octave or so lower than the bass E string.
I am wondering and investigating if it is the brain filling in the "missing fundamental," which is a known phenomenon of human hearing. I cannot find an example of a mechanical system that will produce subharmonics.