I have this experiment. I haven't documented it, but what is basically amounts to is this: I take out my tuner (an app on my phone) and close my eyes and sing the note that I think represents the center of my voice, the note that feels most natural and closest to my speaking voice. Since I don't have perfect pitch it will not be the same note every time. But it is always close to G3, below middle C. Sometimes it is F# or F, something Ab or A. I am not trying to hit G, or any particular note, and I haven't played a G on the piano in the half hour before I do it, or done anything else to orient myself.
To document it, I would do it every day and see how stable it was. For example, today, after not doing it for several months, I notice it is a lower note, F#. Am I just feeling and hearing my voice a bit lower, or is this random variation?